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Mechatronics is a field of engineering where the integration of mechanical, electrical and control systems occurs. While mechatronics has no defined meaning apart from its description as an integration of all the main engineering fields it has been defined as a synergistic combination of precision engineering, electrical control and systems thinking in the design of products and manufacturing engineering.
Mechatronics is nowadays heavily utilized in a variety of industries both in the manufacturing processes used in the industry as well as in the actual products being manufactured. This is due to the fact that electronic control and monitoring systems are being inserted in all kinds of mechanical systems and microcontrollers in particular are being used in these systems and products to increase the flexibility and control possibilities of the system.
The main reasons for its relevance in the engineering and manufacturing industry are that it lowers the cost of production, improves the product quality, allows for complex qualitative decisions, adapts to changing production conditions and allows for the integration of manufacturing processes. In the actual product and design perspective, mechatronic allows for greater control over the product through the use of sensors and controllers as well as allowing these products or processes to perform far more complex tasks than can be possibly achieved using only electrical or mechanical systems.
Many modern engineering designs can be classified as mechatronic systems due to the ever increasing amount of electronics, sensors and control systems being placed in the simplest of objects. This can be seen in a variety of objects from household items such as washing machines, office equipment such as photocopiers and automobiles such as cruise control systems.
This move towards mechatronic systems is due to the innate advantages that mechatronics provides to the product such as flexibility, usability and robustness. Another major reason this is being done is that it makes it far easier to control the product and thus much more user friendly as all a user needs to do is tell the device what they want and the various mechatronic systems will complete the task without having to worry about anything else. One example of this is in washing machines where one can input the type of clothes, quantity and material being washed and the washing machine will configure itself accordingly to the optimal settings for that situation.
In general modern engineering designs are being classified as mechatronic systems as the use of mechatronic systems in these devices allows them to perform greater more complex function with greater user ability. This increases the value of the product and allows the manufacturer to remain competitive in a world where consumer are always expecting more from their devices.
In order to be able to control a mechatronic system, the processor performing the control procedures needs to receive signals to be analysed from the sensors and it needs to send signals to the actuators telling them what to do.
There are two basic types signals regardless of whether the signal is an input or output signal. These are analogous or digital signals. Analogue signals are continuous signals that transmit data in a constant stream. Processors are not capable of using or sending these types of signals and thus an analogue signal must be converted to a digital signal when it enters or leaves the processor, using an analogue to digital converter for input signals or a digital to analogue converter for output signals. Digital signals on the other hand are signals that only have two levels' high or low such as 1 or 0, unlike the varying levels of an analogue signal. Digital signals are the signals that can be processed by the controller's processor.
Most input signals are analogue signals such as those coming from a thermocouple and can be one of three types. These are a voltage level with some form of correlation to the input condition, a pulse with modulated signal or waveform that can have its amplitude, frequency or both modulated. Some input signals however such as those coming from an encoder are not analogue but digital signals. Digital signals come in the form of a square wave representing the high and low values of the digital signal. Output signals coming from the controller are all digital signals that must be amplified and/or converted to be used by the actuators. These output signals can come in the bit, byte (8-bit) or word (16-bit) form.
There are a number of different types of industrial sensors each with their own function and properties. These can include sensors for the measurement of position, temperature, force, acceleration, pressure and flow.
One example of such a sensor is the LVDT or linear variable differential transformer, which is a type of position measurement transducer. Its advantages are that it is accurate over linear range, has an analogue output that does not require amplification and it is less sensitive to changes in temperature. However on the downside it has a limited range of motion and limited frequency response.
A potentiometer is a manually adjustable resistor used to control current entering a system and can be used for applications such as speed control in a motor. The advantages of these devices are that it is easy to use, cheap, has a high amplitude output, is readily available and easy to instal. The disadvantages are that wear can occur due to friction, it has a limited bandwidth, has inertial loading, limited power and is noisy.
A thermistor is an industrial sensor used to measure changes in temperature through the measurement of change in resistance of a semiconductor. The advantage of this sensor is that it is accurate (about 0.01oC), more than other temperature sensors such as RTDs. It also has a low cost, sensitive and rugged. However it also has a narrower operating range, it is non-linear, self-heating and it needs to be well calibrated.
Piezoelectric accelerometers use a piezoelectric crystal to measure acceleration through the deformation of the crystal. The advantages of using this type of sensor are that it does not need an external power supply to produce measurements and that it can measure dynamic loads. However piezoelectric sensors are not suitable for measuring constant or slowly changing accelerations and they can pick up stray or static voltages.
There are eight main characteristics of sensors and actuators. These are:
Range - The difference between the maximum and minimum value that will give the desired output.
Resolution - The smallest increment on the input that can be detected reliably or the smallest increment in the signal that will produce a reliable movement.
Sensitivity - The change in output per change in input.
Error - The difference between a measured value and the true input.
Repeatability - The sensor's or actuator's ability to give identical outputs for the same inputs.
Linearity and Accuracy - Accuracy is the inverse proportional to the error and the linearity is the percentage of the full scale (maximum valid input).
Impedance - the ratio of the voltage and current flow of the sensor or actuator.
System Response - The way that the sensors or actuators respond to the change of inputs in time.
An electromechanical sensor is a sensor that uses electrical and mechanical components to create a signal that shows what in in the area of a sensor. This is usually done by having the mechanical component of the sensor complete an electric circuit that sends out the signal. Such electromechanical sensors are often used in robotics as safety sensors for instance. One type of sensors that are electromechanical sensors is proximity sensors and switches. There are three main types of electromechanical proximity sensors and switches and these are inductive, capacitive and magnetic proximity sensors and switches.
Inductive proximity sensor and switches are devices that detect proximity of metallic object through the use of a coil. An electromagnetic field is generated in the coil at a certain frequency. When the field get into contact with a metallic object some of the field is absorbed by the metal changing the oscillation field which is detected by the sensor when a certain threshold is reached.
Capacitive proximity sensors and switches detect both metal and non-metal and works by detecting the change in capacitance between the sensor and the object. The device has a plate that acts as half of a capacitor with the object being the other half. As the senor moves closer to the object the capacitance increases which can be detected by the sensor when a certain threshold has been reached.
Magnetic proximity sensors and switches work through the use of a permanent magnet found in the device. At its most basic level a magnetic proximity sensor or switch works by having the permanent magnet make or break an electrical circuit (according to whether the sensor or switch is normally open or normally closed) when a metallic object moves close to the sensor thus sending a signal to the system.
Nanomachines are device that are currently under development that fall into the bracket of devices that are in the range of the smallest MEMs to machines made up of individual molecules. These kinds of machines are manufactured by assembling one molecule at a time to create molecular component that in turn can be assembled to create supramolecular structures called molecular devices; where each component has its own function. The combination of these components acting together allows the device to operate and perform its various functions. It should also be noted that to power these devices one is limited to using power sources that can act on a molecular level to power these devices such as photochemical reactions.
Materials being used for such designs or manufacture include single-walled carbon nanotubes (SWNTs) and multi-walled carbon nanotubes (MWNTs). These are both materials whose diameter is just a few nanometres. These materials are used for such devices due to their excellent material properties such as high modulus of elasticity and high mechanical strength.
While they are still largely in development several examples of such devices have emerged such as nanobearings, nanosprings and nanotweezers as well as designs for nanomachines as electromechanical actuators. Some fields have been identified where such devices could be or have been applied and these include health and food.
There are several kinds of smart materials used in mechatronic applications. Three types of smart materials used in sensors are optic fibres, piezoelectric, and magnetostrictive materials; while there are four types of smart materials used in actuators which are shape memory alloys, piezoelectric, magnetostrictive, and ion exchange polymers.
Optic fibres are fibres of glass that use changes in the intensity of the light travelling through the fibre to calculate the strain, liquid level, force or temperature changes proportional to the change in light.
Shape memory alloys are allows that at a certain specific temperature range change their shape to a predesigned shape. They are used in applications such as medical devices, computers, consumer products, the automotive industry and other industrial products.
Piezoelectric is a smart material that consists of a crystal that creates an electric charge when the crystal undergoes deformation or that expands or contracts when a charge is given to the crystal. These crystals are used for both sensors and actuators such as accelerometers, pressure transducers and actuators for micro or nano machinery.
Magnetostrictive materials are used for both sensors and actuators. They an alloy of terbium, dysprosium and iron and work by generating a strain witghin the alloy of up to 2000 microstrain when a magnetic field is applied to the material. It comes in the form of washers, rods, plates and powders.
Ion exchange polymers are
Microactuators and microsensors are both types of MEMs used in mechatronic systems in the place of large actuators and sensors in components or products where small sized components are required.
Microactuators are MEM components that provide the movement or output required by the system being controlled. They are usually made up of functional parts that are 1-15mm in size. These devices are usually manufactured using either lithography or micro machining. There are several kinds of microactuators available though they fall into two main categories rotary or linear microactuators. They can function using one of three kinds of methods, electrostatic, electromagnetic or piezoelectric with electrostatic being the most commonly used form of microactuators.
Electrostatic microactuators work using electrostatic transduction and are regarded as variable capacitance type actuators. Types of electrostatic microactuators include variable capacitance motor, wobble/harmonic drive motor, normal-drive linear actuators and tangential/comb-drive linear actuators. Electromagnetic microactuators work using a current carrying conductor moving through a magnetic field much like a normal electromagnetic actuator while piezoelectric microactuators work using piezoelectric crystals that deform when a current is applied.
Microsensors are MEM devices that are used to monitor the environment around the microsensor and relay that information to the controller. Microsensors are in essence scaled down version of ordinary macrosensors that are manufactured using silicon micromachining technologies to make the sensors smaller with better performance and lower cost. These sensors are being greatly used nowadays due to their excellent properties such as sensitivity, accuracy, dynamic range, reliability and low power consumption. They are used to detect a variety of different variables and send an electrical system to the controller about any changes in these variables.
Microsensors are most often used to monitor pressure, strain, acceleration or force. These kinds of sensors are called mechanical microsensors and are the ones most often used in industry. Other types of microsensors however can be in chemical or biological application to detect pH for instance.
Actuators are the muscle of the mechatronic system. They are devices that create motion by turning the electrical signals coming out from the controller into some form of mechanical power.
There are several classes of actuators used based off of several different power sources. Actuators for instance can be rotary or linear. One main way of classifying actuators however is by their power source. There are three main power sources used by actuators and these are electrical, hydraulic or pneumatic. The main types of electrical actuators are solenoids and motors.
Solenoids are electromechanical switching devices used in applications such as appliances, automobiles and automation. Relays are a special kind of solenoid used to make or break electrical circuits by connecting or disconnecting electrical leads. They are often used in applications such as power switches and electromechanical control elements as they can withstand high currents. The basic components of a solenoid/relay are a movable iron core and a coil that when energized creates magnetic field that moves the iron core.
Electric motors are one of the most common devices used and are electromechanical devices used to create rotary motion. There are two main kinds of electric motors each with their own subtypes. The type of motor differs according to the type of current passing through it. The two types are DC motors and AC motors. The main subtypes of DC motors are brushless and brushed while the main subtypes of AC motors are single phase or polyphase. All electric motors consist of a stator, which is the outer housing of the motor that supports the magnetized poles that generate the rotation and the rotor, which is the part of the motor that actually rotates and coils called armatures and an iron core to intensify the fields of the armatures. The types of DC motors available are permanent magnets, shunt wound motors, series wound motors and compound wound motors. Another important type of DC motors are stepper motors that are a type of permanent magnet motor or a variable reluctance DC motor that moves in steps with a digital output rather than continuously with an analogue output. The AC motor types are induction motors and synchronous motors.
The next types of actuators are hydraulic and pneumatic actuators. While these two types are usually separated the fundamental methods of operation and the type of actuators used are almost identical; as both hydraulic and pneumatic actuators use a fluid, usually oil or some other liquid for hydraulics and compressed air for pneumatics, to push against the actuator to produce motion. Each has two main types of actuators, hydraulic/pneumatic cylinders/pistons for linear motion and hydraulic/pneumatic motors and rotary actuators for rotary motion.
There are several types of cylinders that can be used in either hydraulics or pneumatics. The most basic types are either single acting or double acting pistons. Single acting pistons have the fluid pressing on only one direction out of the cylinder to provide motion and power. To retract the piston would usually have something like a spring to retract the piston. Double acting pistons have the fluid acting in both directions in and out and thus the cylinder can move under power in both directions.
Hydraulic motor are motor that work using hydraulic or pneumatic power to provide continuous rotary motion. There are a variety of motor that can be used and these in general are the reverse of hydraulic pumps. One example is a gear motor that has the fluid turn one gear that meshes with another attached to a shaft to provide the rotary motion.
Rotary actuators are actuators that provide angular rotary motion, which means that unlike the motor the motion here occurs in steps. Two types of rotary actuators include the double-vane rotary actuator and rack-and-pinion type rotary actuator.
A microcontroller is a unit used in mechatronic as the brain of the system, which means that all the control, arithmetic and logic operations happen through the microcontroller. A microcontroller consists of a central processing unit (CPU), read only memory (ROM), random access memory (RAM) and an input/output (I/O) module as the main components. The microprocessor also the data bus, the address bus and the control bus which are used to connect all the components together and transfer data between them.
The CPU is the processing unit of the microprocessor that performs the arithmetic and logic operations required by the system as well as reading and implementing the programs required to perform the control operations. The CPU consists of several components. The component used to process the arithmetic and logic operations is the ALU or arithmetic logic unit; it also possesses registers which are temporary storage devices that are used for data storage and for addressing memory. A program counter is used to pinpoint the location of the program memory after which the instructions are fetched from the program memory and decoded using the instruction decoder. After the decoding has been completed, the data is used by the sequence controller, which generates the control signals that are sent to the actuators to execute the instructions in the program.
The programs and their variables are stored in the two types of memory found in a microprocessor. These are volatile memory or RAM and non-volatile memory or ROM. Non-volatile memory is memory that does not lose the data stored upon when the power is removed. This type of memory is the memory upon which the control programs are stored. While all the non-volatile memory is ROM memory there can be several types of ROM used such as PROM (programmable read only memory), EPROM (erasable programmable read only memory) or EEPROM (electrically erasable programmable read only memory). Volatile memory is memory that loses the data stored in it when power is removed. This type of memory is used to store the program variables used that are generated during use of the program such as the counted value of a counter.
Data can either be read or written from the memory; be it ROM or RAM. The data is read using a memory read cycle where the address of data to be read is placed on the address bus to the CPU and a read signal is sent. After this the data in that address is placed on the data bus which is sent to the CPU. The memory write cycle operates in a similar manner accept instead of the data being sent to the CPU, the data is placed on the data bus by the CPU and is sent to the memory to be written into that address.
The I/O module is an interface module used to convert and interpret the signals entering the CPU from the input devices and convert the signals leaving the CPU towards the output devices. It is made up of logic circuits used to safely transmit the data as well as converters to convert the signals to analogue from digital in the case of input signals (if needed) and from digital to analogue for output signals. It also contains amplifiers used to convert the signal into a form that can be read by the CPU or actuator.
The data bus, the address bus and the control bus are all ways for the CPU to connect with the various components of the microprocessor. The address bar is used by the CPU to select a location or address where data is stored in the 216 locations available in the memory map. The data bus is used by the CPU to move information to/from the CPU from/to the location where the data is present or needed. The control bus is used by the CPU to send out the various control signals such as the read or write signal to the memory or the I/O module.
Op amps or operational amplifiers are an integrated circuit used in a variety of components and applications due to its low cost and to the versatility of its circuits. It consists of a variety of internal components joined together to form a variety of circuits. These components include resistors, transistors and capacitors. Op amps are a type of differential amplifier which means that this amp works by amplifying the difference between two input voltages. Op amps can be used with other external components to perform a variety of operation. This is done through the creation of a variety of signal processing circuits. Some examples of these operations and circuits are amplifiers, integrators, summers, differentiators, comparators, A/D converters, D/A converters, active filters, sample amplifiers and hold amplifiers
Op amps are active sensors that have two input signals and it requires both inputs to function. As the op amp is an active sensor it requires an external power source to power the sensor; this is usually a +/- 15V power supply. These inputs are called the inverting input and the non-inverting input. The inverting input are designated with a negative sign and have a negative voltage gain, whilst the non-inverting input consist of a signal with a positive voltage gain and it is designated with a positive sign. For the op amp to work properly these inputs both need to have the same value as if the value differ the output will become positive or negative according to the higher input value. The output wave of the op amp consists of an inverted waveform of the input signal waveform. The op amp can be set up in either a closed loop or open loop configuration. The closed loop configuration can be used to control and stabilize that amplifier gain and this can be achieved by adding a feedback loop to the inverting input.
In order to help in analysing and designing circuits of which op amps are a component ideal models of op amps are used. To make this ideal model, five assumptions are taken. These assumptions are that the op amp has infinite gain for the differential input signal, that it has zero gain for the common-mode input signal, that it has infinite input impedances, that it has zero output impedance and that the op amp has infinite bandwidth. When op amps use closed loop control systems they use feedback loops. The feedback loop used is usually if not always negative feedback loops. This means that the feedback coming from the output signal returns to the input signal in opposition to the source signal. When considering this in terms of the ideal op amp it means that the value sent to the input signal by the feedback loop will force the differential input voltage and input current to be equal to zero. This is defined as the summing point constraint. This is done so that in all circuits in which op amps are used the output are adjusted to keep the values of the inverting input and the non-inverting input are equal. There are a variety of op amp circuits that can be created when the feedback is added and these include the inverting amplifier, the non-inverting amplifier, the summing amplifier, the integrator and the differentiator.
Photoelectric sensors and laser sensors are both examples of proximity sensors used within industry to either how close an object is to the sensor or to detect whether an object has passed in front of the sensor. Both operate in a similar manner though there are differences between them.
The fundamental operation of both these types of sensors operates in a similar manner. At their most basic, both of these sensors work by sending out a beam, of light in the case of a photoelectric sensor and a laser in the case of a laser sensor, and then seeing either how the light returns to the receiver on the sensor or whether the beam is broken or not. A 'light' sensor works by receiving back a signal or beam while a 'dark' sensor transmits a signal when the beam is not received by the signal.
A photoelectric sensor can be one of three types, an opposed mode alignment sensor, a retroreflective mode alignment sensor or a diffuse/proximity mode alignment sensor. The opposed mode alignment sensor and retroreflective mode alignment sensor are both examples of 'dark' sensor in that both transmit a signal to the controller when an object breaks the light beam between the receiver and the transmitter; though the retroreflective mode alignment sensor can be used as a 'light' sensor if it is used to detect proximity by calculating how long it take for the beam to return to the receiver. The opposed mode alignment sensor works by having the transmitter and the receiver separate and the beam is sent directly towards the receiver. In the retroreflective mode alignment the transmitter and the receiver are both in one unit and beam is transmitted onto a surface after which the beam reflects off the surface back towards the receiver. The diffuse/proximity mode alignment sensor is an example of a pure 'light' sensor. The beam here is transmitted towards a surface upon which the beam is broken up and diffused in various directions. The receiver then detects how much of the light is received back.
Laser sensors on the other hand work through triangulation and are primarily used to detect proximity and to determine the distance from the sensor to an object. The laser emitter sends out a laser that hits the surface of the object. This laser is then reflected of the object into a collection lens located near the emitter. This lens tend focuses the image onto a camera that is used to determine the distance from the sensor to the object. This method is called triangulation due to the fact that the beam path forms a triangle with the front of the sensor.
The two main differences between these two sensors is the mode of operation as described above and the type of light sent out by the sensor. In photoelectric sensors the beam transmitted is a beam of light while the laser sensor sends out a laser beam. This also means that laser sensors are in general more accurate than photoelectric sensors as they are not susceptible to noise arising from ambient light as they use completely different frequencies of light and as such are not effected by ambient light.
The design of a mechatronic engineering system is a complex matter as there are many variable that enter into a mechatronic system to ensure that it is designed properly. Therefore a devising model procedure is use that utilizing basic mechanical building blocks. There are four phases to this design procedure.
The first phase involves the listing of ideas and is called the requirement analysis phase. This phase is used to identify the mechatronic system requirements; which means that the mechanical, electrical and control requirements of the system need to be identified here. However the point of this phase is not just to identify the technical requirements but also the customer requirements, safety requirement and company requirements as well as any other requirements that may be expected but not specifically stated such as maintenance. This phase is also used to identify design problems that may crop up as well as the requirements. The purpose of this phase is to gather enough information and data to eventually be able to recognise when the design has fulfil all that is required of it. Some tools used in this phase include specification listings, use case diagrams, sequence diagrams and context diagrams
The second phase is the concept generation phase. This phase is used to multiple generate conceptual designs that could satisfy the requirements identified in the first phase. The point here is not to come up with full solutions to the mechatronic system but rather to develop concepts for the various parts of the system in varying levels of detail as needed thus fulfilling some of the requirements in each concept. This is done so that the concepts can be combined together at a later stage to create a complete solution. After these concepts are create they are combine together to form multiple possible solutions. Tools used in this phase include block diagrams and various modelling techniques.
The third phase is the solution evaluation phase. In this phase the possible solutions devised in the second phase are gathered and using model analysis tool are analysed to identify the optimal solution for the mechatronic system. The optimal solution can be selected using multiple methods according to the preference of the designer but it at the very least must satisfy all the requirements identified in the first phase as well as any addition criteria selected to be able to decide between the possible solutions. Some methods used for selecting the optimal solution include the minimal overall cost function or the maximum overall value function.
The fourth phase is the detailed design phase. Here the final design of the mechatronic system is created and any unresolved design details are resolved. This step involves repeating the process described here in order to simplify the design through successive iteration while still fulfilling the design requirements.
Control is one of if not the most important parts of mechatronics. All control systems regardless of the kind of control architecture used are made up of three basic components. These components are the input signal coming from the input device such as a sensor, a switch or a valve, a controller or microprocessor that contains and runs the control program for that system based on the inputs it receives and the output signal sent from the controller to the output devices such as a motor or cylinder with instructions for the actuator to perform. Other components that can be found regardless of the type of control system include parts like amplifiers to amplify the electrical signals being transmitted and ADCs or DACs that convert the signals entering or leaving the controller into the type the controller or the actuator requires. The true differences begin when it comes to the type of control used. There are two types of control architecture that can be used in a mechatronic system: open loop and closed loop control systems.
Open loop control systems are the most simple of the control systems available and are also called non-feedback systems. This is because no feedback is gathered by the system to control or modify the output and therefore the output is calculated using only the current state of the input and the programmed model of the system. This means that the system cannot react to any changes or errors in the system. Such control architecture is only used for simple systems where complex control or feedback is not required and for systems where the relationships between the input of the system and the resultant output can be easily and well defined by a mathematical formula or model such as a conveyor travelling with constant speed. Another application were open loop is used is for the control of stepper motors in simple devices such as inkjet printers or simple robots.
Closed loop control systems on the other hand are the exact opposites of open loop systems and are more commonly used in industrial purposes despite being far more complex. In closed loop systems there is some form of automatic control as through sensors the performance of the actuator or the surrounding environment is monitored to detect changes or errors. These are then used by the controller to modify the output signal to react to these changes thus keeping the output within the desired limits. One example of this is the cruise control system in a car which detects the speed the car is travelling and adjusts the fuel injected into the system accordingly. The most common type of closed loop system uses the feedback method where the output of the system or its effects are monitored and feedback into the controller. If any changes or error are detected the signal is changed to account for these changes thus keeping the output in the desired range. The aforementioned cruise control is an example of feedback control. The other method of closed loop control is far less commonly used and is called feedforward control. In feedforward control the system monitor the environment directly and responds to changes before they have time to affect the system. Such control occurs on the system input rather than the system output and is based off predicted models of the system's performance. An example of a feedforward system is a heating system that detects that a door is open and begins heating before the room gets cold. Feedforward systems unlike feedback systems are not error driven. Many modern system nowadays utilize both forms of closed loop control in order to benefit from the advantages of both while limiting their drawbacks.
An important part of designing a mechatronic system is the proper selection of the sensors and actuators needed for the system to work. As such a proper determination of the selection criteria needs to be identified.
The most basic elements of the selection of a sensor or an actuator begins by analysing the requirements of the system and compering them with the sensor/actuator characteristics listed in P1.5. These characteristics are range, resolution, sensitivity, error, repeatability, linearity and accuracy, impedance and system response.
These however are only the most basic selection criteria common to both sensors and actuators; other criteria exist specific to sensors or actuators. However, first one of the most obvious and basic selection criteria is of course cost or rather the users budget. This is due to the fact that while everyone would like to select the most complex, most advanced and precise sensor or actuator available, the reality is that every industry has its budget and therefore one has to select a sensor or actuator that while fulfilling the other criteria such as the range of motion/sensing and accuracy required the selected device must fall within the budget for that device.
Including the basic criteria mentioned above, the other criteria involved in the selection of sensors include:
• Range-Difference between the maximum and minimum value of the sensed parameter
• Resolution-The smallest change that the sensor can detect reliably
• Accuracy-Difference between the actual value of the parameter and the value sensed by the sensor
• Precision-The ability of the sensor to repeatedly and accurately produce the same value
• Sensitivity-The change in unit output over the unit input
• Zero offset-A nonzero value output for no input
• Linearity- A percentage of the maximum value best-ï¬t linear calibration curve
• Zero Drift-How much the output will drift from the zero position over time despite having no input
• Response time- The time it takes for the sensor to produce an output when an input has been applied
• Bandwidth-Frequency at which the output magnitude drops by 3 dB
• Resonance-the frequency at which the maximum output value is detected
• Operating temperature-The temperature range at which the sensor operates as specified
• Deadband-The sensing range for the input into the sensor produces no output
• Signal-to-noise ratio-Ratio between the magnitude of the signal produced by the senor and the magnitude of the noise detected by the sensor at the output signal of the sensor
Besides the above mentioned criteria, other criteria for the selection of actuators include:
• Continuous power output-The maximum torque or force the actuator can achieve without exceeding the specified temperature limits
• Range of motion-The range of motion that can be reached by the actuator example the stroke of a cylinder
• Resolution-The smallest force or torque the actuator can create
• Accuracy-Linearity of the relationship between the input signal and output of the actuator
• Peak force/torque-The maximum force/torque the actuator can produce which occurs when stalling occurs
• Heat dissipation-How much heat the actuator can dissipate during continuous operation
• Speed characteristics-The relationship between the force/torque and the speed
• No load speed-The operating speed/velocity when the actuator is under no external load
• Frequency response-The range of frequency in linear actuators when output is proportional/linear to the input
• Power requirement-The type of power the motor requires - voltage, type of current (AC/DC), number of phases etc.
For instance when selecting an accelerometer for an impacting cylinder one needs to consider the theoretical acceleration range of the cylinder, the type of acceleration experienced (dynamic for example), the required accuracy and sensitivity, how quickly does the sensor need to react and what are the operating conditions of the sensor.
In the case of a motor selection one needs to consider the type of power used by the motor, hydraulic, DC or AC (hydraulic motors for instance cannot be used in clean environments and AC motor are more versatile in industrial applications), how much speed is required, how much torque must it generate, how long will the motor be continually running, what are the operating conditions of the motor and what type of motor is require (torque speed characteristic for instance).
The first step of sensor selection is to identify the type of sensing required and from this to decide the type of sensors required.
From a study of the case study selected it can be determined that at the very least two or three types of sensing is required to be able to control the system. The first type of sensing the systems requires is the myoelectric voltage sensing. This is need so that the controller can detect and measure the muscle contractions the robot needs to replicate. The second type of sensing needed is position sensing within the robotic joints. The controller needs this data to be able to know whether the robotic arm is moving in the same manner as the human arm. The final type of sensing that may be required depends on the application of this robotic system, with the sensing in question being vision sensing. This may be required to allow the human to direct the motion of the robot through his own motion by seeing how the robot is moving and thus directing it exactly where he wants the arm to go.
From this analyse it can be concluded that myoelectric sensors are required for each of the major muscles in the arm, encoders are required for the joints in the robotic arm (between 3 and 6 depending on whether the gripper is included in the system and what type of gripper is used) and a video camera to allow the human to see how the robot is moving. In the case that the gripper is included in the system a tactile force sensor is also to be included to ensure that the gripper can pick objects up without breaking them.
To select the type of actuators to be used in this system, the type of motion being done within the system needs to be identified.
From an analysis of the movement within the system it can be deduced that the only primary source of motion within the system is that of the robotic arm itself or more specifically of the arm's joints. Seeing as the robot configuration in the Case Study is a SCARA robot is can be stated that the arm possesses three joint with two joints being revolute joints and the other joint being prismatic. The gripper mechanism appears to only have one revolute joint. This brings the total amount of actuators needed by the system to four.
The motion in all four joints can be accomplished using an electric motor as the actuating device. The type of motor selected depends on the size of the robotic arm. If the arm is a large mechanism then AC motors would be the best choice as the actuators. However if the arm is not that large a servomotor would be the best choice as they are easy to control, they are reversible and they have a linear torque-speed relationship.
Microprocessors and microcontroller/microcomputers are both devices used to perform similar functions, i.e. to perform control, arithmetic and logic functions within the system they are integrated within.
A microprocessor is a device printed on an integrated circuit that is used together with external components to take an input signal and using preprogramed instructions send out an output. Microcontrollers have a similar purpose with an input being received and an output being sent out according to a preprogramed set of instructions. However as said there are differences between the two starting with the general purpose and construction of the devices. A microprocessor consists of just a CPU with the computing sections and some basic registers for memory. This means to accomplish any task all the other components required to process the data it receives and sends need to be acquired and attached separately. This includes components such as memory; both ROM and RAM, serial ports, timers and I/O ports. Microprocessors are also intended as general purpose devices where the tasks the processor will perform are unspecified and the input-output relationship is undefined. Therefore these tasks generally require a great deal of ROM, RAM and I/O ports.
Microcontrollers on the other hand while also being an integrated circuit being used to produce an output based off preprogramed instructions and the input into the device, the difference is how the resources are used and acquired. In the case of a microcontroller the components such as the RAM, ROM, timer, serial ports and I/O ports are already integrated into the microcontroller circuit along with the CPU. This means that no other external devices are required for the microcontroller to work unlike the microprocessor which cannot function on its own. Microcontrollers are also intended as special tasks device, which means that the application where a microcontroller is used is one with specific tasks. These tasks are ones where the relationship between the input and the output is clearly defined and thus not as much processing is required to reach the required output. It is due to this input-output relationship that the other components are integrated directly with the CPU as the resources needed for these specified tasks are relatively small and therefore can be integrated directly in one chip.
There are a number of other differences between the two devices. Microprocessors for instance operate with far high clock speeds than that of microcontrollers as modern microprocessors have clock speeds of over 1GHz in complex operations while microcontrollers operate as clock speeds in the range of a few MHz to a maximum of around 30-50MHz. The way memory is stored also differs between the two devices. Microcontrollers can store programs and data within separately within the integrated ROM and RAM, whilst this is not possible for just a microprocessor with no external components which means that data and programs would have to be stored in the same memory. Microprocessor also cannot process Boolean functions without the use of external devices unlike microcontrollers which have the necessary components already integrated. Microcontrollers also have more multifunction pins on it integrated circuit than there are on the microprocessor integrated circuit. Finally programming on a microcontroller takes far less instructions to accomplish a task such as retrieving or sending data to/from external storage than using microprocessors which requires far more time and instructions.
In general microcontrollers are cheaper than microprocessors but the reality is that one cannot compare them in this manner. This is because each has their own task and one cannot accomplish the tasks of a microprocessor with a microcontroller and accomplishing a microcontroller's tasks with a microprocessor would be too expensive and impractical. It should be noted however that a microprocessor is a component of a microcontroller.
Amplifiers are an important part of signal conditioning that is used within mechatronic systems. Linear amps are circuits whose output is proportional to the circuit's input, yet still has the ability to provide more power for a load. When analysing the input/output characteristics of a linear amplifier such as op amps, one must first analyse the model for an ideal amplifier.
Ideal amps are theoretical amplifiers that have certain assumptions as characteristics. These assumptions are that the ideal amp has infinite gain for the differential input signal, that it has zero gain for the common-mode input signal, that it has infinite input impedances, that it has zero output impedance and that the op amp has infinite bandwidth. Ideal amps also operate under two so called Golden Rules: the voltage rule and the current rule. The voltage rule states that the output of the amp will always try and make the difference between the two inputs to the amp equal to zero. The current rule is that the inputs draw no current. An amp such as this even if it is not an ideal amp, such as an op amp, is never used on its own and is always used as part of a circuit. Some of these circuits include inverting circuits, non-inverting circuits
Inverting circuits are one type of linear op amp and digital circuits use this amp as an inverting buffer. In this particular circuit the inputs inverts and scales the input signal. This means that there is an 1800 phase shift between the output signal and the input signals.
DC motors are one of the largest branches of electrical motors and are used in a number of applications within mechatronics such as robotics. Whilst there are a number of different types of DC motors available such as permanent magnets, shunt wound motors, series wound motors, compound wound motors and stepper motors; DC motors can be broadly classified into two main types: brushed motors and brushless motors.
The basic operating procedure of a DC motor varies with the type of DC motor being used; however some basic similarities exist within the basic structure with the differences occurring in whether permanent magnets are used and how the windings in the stator and armature are connected. The two basic components of a DC motor are the rotor and the stator. The rotor is the piece of the DC motor that actually moves and consists of coil windings called armature winding where electricity passes through to create a magnetic field and an iron core, usually a laminated iron core to reduce eddy currents, which is used to intensify the magnetic fields created by the armature windings. The rotor is supported within the stator using bearings. The stator is the outer housing of the motor and like the rotor it has its own windings called stator poles and an iron core, also usually a laminated iron core to reduce eddy currents, which is used to intensify the magnetic fields created by the stator poles. These poles are used to create radial magnetic field and it is the interaction between these two fields, the stator fields and the rotor fields that creates the rotatory motion within the rotor.
Brushless DC motors consists of windings in that stator and permanent magnets in the rotor. When the poles are the same the repulsion between the poles causes the rotor to rotate. In order to maintain this rotation however the poles of the stator magnetic field need to be constantly changing in order to have the required repulsion between poles. This is done through the use of proximity sensors in the brushless motor that trigger as the rotor rotates switching the DC currents in the stator windings.
Brushed DC motors on the other hand have windings in both the rotor and the stator and thus the magnetic fields are created through magnetism. This means that one set of windings needs to maintain the same poles while the other switches them as the rotation occurs. In the case of brushed motors it is the rotor's magnetic field that does the switching. This is done through the use of graphite brushes and a commutator that make contact with armature windings to transmit the electricity. As the rotor rotates each side of the armature windings connects with a brush that provides the armature with its electricity and thus pole. As the rotor rotates at a certain point the windings loses contact with the brushes losing its magnetic field, though the rotor keeps moving due to momentum. As it moves past the no contact zone the sides regain contact with a brush but this time it is the other brush which means that the sides have now switched magnetic poles and this process continues as the rotor rotates in order to generate the motor rotation.
Now apart from the brushless or brushed motor divide, motor can be classified as permanent magnets, shunt wound motors, series wound motors, compound wound motors or stepper motors.
Permanent magnet motors are the only type of motor that can be reversed and consists of permanent magnets on the stator instead of windings. They are considered perfect for control applications as the torque-speed relationship is perfectly linear.
Shunt wound motors are DC motors that have the armature and stator windings connected in parallel and powered by the same power supply. This results in speeds that are near constant over a large range of loadings with starting torques that are 1.5x the operating torque. The speed in this type of motor can be easily controlled using a potentiometer connected in series to the field windings.
Series wound motors are DC motors that have the armature and stator windings connected in series and powered by the same power supply. Series wound motors have very high starting load and the speed of the motor varies highly according to the load place on the motor with the highest and potentially catastrophic speeds occurring when the motor has no or little load. the torque-speed relationship of these motors is hyperbolic which implies that there is an inverse relationship between speed and torque as well as nearly constant power over a wide range of speeds.
Compound wound motors are DC motor that are a combination of both series and shunt winding thus resulting in characteristics that are a combination of the two. This results in a maximum speed and speed control that is not as good as shunt would and torque which is not as strong as series wound motors.
Stepper motors are a special type on permanent magnet motor that produces a digital motion rather than an analogue motion like all other motors. This is done by moving in steps according to the pulses that it receives from the electric drive circuit which controls the motor's position and speed according to the number and rate of pulses. These steps can occur in 30°, 15°, 5°, 2.5°, 2° and 1.8° increments according to the type of motor selected.
Over the year a number of products and systems have been converted from purely mechanical or electromechanical to true mechatronic systems. This has occurred for a number of reasons such as to keep competitive in an ever advancing market, to increase the capabilities of the product and to simplify its use for the user. One example of this switch can be seen in the modern automobile.
Automobile of a couple of decades ago used to be purely mechanical devices where every part of the car from the pedals to the steering to the gear shift was done purely through mechanical means. However as time passed by more and more controls, electronic and sensors entered the system in an effort to improve and modernize it as well as to maximize profit. This can be seen from a number of modern car systems such as the humble auto lock and electric windows to more modern and complex systems such as cruise control, anti-lock braking system (ABS) and traction control thus turning the modern car into a true mechatronic system.
This is not to say that the car could not function without these advances as it can and certain people even prefer it without them to all the new complex and expensive systems but rather that these were developed to make life easier on the user.
One example of this is traction control. Traction control is a system used in modern car to prevent a loss of traction or tyre slip when the car is accelerating on a slippery surface. Before the introduction of traction control, drivers had to drive very slowly while putting as little pressure on the gas pedal to prevent the car from slipping and losing traction. Traction control was introduced to make life easier on the driver by controlling the power or brake automatically without input from the driver. Through the use of sensors spread around the wheels the car can detect the rotational speed of the wheels in a system also used by the ABS. When the car detects that a wheel is spinning faster than the other wheels the traction control automatically kicks in to prevent the car from losing traction by intervening in one of a number of systems. The systems the traction control can affect include the brakes, the throttle or the fuel supply to cylinders. This is done so that the power being supplied to the wheel in question is reduced and thus the wheel will return to the same rotary speed as the other wheels thus preventing the loss in traction. As can be seen the implementation of a mechatronic system rather than a purely mechanical system such as there was before the implementation of traction control means that the driver in this case has a far safer and pleasant driving experience as he has less to worry about while driving and is less likely to slip and crash the car.
Another mechatronic system tied to the traction control is the ABS or anti-lock brake system. While traction control prevents slipping whilst one is driving, the ABS prevents the car from slipping whilst one is breaking. The ABS is in place in modern car as a safety feature to prevent the car's brakes from locking up on slippery conditions and thus crashing the car. In a vehicle without this system a driver would need to feather the brake (press lightly on the brake) to prevent slipping or pump the brake to unlock brakes that have locked due to too much pressing to the brakes. The ABS prevent the need to do this by ensuring that the brakes never lock up in the first place and pulsing the brake pumps to ensure that the car never passes the lock up threshold and thus breaking in the shortest distance possible. An ABS is made up of four main components the sensors, controller, pump and valves. There are speed sensors that detect the rotational speed of the wheels. When the ABS controller detects that there is a sudden rapid and impossible (for the car) deceleration in the wheel the ABS kicks in by sending a signal to the valves and pump. These reduce the brake pressure until the wheels begin to accelerate upon which the controller sends a signal to increase the pressure until the wheel begins to decelerate again. It continues to do this in quick pulses to ensure that the wheels slow down at the same deceleration rate as the rest of the car. The cycling of pulses also ensure the shortest braking time possible by keeping the brakes at a point at which they are almost going to lock up as at this point there is the maximum braking power. The advantages of this system over a system without ABS are that one doesn't have to worry about the brakes locking up and the driver losing control and it allows the driver greater control of the car whilst braking (the driver can steer better).