Basics Of Embedded Systems

The term embedded systems is quite a complex one. Simply put, it is a combination of hardware and software that performs the component of a larger system. A few years ago embedded technology existed in stand alone devices such as vending machines and copiers that did their jobs with little regards for what went on around them. But as technology advance to connect devices to the internet and to each other, the potential of embedded technology has increased. Home appliances, mobile phones, cars, tiny micro chips, avionics etc.., are all using embedded technology.

High-profile embedded chips are scaleable, generate small amounts of heat, and consume less power. These are generally preferred for their speed, accuracy and reliability. As they are compact in size and ability to perform time-critical and task specific operators, embedded devices find application in all segments of industrial and commercial market places and home appliances.

In recent years,it became apparent that control systems as integral components of larger systems, should be developed and designed concurrently with mechanics, hydraulics, and electronics. It is important that engineers have a good understanding of the implications of software technology embedded into traditional engineering systems. Current machines consist of physical components providing the means and a control system employing those means to fulfill the machine’s function. Together, they build up the controlled machine, which can also be called an embedded system. . New innovative applications in different areas will make embedded systems as one of the fastest developing technology of the near future.

This paper deals with concepts and developments of embedded systems in control of machines and gives a general overview of the basic components of control systems, ranging from sensors to actuators.

Embedded Systems

An embedded system employs a combination of hardware & software (a “computational engine”) to perform a specific function; is part of a larger system that may not be a “computer”; works in a reactive and time-constrained environment.

Software is used for providing features and flexibility

Hardware = {Processors, ASICs, Memory…} is used for performance (& sometimes security)

The term ’embedded system’ can be used for a wide range of applications and devices. A useful definition is not easy to formulate. Boasson mentioned one characteristic that applies to all embedded systems: Neither the computer system without the special environment in which it is embedded, nor the environment without the computer system has any significance in itself.

An embedded system employs a combination of hardware & software (a “computational engine”) to perform a specific function; is part of a larger system that may not be a “computer”; works in a reactive and time-constrained environment.

Basics of Embedded systems

An embedded systems typically comprises the hardware, embedded RTOS, device drivers, communication stacks and embedded application software.

Embedded hardware: The embedded hardware mainly consists of a microcontroller with various peripheral ICs. A fixed size volatile memory such as DRAM or SRAM and non volatile memory such as Flash or EPROM, connected to the microcontroller, are an integral part of the device. Depending on the targeted application of the device, the peripheral can include communication device such as serial controller, Ethernet controller, or a wireless communication controller and other application-specific ICs (ASICs). Many handheld devices these days also have sensors, actuators, keypads and graphical LCD screens as user interfaces.

The only way a embedded machine control system can get information about its surroundings, is through the use of sensors and/or sensor systems. Control signals from the embedded control are converted into power and/or movement through Actuators.

Sensors: During the past years a shift has taken place from mechanization towards automation. This implies the extensive use of sensors (and actuators) in order to be able to actually control (and influence) the actions that are performed by the controlled system.In principle the task of a sensor is fairly simple. It transforms an input signal that usually is difficult to handle in its original form to a more manageable form. Between input and output of the sensor a number of processes take place to obtain the desired result, as schematically shown in Figure.

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Actuators: Actuators come in many forms and shapes. They act as the ‘arms and legs’ of the machine. Actuators convert control signals into power and/or movement,as schematically shown in Figure below. Control signals do not have to be of electrical nature, also other kinds are possible. The power conversion can be done in a number of ways.

The most common energy sources for actuators are:

• Compressed air, pneumatics

• Pressured oil, hydraulics

• Electricity, electro mechanics

Embedded RTOS: The concept of real-time operating system (RTOS) is inseparable when we talk about embedded systems. All intelligent devices that perform complex functions have an embedded operating system inside. A real-time operating system (RTOS) is built for specific applications and guarantees response to an external event with in a specified time constraint. This operating system is typically real time in nature, i.e. it is capable of responding deterministically to time-critical external events.

For example, when you suddenly apply brakes for your car to avoid an accident, the ‘intelligent gad-get’ responds immediately. Imagine the plight of a driver if there is no response… the result is obvious.

Device drivers: The lowest-level software that acts as glue between the operating system and the peripheral devices is called the device driver. The device driver software controls every peripheral device that is connected to the micro controller.

Communication stacks: If the embedded device is capable of communicating to the external world, it has a communication software stack running on the top of the operating system. In order to connect to the Internet, the embedded device needs a TCP/IP stack.

Characteristics of Embedded systems are:

Small, low power, high performance

Compact efficient use of PCB / IC ‘real-estate’

Maximum output for minimum size

High MIPS to power ratio

High bus bandwidth

Low interface bottlenecks

Advantages:

Customization yields lower area, power, cost…

Disadvantages:

Higher HW/software development overhead.

Design, compilers, debuggers… May result in delayed time to market!

Control Systems

Control system is the section where the system senses the input by sensors and compares with the internal reference of the system and gives the output obtained through the actuators as shown in figure below.

Control systems implementations:

Looking at the history of controllers, we can distinguish six types of control systems:

• Black box or embedded control

• Relay-based logic control

• Single-board control (SBC)

• Programmable control (PLC)

• Computer numerical control (CNC)

• Distributed control systems (DCS)

Black box or embedded control:

Embedded control is all around us. More and more household appliances are being designed with some form of embedded control in it. Washing machines, microwave ovens, car radios, cellular phones, VCRs, and digital photo camera’s are just a few examples of ordinary devices with embedded control in it. Embedded control is used in product itself, not in the production system with which the products are manufactured. The use of embedded control software enables product designers to design ‘smarter’ products with a large portion of its functionality embedded in the software of the product. This results in increased product flexibility and the possibility to change or add new functionality without having to redesign the physical product. So, in a way ‘the software has become the product’.

Relay-based logic control:

Before electronic control was developed, relays (as well as pneumatic and hydraulic components) were widely used as control elements. They serve as switching, timing and multiplying mechanisms for input devices such as switches, push buttons, photo-electric sensors, etc. Since the control is hard-wired, flexibility is low and troubleshooting difficult. Today, they are less used in the actual control functions but many control applications use relays in conjunction with the more sophisticated forms of control for isolation and other specialized electromechanical functions.

Single-board Control (SBC):

Electronic controllers on circuit boards first appeared in the 1960s. The early ones consisted of ‘logic modules’ with lots of discrete components like transistors, capacitors, resistors, etc. on them with which the desired control functions were implemented. Later on, integrated circuits (LSI, VLSI) are applied. Since no moving parts are used they are inherently more reliable than relay-based control systems. Because they are custom-made, maintenance and repair can be a problem. Many original equipment manufacturers still choose to design their own single-board controllers for their own unique machine applications.

Programmable logic controller (PLC):

The need for more flexibility in control systems has led to the development of the programmable logic controller, the PLC. In the early seventies, the automotive industry was growing rapidly. In order to be able to react more quickly to a changing demand in the marketplace, one had to have a more flexible, easily adaptable and expandable control system; therefore it had to be a programmable control system. The early PLCs were programmed in so-called ‘relay ladder. Today, high level programming languages like Pascal, C/C++ and even Java are being used to program PLC(-like) systems.

Computer Numerical Control (CNC):

Essentially, numerical control is nothing more than sending a sequence of

commands to a machine, that in turn interprets them and performs the desired movement

and machining actions. Before the introduction of NC these commands were issued manually by pressing buttons and switches, turning handwheels, etc. The path information is presented in numeric coordinate values (X, Y, Z), hence the name numerical control.

Distributed Control System (DCS):

Relay-based control, SBC and PLC are widely used in the discrete production area. The process industry is the domain of distributed control systems, where the number of analog I/O points exceeds the number of discrete I/O points. DCSs are used where the controlled process is continuous, has a high analog content and throughput, is distributed across a large geographical area and where down time is very expensive.

Embedded Systems in Control of Machines

In early days, the major part of the control was built into the physical machine, using mechanical parts. Although hardware-based control is still widely applied, major developments take place in the field of software-based control. An increasing part of the design process deals with the software of the control system. The development of the microprocessor and subsequent gain in flexibility has contributed a great deal to this

Controlled machine

Current machines consist of a physical machine providing the means, and a machine control system employing those means to fulfill the machine’s function. Together, they build up the controlled machine, also called an embedded system. The term embedded system is used for a wide range of applications or devices. The physical machine can be considered to consist of three subsystems: the main structure, actuators and sensors. The main structure physically connects the parts of the two remaining sub systems. The machine control system sends information to the actuators and receives information from the sensors via the I/O-interface.

The machine control system can be divided into five functional subsystems:

• Regulative control, also referred to as direct control or feedback

• Error-handling control, also referred to as fault detection and isolation (FDI) or exception

• Supervisory control also referred to as logic control.

• The data processing subsystem stores and manipulates gathered data.

• The user interface subsystem allows the user to interact with the machine-control

system.

The machine control system can also be regarded as a layered control system, as shown in Figure. The bottom layer interacts directly with the actuators and sensors, that is, the physical machine. Some components are controlled directly (for instance, pneumatic components).Some components are controlled by regulative control (for instance, motorized manipulators) or by both regulative and error-handling control subsystem (for instance, robot arms). Some components are controlled by an error-handling subsystem only (for instance, warning lamps or safety locks). The intermediate layer is involved in scheduling, coordinating control of individual machine components, gathering and processing data, monitoring and compensating possible machine failures, and providing the top layer with the required information on the machine status. The top layer allows the user to interact with and to monitor the machine.

Machine control is closely related to manufacturing control. The goal of machine control is to perform certain manufacturing functions in a controlled manner. Machine control enables us to influence production means in such a way that the manufacturing process produces the desired products of the correct quality at the planned time in the required quantity.

Over the years, a lot of developments have taken place in production methods, machine design and machine control design. Not surprisingly, they all influence each other. In the route from manual work to automation we can distinguish five phases of mechanization or automation. The meaning of the term ‘phase of mechanization’ is the extent to which a machine or production system can function independently, without human intervention.

Manual labour with tools

Specific machines

Universal machines

Multiple link specific machines

Multiple specific Intelligent control

In universal machines, control is embedded in the machine itself, through the use of mechanical parts like eccentrics, cams, camshafts, springs, gearboxes, drive axis, etc. The result depends less on the quality of the worker and more on the quality of the machine. To avoid unnecessary delays, attention has to be paid to operating procedures, work preparation, material handling and tool preparation.

An example of an embedded control system: Dryer

Different sub systems in the dryer are –

Main structure: The motor

Sensors : Temperature sensor, humidity sensor

Actuators : Motor driver control, fan control

Machine control system : SAB-C504

Examples: Consumer electronics: e.g., cameras, camcorders ….

Consumer products: e.g., washers, microwave ovens …

Automobiles (anti-lock braking, engine control …)

Industrial process controllers & avionics/defense applications

Computer/Communication products: e.g., printers, FAX machines …

Emerging multimedia applications & consumer electronics: e.g., cellular phones, personal digital assistants, videoconferencing servers, interactive game boxes, TV set-top boxes…

Multimedia: Increasing computational demands, and increased reliance on VLSI, HW/SW integration.

Embedded software can support such applications as the Internet, e-mail and MP3 decoders etc. They also support sophisticated graphical user interface screens.

The automatic DAM DOOR opening systems is a system where the DAM DOORS is controlled on the speed of the raising water. A situation araises where the raising water in the river may be very fast as the rainfall increases. If the river is blocked by a DAM then the speed of the raising water will obviously rise very soon. To prevent this raising and to prevent flooding of the river bank the DAM DOORS has to be opened but with a controlled speed because there is no point on opening the DOOR slowly if the water is raising at a quicker pace.

It is a circuit to measure the flow rate of water .Using this device one can determine the total volume of water raised in the river. The instrument is a microcontroller based system. It can have a optional of manual operation.

In manual method of measuring the flow rate of water, we need to observe the rise of water in river .As soon as water reaches a fixed point in the river , we press start button on the stopwatch .After the water reaches another fixed point, we need to press stop button on the stopwatch. The observed time and level are used to calculate the flow rate of the water.

Flow rate = volume/time

But in this procedure there are chances of errors . The device eliminates the errors and has the following features.

Automatically senses the level and generates triggering pulses for counting of the elapsed time there by providing basis for calculation of the flow rate of waters.

Includes the circuit for digital display of the elapsed time using MICROCONTROLLER as well as a 7-segment display.

DESCRIPTION :

The circuit can be divided into four blocks ,namely ,sensor ,logic controller ,pulse

generator ,switching module.

The sensor section for conducting waters:

Sensor 1 is permanently connected to circuit ground, while sensors 2 and 3 should/might be connected to positive supply via some pull up resistors. When the water level touches sensor 2 and/or sensor 3, 2 and/or 3 are pulled low towards ground potential.

As a MODEL PROJECT we can use small spherical stainless steel probes as sensors and screw the wires coming from the circuit to them . To avoid oxidation and sulphation of the naked portion, we can cover the joints using araldite or any other suitable epoxy compound .For longlife and protection against weather, we can use Teflon-insulated multistrand wires . For proper support wires along with the probes , a PVC support rod may be used for tying the wires to PVC rod to keep them in their proper position.

The logical control section:

This section might comprises a quad two-input NOR gates. When the water level is so slow that both sensors 2 and 3 are out of the water , the voltage at input of NOR gates may become logic 1 ,while their other input terminals may be logic 0. So the output pins of NOR gates are at logic 0. The output of those/that NOR gates connected to sensor 2 and 3 and the output of those/that NOR gates from sensor 1 are terminated at another NOR gate where in we get logic 0. The logic controling cuircuit is proposed to be designed to give an apropriate logical output acording to the the sensor inputs. The output This final output of the logic control unit is proposed to keep the pulse generator formed by timer IC. It is decided that the timer IC will be 555.

When the water level rises to touch sensor 2 the output of the NOR gates combination should be logic 1,which initiates the operation of the pulse generator.

When the water level further rises to touch sensor 3, the output of NOR gate will be logic 0, which should be in a position to terminate the operation of the pulse generator.

In this way, the pulse generator can be automatically controlled as per other requirement.

Pulse generator:

Timer IC 555 will be used in out project to generate pulses with pulse recurrence frequency of 1 Hz. The frequency /period of the astable should depend on combination of resistences , capasitences and variable resistences also.

The switching section:

The switching module is used to make and break switch contacts at 1 Hz rate, using the output of the timer IC555 . This function analogous to pressing/releasing of a push-to -on switch once a second . An optocoupler is proposed to be used for making/breaking contact between to points.

Whenever the output of timer IC555 goes high, the optocoupler conducts . this optocoupler is connected to the microcontroller KIT.

The road ahead

Telematics:

The impact of telematics would be really innovative. With access to e-mail the internet, and telephone services, car occupants could shop and bank online, receive traffic and navigation information, and avail of remote diagnostics facilities

Automotive electronics:

Remember the talking car in the serial knight rider, a fully computerized car capable of doing almost everything a car lover would want to. Seems like a fantasy but the day is not far when almost all automobiles would interact with computers on dash- boards. From ordering a pizza to booking tickets at the nearest theaters, things would be as easy as giving orders to your servant. Whole of which would be possible with embedded systems.

Mistral software is in the process of developing text to speech and speech reorganization technologies to give the car occupants the ultimate comfort. Whenever there’s a call on your mobile, you need not get jumpy at the very onset of the call. The computer in the cars dashboard would do the job for you. GPS navigation guides you safely through the traffic. The GPS interface in the car pinpoints your exact location on a map. In case GPS signal can’t be received due to high density of tall buildings or other magnetic interface, the ‘dead reckoning’ technique, which works for short durations, guides you effectively. The system is also loaded with GSM/CDMA protocol standards.

Biomedical solutions:

The biomedical chord developed by mistral is a centrelised patient monitering system that allows remote monitering of up to 32 patients at a given time through a central computer. It can process of maximum of 32 channels usin the state-of-the-art DSP in a PC environment.

Economy:

The world of embedded systems is a dreamer’s paradise with unlimited possibilities. According to the global market size for embedded software development alone was $7 billion in 2001, which is expected to reach $20 billion in 2003 and $31 billion by 2005.For India are $400 million. $750 million and $1.1 billion respectively. In India R&D in embedded system was worth $1.1 billion in 2001, which would grow to $8 billion by 2008.

CONCLUSION:

Further more, embedded systems are rapidly becoming a catalyst for change in the computing, data communications, telecommunications, industrial control and entertainment sectors. Automatic systems in any field will be useful and will save the people and organizations. New innovative applications in these as well as other areas will make embedded systems as one of the fastest developing technology of the near future.Thus the embedded system plays an important role in our day today life.