Methods Of Mechanical Indexing Information Technology Essay
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Published: Mon, 5 Dec 2016
Indexing is an application where a motor is used to move a load for a certain predefined distance, slow to a stop, wait a predetermined interval and repeat the cycle. Indexing systems are used in numerous applications such as conveyor belts, feed to length rolls and rotary tables. Indexing is used in many different industrial processes some of which include but not limited to; inspection lines, assembly lines, packaging processes and food handling processes.
These processes all require parts to be progressively moved from one point to the next where an incremental processing step will occur.
The length of the distance a part is to be moved can be set to a fixed length or could just as easily be a variable parameter. This distance is also sometimes referred to as a ‘step’. The dwelling time between steps may also be a fixed or variable parameter depending on the specific application. Indexing is commonly associated with applications where motion control is incorporated, commonly using solutions such as stepper positioning systems and servo positioning systems. Fig 1 shows some of the variations of speed control.
Figure Speed Profile
1.2 Automated assembly:
Automated assembly is the use of a mechanised or automated device which is used to perform a number of assembly tasks in an assembly line or cell. Most automated assembly systems are designed to perform a fixed sequence of steps on the specific product. An automated assembly system is considered applicable if the following criteria are met:
High product demand: automated assembly is only applicable to situations where the particular product is manufactured in vast quantities.
Stable product design: the product design needs to be maintained and not changed regularly. If a design change happens it may require changes to workstations, tooling etc. which will be expensive.
Limited Components: most automated assemblies will be limited to no more than twelve separate steps in the process.
Figure an automated assembly line, mass productionAn automated assembly is expensive to design and implement but are comparatively cheap when compare with automated transfer lines. Automated assemblies are produced on a small scale and will not require the same amount of space consumption. This is mainly due to the fact automated assembly systems are used to produce products far smaller than products being built on a transfer line. They also require less power consumption due to their smaller scale physically. http://www.nanowerk.com/images/robotic_assembly_line.jpg
2. General Solutions
Some mechanical systems which are used employ the use of a cam wheel or an eccentrically shaped rotating part to produce a certain desired motion.
Some solutions are shown below in Fig 3.
It can be seen that as the cam wheel rotates it will engage with the toothed gear at only certain positions during each revolution. This will produce a particular desired indexing pattern; the pattern obtained produces a very reliable repetitive motion, with the response being high as the cam’s speed is increased or decreased.
Figure eccentrically shaped Cam wheels
Disadvantages of these particular systems may include:
Mechanically machined cams, chains and belts can suffer from mechanical wear due to the motion of friction as the parts grind against one another. Such parts may need re-machining or even replacing at numerous intervals during the applications lifetime.
If a change to the indexing pattern is required, a mechanical change-out of parts will have to take place, the effects of such a change-out may cause excessive down-time to the process, which in turn may affect productivity and even financial loss during these periods.
If the process requires a variable pause time, the clutching in and out of the cam may be necessary.
This clutching in and out can be relatively difficult to achieve and time smoothly.
2.2 Stepper Motors:
The use of steppers motors is also a common solution for indexing. A stepper motor consists of a permanent magnet motor containing a number of poles. A controller can be used to apply a pulse or a phase step, with each pulse the motor will move due to the effect of magnetic repulsion. This movement is predictable due to the confirmed number of poles and magnetic field principles. The pulse to step association from controller to motor will result in a defined angle of rotation for the motor.
As a result of this, accurate positioning is capable and the use of an encoder will not be required. As stepper motors are also relatively rugged devices they do not require a great deal of maintenance which in turn saves down-time.
Disadvantages of stepper motors for this process:
Stepper motors have relatively low resolution (200-400 steps per revolution). This may often require the stepper motor to be geared. Over time this can result in drift due to precision limitations. Whereas with encoder positioning systems with higher degrees of resolution and correction capabilities this can be resolved.
Stepper motors are also limited to seven horse power. At low speeds the motor s response may be unsmooth and may lead to stalling. Also if the load being indexed is not at a constant the stepper motor’s performance may decrease.
When stationary the motor will still draw a current, therefor energy expenses are still being accumulated while no work is being achieved. This is due to the fact that there is no actual position feedback, so excess power is needed to prevent any unwanted movement.
If the load applied to the process is higher than the torque rating of the stepper motor (due to jammed part etc.) the motor may become out of sync.
2.3 Variable Speed Drives:
Positioning drive/motor’s with the use of an encoder to provide feedback, which use either a servo motor or a flux vector motor are seen to provide the greatest flexibility within indexing systems. The encoder is used to provide both positional and speed feedback which in turn offers controlled index moves. A benefit of this type of system is that it provides for easy change to dwell time and index distance which in turn results in no or very little down time. PLC and photo sensing inputs can also be incorporated with this type of system which allows for indexing with event driven operation.
Advantages of a positioning drive system:
Encoders will provide for extremely high resolutions.
Closed loop positioner can be incorporated to provide over-shoot protection.
Numerous indexing profiles may be stored and retrieved when required. In other systems this would require down time to allow for adjustments to belts and gearing etc.
A PI loop can be incorporated to provide velocity control, this will allow for smooth, predictable velocity profiles.
The capability for registration input will provide accommodation for any changes due to slippage of the motor or belts or wear of belts/chains. This is a correction loop external to the positioning loop which will tie the system to a number of absolute points.
Positioning drive systems use Ac vector and servo motors which allow for a wide range of choices. Servo systems will provide a greater dynamic performance due to construction of the motor itself. Servo motors have lightweight rotors with extremely low inertia; this is a huge principle of dynamic response. For example there are cases where servo motors are being successfully applied to mail sorting machines which require fifteen indexes every second.
Disadvantages of Servo motors for positioning systems
Servo motors incur a very high financial cost. The magnetic materials used in the motors construction are expensive and require costly processing while being applied to rotors.
Servo motors are only available at fifteen horsepower or less.
Permanent magnet motors will not generally operate above base speed.
2.4 Ac Vector Drives:
Ac vector drives incorporate the use of induction motors which provide a promising alternative to servo motors. However this solution is not quite as dynamic as servos because there is generally larger rotor inertia associated with inductor motors, even so they are still used in many indexing applications. A general rule of thumb which is normally adhered to is that; a vector solution is considered applicable if the given indexing cycle is no more than twice per second.
A real advantage of using a vector solution for indexing is that it provides the ability to handle applications of 20HP and more. Vector systems which are greater than 2HP are also less expensive when compare to servo systems.
The disadvantages of positional control systems compared with stepper motors:
Controllers and drives are relatively inexpensive
They can often be far more complex systems, which in turn will require greater engineering, programming, configuration and tuning etc.
Controllers both vector and servos constantly need to ‘hunt’ to stay at zero, with the net effect of continually oscillating by a few encoder counts.
Variable speed motors can be put under large pressures from indexing cycles and measures may need to be taken to remove excess heat generated during the cycles. Motors provided with individual cooling blowers may be required.
If the decision has been taken to incorporate the use of a drive with a variable speed motor, the selection of the right controller, drive, motor will now need to be assessed under the following criteria:
Accuracy and repeatability: the indexing process is progressive and any inaccuracies will be continually magnified over time.
A well designed system should be able to move from location to location very predictably. Any overshoot will need to be minimised as this will only cause delays to the process. Example: if the application is being used in a bottle fill process where each bottle is indexed to a nozzle for filling, if there is any overshoot it will lead directly to spillages. These spillages may in turn cause the bottles to be out of spec with the required product weight. Also the machine operations taking place during the indexing dwell may require the indexer to be absolutely stationary, if there is any movement this may lead to machine damage. Proper tuning of the controller and drive is required to provide a good dynamic response.
A well designed system needs to be rugged: a tightly tuned system. These systems are very demanding on the drives, motors, and gearing equipment due to continual and repetitive acceleration-run-deceleration cycles. Heat generation due to friction and continual speed change may cause the premature failure of some components in the system. This may be avoided by selecting thermally-rugged equipment.
Due to the rapid deceleration associated with indexing systems energy transfer from load to the drive may occur. Because PWM (pulse width modulation) drives are used this energy will accumulate on the Dc bus. High voltage tripping will occur if this is not removed. The use of a braking transistor and resistor may be a solution to this adverse effect. A regenerative braking unit may also be a cost effective solution if there is a very high level of energy dissipation. This unit will be used to put the excess energy back onto the line.
It is vital to analyse the intended indexing system to determine its regenerative needs during the design and build stage. Factors to be taken into consideration during this analysis include:
3. Intermittent motion
An intermittent system will pause the process periodically to perform a particular action. Intermittent systems therefor cannot achieve the same high cycle count of a continuous system. Intermittent motion can rarely achieve more than 250 cycles per minute where as a continuous system may achieve many more times that. Also if the system becomes over complicated there is a good argument that rotary indexing may be more productive.
3.1 Geneva Mechanism:
The Geneva indexing system obtained its name from its earliest application when it was used in mechanical watches with Geneva Switzerland being an important centre for the watch building industry. The Geneva drive is also commonly referred to as the Maltese cross mechanism because of it visual similarities. Another reason for it name may also be because it was a watch maker who actually invented the system. A limited number of slots were put into one of two rotating discs so that the system could only rotate through a certain number of rotations. This system provided the watch with a mechanism to prevent it from being wound excessively tight. This also led to it being known as the Geneva stop mechanism.
The Geneva drive mechanism is relatively simple in its operation, with little complicated design features. The mechanism does require however the correct dimensions and a close tolerance to allow for correct engagements and disengagements.
The Geneva mechanism uses intermittent rotary motion, this means that the motion is not continuous but will stop a definite intervals. Intermittent motion is generally required in machine tools.
There are three basic types of Geneva motion in use; external motion, internal motion and spherical motion.
This is the most common type of Geneva motion and is widely used in modern day industry. This mechanism is used to transfer rotary motion into intermittent rotary motion. Due to the fact that the driven wheel in a Geneva motion is always under full control of the driver the problem of over-running does not occur. The rotating drive wheel has a pin which slides into slots cut into the driven wheel as it rotates; this advances the driven wheel by one step per revolution.
In its most common arrangement, the driven wheel has four slots cut into it, thus for every revolution of the drive wheel the driven wheel will be advanced by 90 degrees, see Fig 4.
Figure Geneva Mechanism with 90 degree step
A common use of the Geneva drive mechanism is in projectors which are used in the movie industry. When a film is being shown it does not run continuously but actually frame by frame with each frame stopping still for 1/24th of a second, each frame is stopped twice per second to give a frequency of 48 Hertz. To achieve this intermittent motion the solution is to use a Geneva Drive mechanism.
Geneva wheels are also still commonly used in mechanical watches to limit the tension of the internal spring so as to achieve an elastic force which is almost linear. Other common usages include; indexing tables, plotters, assembly lines and many more.
3.2 Mutilated Gears:
Gears are used in many different ways to produce intermittent movement, a typical gearing ratio to produce this type of motion is the ‘mutilated gear unit’ shown in Fig 5 below. In this configuration some of the teeth are removed from the driver gear. A partial holding ring is also added to each gear to help prevent slippage of the drum gear during the indexing dwell. When operated under light load and slow speeds this system is a very simple and versatile intermittent motion mechanism. Both the dwell and motion periods of the system can be easily varied depending on the sizes of the two gears being used. Selection of the proper gearing ratio also offers easy control of the velocity ratio.
Figure Mutilated gearshttp://www.roymech.co.uk/images/mechanism_27.gif
3.3 Cycloidal Gears:
Cyclodial gears are another gearing mechanism unit used to produce an intermittent motion. In Fig 6 below the hypocycloidal gear train is shown, it consists as a ‘small planet’ gear which runs around the inside of a larger fixed ring gear. A pin in the planet gear is will engage a slot in a in an output crank which will rotate about the centre of the ring gear, with periodic dwells as it rotates. The hypocycloidal gear mechanism does not produce impacts and only produce very short dwells (almost instantaneous), and there for differs to mutilated gears significantly. From Fig 009 the schematic drawing of the gear train can be seen showing the path taken by the drive pin as the planet gear circles the inside of the ring gear. As the diameter of the ring gear is an even multiple of the pitch diameter of the planet gear the drive pin will always follow the same path.
Figure Cycloidal gears
Cycloidal gear trains can be used as a standalone mechanism or can be incorporated with other intermittent motion devices such as Geneva’s or ratchets.
3.4 Harmonic Drives:
Harmonic drive mechanisms are noted for their ability to provide precision motor control with very low backlash and vibration. Harmonic drives provide high reduction ratios through concentric shafts in lightweight but robust assembly’s. They consist of a very simple design consisting of just three concentrically mounted components: a wave generator, a flexible spline and a circular spline. The wave generator is mounted on the drive shaft normally a servo motor; the gear output shaft is attached to the bottom of the flexispline. This then rotates within the rigid circular spline.
Figure Harmonic drive C:UsersAlan.sarah-HPDesktophd.jpg
3.5 Cylindrical Cam / Barrel Cam:
As the cylindrical cam rotates the follower gear will move in an upward direction. When the follower reaches the top, the cylinder cam can be rotated in the opposite direction which will allow the follower to move back in the downwards direction. These are relatively unusual cams and normally consist of a cylinder which has a groove cut from its surface allowing the follower to run up and down along it.
This type of cam is normally seen in sewing machines and older clock mechanisms, typically where repetitive movements are required.
Figure Cylinder Cam
3.6 Walking beam conveyors (in-line transfer):
Rotary beams are another method of providing synchronous automation, a rotary beam works by moving product sequentially in straight-line steps. To advance the product the beam will usually engage with them from underneath, and can move the product in numerous directions, up, down, forward and backward. The beams motion is used to replicate an arm that rise and falls similar to a see-saw. These machines are based on high performance servo technology and provide stable and fast transfer system for rigid containers. These systems are used where alternative in-line transfer systems such as rope or chain conveyors would be unsuitable. In an intermittent walking beam system all tracks will move simultaneously. They are sometimes referred to as: ‘Axil Horizontal’ transfer mechanisms.
Figure Intermittent walking beamhttp://t0.gstatic.com/images?q=tbn:ANd9GcQxpaf7MyMQuglzlnd4td9ZEDWinBIMIO5X65516UaNZcGfcBKslw
Shown in Fig 10 below is a series of workstation located along a line transfer system typically a conveyor. Synchronous and asynchronous transfer systems are used to transfer product awaiting assembly from workstation to workstation.
Figure In-line transfer system
3.7 Dial-type assembly:
In Fig 11 we can see a dial type assembly with six stations. Base parts are loaded onto the stations around the edge of the circular dial. As the table turns through each station components are assembled sequentially into the base part. This is a synchronous transfer operation as all nests are rotated at the same time. This can be done sometimes through continuous motion but more often intermittent rotation
Figure intermittent carousel with 6 nests
3.8 Carousel systems:
Intermittent carousels operate in a very similar fashion to the inline beam transfer already discussed. A carousel system is sometimes referred to as a ‘vertical axis’ process.
Tasks can be performed on both sides of the conveyor when the process is in operation. A carousel system is referred to as a hybrid of both the dial type system and the in line transfer system. The carousel offers both the circular motion of the dial along with the straight work flow of the in line system. Carousel systems can be operated with either continuous synchronous or asynchronous transfer mechanisms. An outline of a carousel system can be seen below in Fig 12.
Figure Carousel type transfer
3.9 Single station assembly:
Single station assembly consists of only one work station where parts are assembled successively onto a base component to complete the finished product. Once all components have been assembled onto the base part, it will leave the system and another base part will enter to repeat the process. This types of system is far slower than the other systems describes as only one product is processed at a time. Fig 13 below shows the outline of a single assembly station.
Figure Single station assembly
4. Continuous Motion Systems
4.0 Continuous Motion
Continuous motion systems provide some of the fastest output times in terms of high speed assembly. Speed isn’t the only benefit of continuous motion technology; these systems help to provide some of the most consistent and high quality performances.
In a continuous motion system multiple there are multiple processes taking place without interruption in every cycle, effectively over-lapping each other.
The results of this are that the processes are run much smoother and efficiently, there is less chance of damage to the machine or components.
Advantages of continuous motion assembly:
Significantly lower maintenance costs when compare to indexing systems.
Can provide multiple and longer processes all automated within the one machine, cycling at 400cpm or higher.
Provides for greater productivity due to less dwell time when compared with indexing.
Machines can be custom designed to cater for the desired application
Figure Continuous motion systems in operationDue to all the processes contained within the one machine, the physical size of space required can be far smaller than an indexing system.http://www.haumiller.com/images/page_images/contmach_03.jpg
Continuous motion machines are used for many different applications including the assembly and functional testing of aerosol valves, sprayers, pumps and medical devices including syringes. Continuous motion refers to the continuous movement at a constant speed. The movement may be rotary or linear. In the case of rotary continuous motion the main element is known as a dial. The components are introduced to the dial and assembled products are output immediately on the fly. Assembly is completed as the components move with the dial, and results in extremely high production rates.
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