Thread Starter

Sudarshan Iyengar

hello everyone,

I have got an interesting project where in servo motor control is involved. I am venturing into this type of control for the first time and would like to know more about this type of control. Also i am interested in knowing about following things regarding Servomechanism/servomotor

1. whether is it always necessary to buy a servocontroller to drive a servomotor or can servomotor motion be controlled through PLC.

2. There is an encoder(Quadrature) provided in the servo motor, i was told that i can branch the output of the encoder and can be connected to two
locations(Controller and PLC). Is this reliable and accurate.

3. Basically what type of signal does servomotor accept?

4. I have to drive two servomotors in synchronisation, but i have been to told that i have to buy two seperate servocontrollers to drive these motors.
Is there any single servocontroller which can drive multiple servomotors?

5.If i can drive the servomotors through PLC, what are different types of inputs servomotors accept?

6. I know that a servomechanism is closed loop, can any one enlighten me with more details with regard to servomechanism concept?

Can anyone over there help me in this regard. Any help in terms reference/books/links regarding the above mentioned subject is highly appreciated

Thanks in advance

Glen Road systems,inc

Daniel Boudreault

Hello Sudarshan

My answer follows your questions.

>1. whether is it always necessary to buy a servocontroller to drive a
>servomotor or can servomotor motion be controlled through PLC.

There are many types of electric motors out there used in servo systems.
- d.c.
- a.c.
- a.c. vector
- step
- servo

Each one has its own type of "drive" to make them turn.

Some PLC's can command the drive to directly, some require special modules to do this, and others can't.

There are specialized controllers for servos. Some are integrated with the drive. Some have multiple axis to drive more than one servo motor. Some require daisy-chaining a serial link to expand the number of axis.

>2. There is an encoder(Quadrature) provided in the servo motor, i was told
>that i can branch the output of the encoder and can be connected to two
>locations(Controller and PLC). Is this reliable and accurate.

I have seen this done, and I would not recommend doing this unless it is for monitoring purpose only.

Places where I have seen it done is where the drive and the controller comes in a separate package. In this case, if the package suggests
this type of connection, then I don't see what else you can do.

Some servo controllers have and auxiliary encoder output that can be used to branch the encoder signal.

>3. Basically what type of signal does servomotor accept?

I assume you mean servo amp (drive).
Control inputs vary.
- analog voltage or current
- digital
- serial communications
- the integrated ones have direct memory access from the controller.

>4. I have to drive two servomotors in synchronisation, but i have been to
>told that i have to buy two seperate servocontrollers to drive these motors.

This type of application is quite common. It is usually called electronic gearing. I would look into getting a single servo controller with the capability of two axis. If you don't need the first motor to run like a servo, and you can allow the second motor to "follow" the first one, then you will only need a "1-1/2" axis servo
controller. The first encoder input will command the controller to follow the first motor, and
the second encoder input will give the controller position feedback for accurate synching.

>Is there any single servocontroller which can drive multiple servomotors?


>5.If i can drive the servomotors through PLC, what are different types of
>inputs servomotors accept?

no comment.

>6. I know that a servomechanism is closed loop, can any one enlighten me
>with more details with regard to servomechanism concept?

The servo controller will compare the encoder feedback position to the desired positon. If the position is lesss than desired, then it will tell the "drive" to start moving in one direction. If at this time the positon error lessens, then the controller will reduce the move command until the desired position is reached. If the position error increases, then the controller will increase the move command until the position error starts to decrease.

The drive has it own closed loop circuitry also.

>Can anyone over there help me in this regard. Any help in terms
>reference/books/links regarding the above mentioned subject is highly

I've looked through my magazines for vendors which follows:
Warner (800) 787 3532
Nyden (800) 806 6554
HD Systems (800) 231-4374
Copley Controls (781) 329 8200
Allen Bradley
Modicon (847) 397 2600

I have used A-B and Warner before, and both have met my requirements.

Good Luck, Dan B.

George Kaufman

In reply to Sudarshan's questions in the message below:

Yes, a PLC can be used to control a servo system. Please refer to the application note "Replace Stepper Systems with AC Servo Systems and
"Step-Up" to Closed-Loop Performance". This can be a very cost-effective way to use a PLC and digital servo drives to perform many types of motion control applications.

Also, for a complete overview of servo system operation including encoder operation, servo command signals, and closed-loop concepts, please refer to the "Handbook of AC Servo Systems".

Both documents above are availble as free .pdf downloads from


Jay L. Mackey

I programmed a system with two dc servo motors working both independently and dependently, so I can tell you how not to do it.

I used a single controller (Parker Compumotor) that took two encoder inputs and provided two servo outputs. The servo outputs went to two dc servo drives that put power to the motors. The controller had its own programming in a basic-like language. The PLC was A-B. Based on the
machine sequence, the PLC would initiate the Compumotor program with a discrete Output. The Compumotor program would perform a motion sequence using one or both motors, pausing at the end of each step and sending a "motion complete" signal to an A-B Input. The PLC would send an
Output signal back to initiate the next "step" or sequence in the Compumotor program.

Things I Learned:

Try to incorporate all programming into the PLC or Main Controller instead of splitting it between two separate controllers (PLC and Servo). I don't know about other solutions, but I know that this can be done with A-B's newish Control Logix platform. You could also accomplish this with PC-based solutions if you are not adverse to PC control.

Try to get most or all of the equipment from the same vendor. A servo controller with integrated drive electronics and a motor obtained from the same vendor/manufacturer will usually function together better, and there will only be one person to call or to point the finger at if something doesn't work right instead of two or three.

If you use a programmable Servo Controller separate from the PLC, use one that incorporates the drive with the controller. Less panel space, less wiring, and less setup headaches. The above system needed the drives calibrated and the servo tuned, and the calibration and tuning was not "automatic". (The servo controller also required a 5vdc +0%/-5% power input, so it flaked out if the power supply was not dead on.) Servo controllers with integrated drives are much easier to calibrate and tune.

If the app is not adverse to servo auto-tuning, make sure the servo is capable of auto-tuning, or that the servo software is capable of auto-tuning. If auto-tuning works for your app, it makes it much easier to setup the servos. This is even more important if you are doing multiple systems. Otherwise manual tuning has to be done every time a machine is shipped or setup at its end location.

The above system was designed by a new engineer who was told to build the cheapest solution possible. And it sure was -- in terms of hardware costs. But wiring, setup, and programming costs were considerably higher, so that if we had used Control Logix, which wasn't available back then, we would still have come out ahead on total cost.

More info below:

> ------- Forwarded message follows -------
> From: Sudarshan Iyengar <[email protected]>
> ....
> 1. whether is it always necessary to buy a servocontroller
> to drive a servomotor or can servomotor motion be controlled
> through PLC.

Either way, but I only know of an A-B Control Logix solution for the latter.

> 2. There is an encoder(Quadrature) provided in the servo
> motor, i was told that i can branch the output of the encoder
> and can be connected to two locations(Controller and PLC).
> Is this reliable and accurate.

I don't know, but if it is not generally true that it is reliable to hook any old encoder to two separate devices, then I would make sure that this particular encoder can do it, and I wouldn't trust a vendor's word about it either.

> 3. Basically what type of signal does servomotor accept?

There are all kinds of solutions now, with some motors incorporating electronics that confuses this issue, but usually you have a motor that needs some fairly high current to do any useful work, a drive that can accept some kind of
low-voltage input and translate that to the motor in the form of higher voltage and current, and a servo controller that takes position info and a command position and spits out a low-voltage command signal. These can be all separate devices
from different vendors (not recommended), or the controller and drive can be integrated, or everything can be integrated into the motor, or the servo can be integrated into the PLC with a matched drive that just plugs a cable right into the PLC servo card. And there are also PC-based solutions that I've never used.

> 4. I have to drive two servomotors in synchronisation, but
> i have been to tld that i have to buy two seperate
> servocontrollers to drive these motors.

Maybe from your vendor, but 4 years ago, Parker Compumotor had servo controllers that would take at least 2 encoder inputs and put out two servo motor control signals. I think they even had product that would do more than 2. I think it
is fairly common for controllers to be able to handle more than one motor, although I don't know if you can get a controller with integrated drive electronics that could do two or more motors.

> 6. I know that a servomechanism is closed loop, can any one
> enlighten me with more details with regard to servomechanism
> concept?

As a mostly-programmer/ee-tech person, I don't know the finer points of servo controller system design, but in general:

Inputs to the servo loop include position (encoder, etc.) and a commanded position. If the current position (encoder) and the commanded position are sufficiently different, the servo controller will send a command voltage to motor drive
electronics to move the motor in the correct direction until the current position and commanded position are the same (or nearly so). The bigger this difference (and perhaps the longer it takes to rectify this difference), the more "juice" the servo will tell the drive to send to the motor. The accel and decel
parameters are usually configurable, for example a slow accel to max speed followed by a slow decel to stop may be desirable.

Depending on the relation ship of encoder counts to real-world distance, the program could initiate a move by telling the system to move to a particular count value which, for example,
might be 1000 counts farther away from the current position. When the hardware is enabled and the controller is told to "go", the system attempts to move to the commanded position.
Of course, once the commanded position has been reached and as long as everything remains enabled, the servo tries to maintain position.

How fast and smoothly this motion is carried out depends on many factors: Motor power, gearing and belting, mechanical backlash, system load size, and various servo parameters.

To help servos operate well with a wide range of end uses, there are many different parameters that affect the way the servo tries to move to and maintain the commanded position. This allows a servo to be programmed in one app to work with
heavy loads, or to be fast and super-accurate with a light load in a different app.

The act of adjusting parameters to get a servo to operate "cleanly" in a given app is called "servo tuning". Get a controller that has auto-tuning built in or included in the servo programming software. However, depending on the app,
auto-tuning may not be so automatic. Manual tuning may be required.

Because manual tuning is often required, most servo manufacturers have some pretty detailed info about how their systems work. If you are investigating the controllers of a certain vendor, ask for the manuals or any other publications about their servo controllers.

I'm no servo-loop expert, so I know there's plenty more that others here could add to the above.

best regards,

Jay Mackey
Spectra Technologies, Inc.
Grand Prairie, TX
At 10:23 PM 2/14/2000 -0600, Daniel Boudreault wrote:
>>2. There is an encoder(Quadrature) provided in the servo motor, i was told
>>that i can branch the output of the encoder and can be connected to two
>>locations(Controller and PLC). Is this reliable and accurate.
>I have seen this done, and I would not recommend doing this unless
>it is for monitoring purpose only.

Why not, I have done this many times for real control purposes. You have to make sure that the encoder has enough drive for the multiple inputs and be careful to connect your commons correctly (use star connections and no ground loops)

Are you just being cautious or are there other reasons not to connect one encoder to two controls?

Bill Sturm

Davis Gentry

One additional caveat - many PLC based servo
controllers are somewhat limited as to the complexity of motion which they support. If your application requires it, watch out to be sure that your controller supports multiple coordinate systems, coordinated motion by 2 or more axes, even some things which you might think simple, like precision circular or arc motion.

Sometimes a DSP or ASIC based system that resides
either in a PC backplane or in an integrated
drive/controller is a much better solution if you
require much complex motion. has some good stuff in this arena. Their stuff is even pretty cheap when compared with A-B - and much more capable.

But if your motion is relatively simple in nature and you already HAVE to have a PLC, then PLC based control might be the way to go.

Davis Gentry
Controls Project Engineer
Carpenter Company

krishnaiyengar sudershan


I just have an apprehension that the output from the encoder may not be sufficient to drive the two inputs(servo controller and PLC high speed input module input channel) and i just wanted to ensure that this should not be a bottle neck for the system.


Glen road systems Inc.
Aside from the groundloop and power supply reasons mentioned by others on this topic, there are two further items to think about when considering the use of motor-mounted encoders. No let me say three:

1. The servo systems under consideration are digital inside. There was mention of servo gearing, also called follower mode. To accomplish this the two axis have to be related in math. There is not an infinite range to the servo controllers mathematical terms. You will typically find a limit of 32:1 - 1:32 or 100:1 - 1:100 for the two axis to be related. With the encoder mounted on the motor you might "give away" some range of the ratio and bump into a limit.

2. Depending on the drive mechanics you might give away a lot of accuracy if the encoder is on the motor. I'm thinking gear backlash, screw wear, slide play.

3. Using the motor manufacturers choice of encoder limits you from a design point of view. Look at what the motor manufacturer has as constraints and compare them to yours. Does it matter to the motor (final drive) if one or a
few pulses from the encoder get swamped by a burst of noise? Not likely, the drive PID function will smooth it over. But if you are controlling a positioning system every pulse is dear to you! So you might want to choose a differential output encoder to give you that extra signal margin.

Hugo Ahrens
Project Mgr., IAI

Guy H. Looney

Well said. I've done this successfully as well.

Most modern drives bring the encoder directly into the drive. Therefore, you get the encoder information from the drive. This allows you to be dependant on the drive's power supply instead of the encoder's power supply.

Further, a lot of encoder manufacurers have up to 24VDC output signals (as opposed to 5VDC) which compensates for voltage drop.

Guy H. Looney
Sales Engineer

Regan Controls, Inc.
475 Metroplex Dr.
Suite 212
Nashville, TN 37211
phone: (615) 333-1940, ext. 322
fax: (615) 333-1941

Guy H. Looney

The discussion surrounding this application has been very interesting. I've only been working in the motion control industry for a little over 4
years, but it's all I do. I feel very comfortable with this question & would like to share my opinions/experiences. I've done a lot of applications involving motion controllers (Galil, Compumotor, ORMEC, AB, Modicon, Yaskawa, Baldor/Optimised Control, etc.) and PLC's (AB, Modicon, PLC Direct, GE, etc.) where they've acted alone or together.

1) A PLC can do motion control w/ a servo card (module), but it's not its strength. The primary reasons are processing speed, PID update rate,
programming, and cost. We could debate price & programming for days, but in general a motion controller wins out on both issues. If you are doing simple point to point moves & tight coordination of axes is not important, a PLC will function quite well. However, if you are doing complicated moves (camming, gearing of multiple axes, high accels, high decels, contouring, high speed latch, etc.), a PLC is seldom the best choice. If settling time is an issue, a PLC may not be the best choice either. Motion controllers are better suited for these applications because of the PID loop closure (62.5 microseconds per axis or better) & command processing ability. Plus the programming language in a motion controller makes contouring, camming, high speed latching, and gearing relatively painless. I am a firm believer in centralized control & do not like to hand shake inputs & outputs, but sometimes the application dicates you must decentralize the control. Many motion controllers have implemented field bus ports or ethernet which greatly improve the communications
process betweent he PLC and motion controller. Mapping registers over Device Net is much easier than doing BCD encoding & decoding across
discrete I/O.

One easy alternative is using servo drives that accept step & direction mode. When you do this, the PID loop is closed in the drive & not the
PLC. By putting a stepper card in a PLC you can accomplish a lot of the simple point to point moves without taxing the ability of the PLC. You
have the option of bringing the feedback from the drive to the PLC (may require a high speed counter card) to let the PLC monitor the position.
If the application is not very I/O intensive (less than 32 in & 32 out) a motion controller could do the whole application without a PLC. Even though motion controllers can provide hundreds of I/O points, it is not always the best choice for handling this much I/O.

I guess the bottom line here is the application dicates the equipment you use. A PLC does I/O really well & a motion controller does motion control really well. Both can do the whole thing, but the application may require a PLC & motion controller to give you optimal results. Lastly, but certainly not least the tech support you have available & your comfort level plays a big part in which way you go.

2) An encoder can be branched to several different devices. All servo motors have 2 types of feedback. Sometimes the same feedback device can be used for both types. The two types of feedback that I'm referring to are position/velocity (velocity is a derivative of position so technically its the same type) feedback and commutation feedback. The position/velocity feedback is what most people think about when they think of an encoder. The commutation is used so that the drive knows were the rotor is. It has to know where the rotor is so that it can "inject" current through the
right windings at the right time. The first commutation device was used in brushed motors. The brushes were used as commutators. With brushless servo motors, the commutation device can be an encoder (incremental or absolute), resolver, or hall effects. Each is effective, but the more resolution you have during commutation, the less torque ripple you have.

That being said, most digital drives on the market today bring both types of feedback into the drive directly. The drive then has a breakout board that allows you to wire the encoder "output" into your motion controller
or PLC. You can easily branch that encoder output into many different devices. As an example, Yaskawa drives have the ability to take +/- 10V or step and direction as a command signal. You could take the encoder output of one drive & wire it to the PLC or motion controller for position feedback. Further you could take that same signal and wire it into several other drives (up to 8 I think, which based on current draw). That encoder output could be used as a step and direction signal to the other drives. Yaskawa also has the ability to gear a commanded input signal. This would allow you to command one drive & motor w/ the PLC and then gear up to 8 additional servos to the first one simply by wiring the encoder output signal of the first into the others. I have seen this done and it
works very well. The gear ratio cannot be changed on the fly, but it may not matter in certain applications. I know other servo manufacturers have these features as well, but I have not personally witnessed anyone else doing it.

Analog drives typically don't bring the position/velocity feedback into the drive. They typically have an encoder that you wire directly into a PLC or motion controller. There's no reason you can't split that signal, but now your limited to the power supply that feeds the encoder as opposed to the power supply in the drive. Therefore, I would tend to think the
number of times you can split it would be more limited.

Splitting the encoder signal is really not a big deal. The hard part is justifying why you want to do it. Does it buy you anything? If you've
got a motion controller and a PLC on the same job, mapping registers will allow you to monitor the position over a field bus or ethernet (if the
motion controller is so equipped) without splitting the encoder feedback. If you just want to make sure the motion controller is done & you don't have a field bus or ethernet, handshaking I/O may work as well.

3) I believe you meant what type of signal do servo amplifiers accept. The motor is controlled by the drive (amplifier) & works on about a 400VDC
bus. The servo amplifier as stated above can take +/- 10VDC or a step and direction. The drive is really as smart as you want it to be. It can work in torque, speed, or position mode. The analog input (+/-10VDC) would control torque or speed mode and the step and direction signal would control position mode. Which one do you use? It depends on the application and the controller you are using.

Torque mode is the most demanding mode for a host controller. You will need to tune the system almost entirely from the host controller. Don't
let tuning scare you though. Most any motion controller worth its salt has some kind of auto-tuning feature that's decent. The drive takes in a +/-10VDC signal and "amplifies" it to the appropriate motor torque. The amplifier closes the torque loop internally via commutation. The host controller is tasked with closing the postion and velocity loop. If you give the drive a 5 volt signal it's going to "tell" the motor to apply 50% of available torque. If the host controller is not monitoring the feedback, the motor will run away at full speed until the signal is removed or the power is turned off. This run away condition makes this mode a potentially dangerous mode if you lose your feedback. If you are in torque mode, your host controller is in full control of the system.
The amplifier's only job is to make sure the motor always has enough torque to do the job. This mode is very useful when:
a) The load is constantly changing and you have to make adjustments in the tuning parameter on the fly.
b) You need the ability to "clamp" the torque (nut runners, load testing, web handling, etc.).
c) Your host controller can close the velocity loop faster than the amplifier could.

Speed/Velocity mode is a little less control. The drive takes in a +/-10VDC signal and "amplifies" it to the appropriate motor speed. The amplifier closes the torque & velocity/speed loop. The host controller is tasked with closing the position loop. If you give the drive a 5 volt
signal, the drive "tells" the motor to apply 50% of the available speed. The drive also makes sure that there is always enough torque available to
achieve that speed. Thus, it's closing two loops. If your host controller loses the feedback in this mode, the drive/motor will not know when to quit. It will continue running at a constant speed until the signal is removed or the power to the drive is dropped. This mode requires tuning in both the drive & the host controller. The lack of the wide open run
away posibilty makes this mode a little safer, but it's not as flexible. Because the drive is tuned to handle the torque requirments (as opposed to the programmable controller), this mode does not handle on-the-fly torque
restrictions & changing loads as well. Some host controllers (PLC servo cards and/or motion controllers) require you to run in velocity mode.
This is because the velocity loop update rate is too slow in the host controller to be effective. For instance the IMC-123 module (I know this
is a very old card, but I have experience with it) stipulates you must have a velocity mode drive. Typically if your drive can close the
velocity loop faster than your host controller, it's a pretty good idea to run the drive in velocity mode.

Position mode is probably the easiest to use, but it also limited. If you don't have a constantly changing load and your motion is simple, position
mode would be my suggestion. All the tuning is done in the amplifier. Some PLCs have the ability to switch outputs at decent rates that may
allow you do to slow moves without the need of a stepper card. Most motion controllers allow you to send analog or step and direction signals
(or a combination of both). The step and direction is interpretted by the drive as follows:
direction bit dictates forward or reverse movement
number of pulses dictates how far to move
frequency of steps dicates how fast to move
In position mode, the drive closes all 3 loops (torque, velocity, & position). The encoder feedback can be passed back to the PLC for
monitoring purposes but is not necessary.

4) There are a number of controllers that can drive multiple servos (Parker Compumotor, MEI, ORMEC, Indramat, Galil, Yaskawa, AB, GE, G&L,
Delta Tau, Baldor, AML, Emerson, and more). There equally as many companies that make single axis drive/controllers (drive & controller in 1
box). Which way do you go? It depends on the application. If you want to have two motors in synchronization, you'll need two separate servo =
drives. However, you'll probably only want one motion controller. It's tough (although not impossible) to synchronize 2 motors w/ 2 controllers.

5) I believe the above information addresses your question here.

6) Most of the concepts of closed loop were addressed in my previous answers. I'll make one more comment here. The term PID (or PIV) is
commonly associated with closed loop systems and thus servos. The term PID has to do w/ how the system responds to change and the abbreviation
refers to Proportional, Integral, and Derivative (or Velocity). I'll give you my interpretation of how PID relates to a closed loop servo system.

When you give a servo a command to move, it instantly has a postion error. It is physically impossible to have instantaneous acceleration & achieve the commanded velocity. The servo will attempt to acheive the commanded postion/speed as quickly and agressively as you allow it to. When you tune a system you are telling it how aggressive it can be in combatting position error.

The proportional term dictates how hard to attack the position error (how much current should it apply). If you tell it to correct too aggressively, the system will overshoot the target. If you tell it to correct for the
error with little effort it may never reach the desired target.

The derivative term dictates how quickly the system will settle after the target has been reached. If you don't give the system any derivative it will never settle out to a constant speed or position. If you give the system too much derivative, the system will go into oscillation & you'll see or hear vibration.

The P&D term work very closely together. Too much P & not enough D will result in a system that either never settles out or is underdamped. Too much D & not enough P will result in a system that either oscillates or is overdamped. The goal is to get both terms as high as possible without making the system unstable (resonance or vibration). Different host controllers have different guidelines for accomplishing this. For
instance, Parker Compumotor normally has a P value 10 times greater than the D (Parker calls it V) term, but Galil's D term is normally 10 times greater than their P term. The difference is based on how each company accomplishes their algorithims.

The integral term is normally the steady state term. If your final position/velocity is wrong or if your position/velocity/following error is
too great you can normally address it w/ the integral gain. Think of the Integral gain as the amount of force it takes to keep a boat on the
Mississippi even with a dock. The boat has to maintain some force to oppose the current & thus keep its steady state position & velocity even
with the dock.

There are other terms involved with tuning a servo system (integral limit, feed forward, etc.) but in my experience I've never had to mess with them.

Please feel free to contact me w/ any other questions. I do teach training classes on servos & enjoy discussing the technology.

Guy H. Looney
Sales Engineer

Regan Controls, Inc.
475 Metroplex Dr.
Suite 212
Nashville, TN 37211
phone: (615) 333-1940, ext. 322
fax: (615) 333-1941

Guy H. Looney

Very good points. I recommend having at least 3 times more commanded resolution than position feedback. If you have more feedback than
resolution, you can't command all positions that are available.

Also it's a good idea to have 10 times more feedback than your repeatability requirement. The two must be at least equal. You can't be more
repeatable than you can command.

As far as loss of feedback goes, noise isn't the only problem. A servo system is always "servoing" (moving). Even if you are commanding it to sit still, it will be moving (unless you have a brake applied). I had an application that used a linear encoder. The encoder had no filtering circuits built in to it. As a result, the "servoing" caused the encoder to send a 3 MHz signal to the host controller at "rest". It was "servoing" at "rest", but was actually moving back and forth around 3 different positions on the encoder (0.1 micron resolution). The host controller had a 2MHz maximum input frequency. As a result, when the system was at "rest" we were getting a loss of feedback signal. We had to install a low pass filter to fix the problem.

Differential encoder outputs are the best way to go. It's sort of like RS-232 vs. RS-422/485. It's better to compare A+ to A- and B+ to B- than
to compare both to the same digital common. This is especially true with long encoder cables.

Another word on repeatability. If you are using the encoder on the motor, then you can't be any more repeatable than the mechanics allow you to be. If you're using a 1 pitch (1 rev/inch) ball screw w/ +/- 0.0005" repeatability and a gearhead w/ 5 arc-minutes of backlash, the best you can hope for is +/- 0.00073".

5 arc-minutes of backlash =3D 2.31481 x 10-4 revolutions
1 revolution = 1"
Backlash of gearhead =3D to 1" * 2.31481 x 10-4 =3D +/- 0.00023"
Total system repeatability =3D +/-0.0005" on screw plus the +/- 0.00023 from the gearhead
Total system repeatability =3D 0.00073"

If you don't use external encoder feedback, it doesn't matter how good your servo system is. You'll never to better than that. It all starts
with your mechanics.

Finally, on the gearing concept. Many servos digital servos allow you to dicate how much feedback you get out. It's called output dividing ratio. For instance, if you have a motor encoder that has 8192 pulses per rev, you can tell the drive to send out any resolution you want up to the maximum. There is some tricks to this & yes, there is some rounding. However, the rounding is rarely an issue (haven't seen it matter yet). This will help you in your gearing if your mechanics don't match up. Steppers allow you to dictate the drive resolution. This works much the same way except this tells the stepper drive how many pulses equate to a revolution. The only limitation here is the pulse width. You have to change the pulse width with you change the drive resolution. The problem arises because when the pulse width gets wider, the maximum velocity (frequency) becomes smaller. Therefore, you sometimes sacrifice speed.

Both of these methods help with gearing, but both sacrifice resolution. The loss in resolution sometimes causes you to loose the very thing you
were trying to improve....better repeatability.

As with all motion control applications the application dicates what you do, and there are trade-offs in everything.

Guy H. Looney
Sales Engineer

Regan Controls, Inc.
475 Metroplex Dr.
Suite 212
Nashville, TN 37211
phone: (615) 333-1940, ext. 322
fax: (615) 333-1941

I have some experience in this !

Rightly said ,most modern drives bring speed feedback in to the drive . In case the same has to be used for dual purpose ( drive & plc), the same can be accomplished by using a pulse multiplier board. The pulse multiplier board may be mounted either in the drive or PLC panel and
will be powered from the same panel.

The encoder pulse train is taken as the input and two identical pulses are generated,one can be for drive and other plc.

This works quite well for monitoring as well as control. I recall in one of our projects, for a Hot Rolling Mill Complex, one Coiler motor encoder was similarly connected and it works very well.


This was supplied by a local vendor G.G.Tronics in Bangalore,India.
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You need to review the data sheets for both the drive and encoder and do the math to see if there is enough drive. Please note that we work with a drive that passes the encoder signals but DOES NOT buffer them. In our case, even though we can get the encoder from the drive, it is the encoder that is driving in the line. Depending on your system (how far is the PC from drive/motor) you may need to buffer the encoder lines.

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