Replace Servo with AC motor

[Big John T]

I'd go with the encoder. You can use it for commutation if needed as well as position feedback. The downfall is the need to reference.

If you don't think you need to operate in closed loop vector mode you could put an absolute multiturn encoder on the back of the motor. You'll need a fieldbus to get the info into a plc, though.

Personally I would go with an incremental encoder and take the hit referencing when needed.

Keith

Can you explain a bit about the need to reference?
If the encoder is connected to a high speed counter interface with quadrature input does that cover that part?

Thanks
John

 

[OkiePC]

I just quickly scanned the recents posts, but I think by "referencing" he is talking about scaling the motor driven encoder into inches for your ballscrew.

The issue that brings up is counting motor revolutions at high speed.

If I were you, I would go with an absolute device on the screw itself and use sensorless vector VFD drive...unless you require motor holding torque at zero speed.

What sensorless vector means is the drive will calculate the motor shaft angle and estimate it's position in software. This is not as accurate as true vector control with a feedback device. It will give you outstanding speed regulation and great torque at low speeds as well as better acceleration.

You don't need true vector control unless you expect the drive to hold full torque at zero speed (braking with the motor). If your current system does that with the servo, you will either want a true vector VFD with motor mounted encoder or an external brake.

On my ballscrew application I had neither, and it still worked fine because there was no requirement for braking with the motor stopped. There was no external force that could backdrive the screw/gearbox and motor.

I actually was able to get rid of a pneumatic brake that was used in the previous controls for position accuracy.

Now, if you go vector with encoder, it is usually possible for the drive to share that position info with the PLC. Two main methods are: 1. through software comms channel (C-Net, E-Net, D-net, RIO, or other...) or by 2. repeating the pulses to a high speed counter card.

With either method, you must be able to count motor revolutions in the PLC, which can get hairy depending on RPM and PLC scan times, but it's usually do-able.

If you don't need motor braking, use a sensorless vector mode for high performance and spend your feedback money attaching an absolute device to the screw, geared down to keep it comfortable for the PLC scan rate, and then you get the additional advantage of being able to detect any failure of the mechanical drive system.

JMHO
Paul

 

[Big John T]

I just quickly scanned the recents posts, but I think by "referencing" he is talking about scaling the motor driven encoder into inches for your ballscrew.

The issue that brings up is counting motor revolutions at high speed.

If I were you, I would go with an absolute device on the screw itself and use sensorless vector VFD drive...unless you require motor holding torque at zero speed.

What sensorless vector means is the drive will calculate the motor shaft angle and estimate it's position in software. This is not as accurate as true vector control with a feedback device. It will give you outstanding speed regulation and great torque at low speeds as well as better acceleration.

You don't need true vector control unless you expect the drive to hold full torque at zero speed (braking with the motor). If your current system does that with the servo, you will either want a true vector VFD with motor mounted encoder or an external brake.

On my ballscrew application I had neither, and it still worked fine because there was no requirement for braking with the motor stopped. There was no external force that could backdrive the screw/gearbox and motor.

I actually was able to get rid of a pneumatic brake that was used in the previous controls for position accuracy.

Now, if you go vector with encoder, it is usually possible for the drive to share that position info with the PLC. Two main methods are: 1. through software comms channel (C-Net, E-Net, D-net, RIO, or other...) or by 2. repeating the pulses to a high speed counter card.

With either method, you must be able to count motor revolutions in the PLC, which can get hairy depending on RPM and PLC scan times, but it's usually do-able.

If you don't need motor braking, use a sensorless vector mode for high performance and spend your feedback money attaching an absolute device to the screw, geared down to keep it comfortable for the PLC scan rate, and then you get the additional advantage of being able to detect any failure of the mechanical drive system.

JMHO
Paul

Thanks again Paul. The screw does not turn. The gear box has a hollow shaft with a built in female acme thread and as it rotates it drives the screw in and out. So to do anything with the screw I would have to attach a linear transducer to that... I don't think I have room on the machine for that... Nothing rotates that I can attach anything too...

The AC drives I have been looking at the encoder inputs are for speed reference not position. That's why I was considering the high speed quad counter card for the PLC and the analog out put card to drive the inverter speed.

John

 

[Big John T]

Just another thought popped into my head. With sensorless vector drive and a home position switch would that be good enough to work by homing the screw each stroke?

John

 

[kamenges]

Originally posted by Big John T:

Can you explain a bit about the need to reference?

Also referred to as 'homing'. I think you know this but just in case. A quadrature encoder can only provide incremental position information, not absolute position. When you first power the system up the control system doesn't know where the assembly is. You need to define a known position and trigger an input to the control system when that position is reached. Then incremental move information from the encoder can be used to keep track of position.

Originally posited by Big John T:

If I use the Max motor with the encoder I would use the 1746-HSCE2 High-Speed Counter Module to determine position of the ram in my PLC.

Correct. You shouldn't have to worry about pulses coming too fast with that bad boy. Just be warned. Alot of people have gotten hung up configuring the VHSC. If you use it follow the programming and config example from the manual and tweak it as necessary to get what you want. You are also correct with the analog out to from the plc to the drive.

Originally posted by OkiePC:

...and spend your feedback money attaching an absolute device to the screw, geared down to keep it comfortable for the PLC scan rate,...

Back in Post 27 John said he coudln't mount anything to the screw due to machine configuration. He is stuck with a feedback device on the motor. One additional idea may be to use a linear position sensor like this:
http://preview.ametek.com/content-manager/files/stp/Gemco%20LDT%20Catalog%20Z144%20955S.pdf

This gets you analog absolute position feedback. No encoder n the motor, no VHSC. If you have an open analog input to the plc all the better. It all depends if you can find a home for this but it's pretty small.

Originally posted by OkiePC:

It will give you outstanding speed regulation and great torque at low speeds as well as better acceleration.

While I generally agree with this I'm not the kind to paint myself into a corner either. At the very least get a motor with provisions for mounting an encoder. Also, look toward drives that will operate either sensorless vector or closed loop vector. I have yet to see a sensorless vector drive perform on par with a clsed loop vector drive anywhere in the speed range. (Sorry, DickDV. I haven't tried ABB yet.) That way if you end up with performance issues you have an easy out.


Keith

 

[OkiePC]

Thanks, Keith, for clarifying the points I missed.

John, one other idea for external feedback which I have used successfully is a string pot. Think of it as an electric industrial tape measure using a simple potentiometer for absolute feedback.

I would only recommend this if the environment is suitable. They are not good if the cable can snag on something, or if the wiring is subjected to lots of noise, but they only require a free analog input and are very easy to mount, troubleshoot and work with.

They are absolute, and nearly infinitely configurable. The only critical restriction is the retract speed. If you don't exceed the retract speed they are very rugged and reliable.

http://www.celesco.com/cet/index.html?gclid=CODbspbp_okCFQgTWAodGGvRQg

 

[swhite65]

Positioning accuracy does not seem to be an issue here. With the gearbox and ballscrew, you are already dealing with a lot of mechanical "slop" as positioning applications go. Why not put a stepper on it? It is certainly cost effective and could be run with the existing PLC.

 

[kamenges]

John's current servo is rated at 145 lb-in of torque. Do they make steppers that big?


Keith

 

[swhite65]

Parker for example has Nema 42 stepper motors with torque ratings that exceed 145 in-lbs. If this is not a Nema frame but an IEC frame then an standard AC motor could be found to fit. And, as mentioned above, this could be done with prox switches and careful setup of a standard inverter.

 

[Big John T]

Back in Post 27 John said he coudln't mount anything to the screw due to machine configuration. He is stuck with a feedback device on the motor. One additional idea may be to use a linear position sensor like this:
http://preview.ametek.com/content-manager/files/stp/Gemco%20LDT%20Catalog%20Z144%20955S.pdf

This gets you analog absolute position feedback. No encoder n the motor, no VHSC. If you have an open analog input to the plc all the better. It all depends if you can find a home for this but it's pretty small.

Keith

That's an interesting linear feedback device. I like the ball attachment... The question is can I find a place to mount something like that...

John

 

[Big John T]

John, one other idea for external feedback which I have used successfully is a string pot. Think of it as an electric industrial tape measure using a simple potentiometer for absolute feedback.

I would only recommend this if the environment is suitable. They are not good if the cable can snag on something, or if the wiring is subjected to lots of noise, but they only require a free analog input and are very easy to mount, troubleshoot and work with.

They are absolute, and nearly infinitely configurable. The only critical restriction is the retract speed. If you don't exceed the retract speed they are very rugged and reliable.

http://www.celesco.com/cet/index.html?gclid=CODbspbp_okCFQgTWAodGGvRQg

I know I would have room to mount that... and as slow as it moves the speed would not be a problem.. I have lots to chew on now thanks guys.

Thanks
John

 

[Big John T]

Parker for example has Nema 42 stepper motors with torque ratings that exceed 145 in-lbs. If this is not a Nema frame but an IEC frame then an standard AC motor could be found to fit. And, as mentioned above, this could be done with prox switches and careful setup of a standard inverter.

This servo is a 145TC mount. The same as a 2hp C-Face.

John

 

[milldrone]

This servo is a 145TC mount. The same as a 2hp C-Face.

John

Big John, finding a 145 in/lb torque motor in a 145tc frame might be a problem

 

[Big John T]

Parker for example has Nema 42 stepper motors with torque ratings that exceed 145 in-lbs. If this is not a Nema frame but an IEC frame then an standard AC motor could be found to fit. And, as mentioned above, this could be done with prox switches and careful setup of a standard inverter.

Proxes won't work in this application. I have to be able to program the stopping points from the PV550. And the stop points change with each item ran on the machine. There are proxes for home, OMG extend and OMG retract on the machine now.

John

 

[swhite65]

Then there are many indexing type setups that could use induction motors or servos. The induction motor has the advantage of price. The A-B Ultra drive can be setup for induction motors but I'm not sure how costly that process is. I know that the ABB ACS800 has an offering with a positioning macro. It also would have communication modules to talk with an A-B PLC if necessary to change position presets. Or, this could be done through the keypad on the drive.

Also, don't always take for granted that the application was originally sized correctly. If possible, take some current readings to see what the actual HP and torque requirements are. I have done several retrofits where the original design was extreme overkill which also means very inefficient. Sometimes the production parameters have changed and less power is needed for the current process. I would certainly verify the requirements of the application since you are facing multiple retrofits.