Servo to VFD

I am not a drives guy so please take it easy on me. I have a customer that wants to switch from a servo to a VFD. This app is a small rotary fill station that is running a maximum of 100 RPM so speed is not a problem. The drum runs in constant speed mode so no stopping and starting. I do need positioning to +/- 2°. The repeatability is really what I need on this job. I am only using this as a programmable cam switch to open and close valve, solenoids, etc.



What family of PowerFlex do I start getting into encoder feedback on. It looks like a 70 put I can't see any supporting documents. Maybe I am just looking in the wrong place.

Anybody have some pointers.

As to constant speed with very little error, a modern vector VFD will be adequate. As to rapid start/stop sequences, unless you are using the most extreme high-dynamics servos with minimum rotor inertia motors, a modern vector VFD will do that too.

Where a modern VFD comes up short is in the positioning department. Unless you can provide prox or limit switches that accurately instruct the VFD where and when to stop, it will not be able to keep track of where it is like a positioning controller on a servo.

It is true that some VFD's are now available with positioning controllers built in but, by the time you buy that, you might as well stay with your servo system.

That's my experience on this subject.

Jeff

Siemens makes a drive called a sinamics s110. You can get different communication interfaces too. It would be perfect for this and I bet you could do it for less than a thousand.
What size is this?

DickDV thanks for the info. It hit me like a ton of bricks this morning look at using a Encoder to close the feedback loop to the drive and feed a high speed counter card in the PLC. Does that sound like a good choice from where you sit???

JRW thanks the customer saied AB only. So Siemens is out

I'm not a positioning guy, Jeff, but, if you do the positioning in the PLC, the drive will follow the positioning instructions quite well if you set the drive up as a vector drive. I've done this with good results subject to the limitations I mentioned earlier.

In fact, if you use a good sensorless vector drive (no encoder) and tune it up nice and tight, usually the performance will be good enough and you've saved the cost of the encoder. If you need the encoder for positioning, I guess that wouldn't matter much. In that case, you might as well set up the drive as a flux vector drive (encoder for speed feedback) and use the encoder pulses for positioning too.

Flux Vector YES that is it. I don't do drives so I have to dig through the cobwebs or/and ask for helpfrom guys like you. I will be playing with this system hopefully in the next week or so. I will let you know what happens. I will probably be asking more questions before I give any good update info.

Jeff: Take a look at this: http://samplecode.rockwellautomation.com/intradoc-cgi/nph-idc_cgi.exe?IdcService=GET_SEARCH_RESULTS&urlTemplate=%2fidc%2fgroups%2fpublic%2fdocuments%2fwebassets%2fsc_search_results.hcst&QueryText=%28dSecurityGroup+%3CMATCHES%3E+%27Sample_Code%27%29+%3CAND%3E+%28%28xSC_Products+%3CSUBSTRING%3E+%27Programmable+Controllers%27+%3CAND%3E+xSC_ProductsLevel2+%3CSUBSTRING%3E+%27ControlLogix+System%27%29%29&SortField=dInDate&SortOrder=Desc&ResultCount=25&SearchProviders=mms&&PageNumber=2&StartRow=26&EndRow=50
With this AOI you convert you Powerflex into a virtual axis, so you can use positioning, i´ve never used it.
The program it´s called: An AOI to convert movement of Virtual Axis in real movement over AC induction motors PowerFlex 700 V.

JeffKiper said:

I am not a drives guy so please take it easy on me. I have a customer that wants to switch from a servo to a VFD. This app is a small rotary fill station that is running a maximum of 100 RPM so speed is not a problem. The drum runs in constant speed mode so no stopping and starting. I do need positioning to +/- 2°. The repeatability is really what I need on this job. I am only using this as a programmable cam switch to open and close valve, solenoids, etc.



What family of PowerFlex do I start getting into encoder feedback on. It looks like a 70 put I can't see any supporting documents. Maybe I am just looking in the wrong place.

Anybody have some pointers.

Click to expand...

Power Flex 40 P have encoder feedback and are a bit cheaper than the 70 series.

Though I do agree with DickDV that the encoder is probably overkill. Sensorless vector will run the drive at a constant rate and you can then base your trigger pulses on that.

You have not said what PLC you are using. In situations like this, the biggest gotchya is usually the comms betwenn the drive and the PLC.

Clay Sorry I am going to use a Complact Logix L32E and I plan on using the 20 COMM E module for the PF family.

JeffKiper said:

I am only using this as a programmable cam switch to open and close valve, solenoids, etc.

Click to expand...

JeffKiper said:

Clay Sorry I am going to use a Complact Logix L32E and I plan on using the 20 COMM E module for the PF family.

Click to expand...

Jeff, this is what has me a little confused. If I understand you correctly, the motor is turning a cam drum that is operating some switches. You also have a compact CLX. I don't understand why you aren't just programming a sequencer in the CLX to perform the drum switch function. You wouldn't need the drive or the extra complexity. Or am I not understanding the application?

Alaric said:

Jeff, this is what has me a little confused. If I understand you correctly, the motor is turning a cam drum that is operating some switches. You also have a compact CLX. I don't understand why you aren't just programming a sequencer in the CLX to perform the drum switch function. You wouldn't need the drive or the extra complexity. Or am I not understanding the application?

Click to expand...

I think I am a bit puzzled as well. Can you give us a description of the process again. I thought you were using an actual rotary valve and you needed to track the pockets in the valve to actuate your other processes.

Clay B. said:

I think I am a bit puzzled as well. Can you give us a description of the process again. I thought you were using an actual rotary valve and you needed to track the pockets in the valve to actuate your other processes.

Click to expand...

Clay you are correct this is a rotary valve and I need to track the position.

The reference to the PLS was just to visualize what was needed out of the system.

So you don't want the encoder on the motor, you want it on the rotary valve. In fact, unless you have an integer gear ratio between the motor and the encoder or you have an encoder that is an integer multiple of the gearing input/output tooth product you don't want the encoder on the motor.

Fundamentally, you don't care how closely the motor is tracking to the speed command (within reason). You jsut need to know exactly where the rotary valve is so you can actuate the other devices.

I would bring the encoder into the plc directly, unless you decide to go with a PF70 or better. Then you can get the $125 quadrature encoder card and pull the position register back across Ethernet.

Keith

DickDV said:

I'm not a positioning guy, Jeff, but, if you do the positioning in the PLC, the drive will follow the positioning instructions quite well if you set the drive up as a vector drive.

Click to expand...

I have done this with very good results, even with scalar control cheap drives. Bring the position sensor into the PLC and use straight proportional control, which is very simple math. Basically, you'll have a position tolerance (LIM instruction) a decel distance (angle? or other EU), max speed, and min speed variables that you can adjust to get great positioning.

Just calculate the velocity command so that it is proportional to the position error, and clamped between min and max speeds, when within the error tolerance, then hold it's position...this is where I am not sure a sensorless vector is adequate...an external brake or a full feedback vector drive may be required.

I did this with AB160 drives and a resolver with 1024 PPR, and could get within 2 resolver counts, all day long, every day, hundreds of times on a dozen machines.

I used a relay enable output and a bipolar speed reference, so if there was overshoot, the PLC output would go negative and automatically correct. I also set the accel and decel rates in the drive very short, which allowed me to have full control over those parameters with a couple of PLC integers.

On some axes, backtravel was not allowed, so I merely disabled it in PLC logic, and widened the decel distance.

We were able to reduce position errors from +- 15 counts to 2 counts with this method replacing a slam-bang clutch (which also went in the dumpster) with an antique programmable limit switch.

I can only imagine with a vector drive, you'll do even better.

Paul

I'd like to meet you someday, Paul. It seems we have similar approaches to problem solving. And an inherent desire to do it simply, if possible.