Sampling Frequency Problem

[erdemsvri]

I have found the problem

The problem is when I get online PLC couldnt manage both so that is not a big problem.

And also managed to get all 3 signals. Not the best sine maybe but I think it will work. After all there is a cascade PID so the answer will change according to it.

The problem seems to be solved now but maybe the PLC answer wont be enough for hydraulic system so I contacted the Delta distributer here anyway.

Will PLCs performance be enough or this job? What do you think?
Or should we start thinking about Delta or something like that?

And also learned know the force feedback is done with a Siwarex FTC which replies at 100Hz so even I get the best sine wave or so the systems sampling point will be 10ms each so if I get another controller what the force feedback will be? What I am trying to ask is there another way faster better than Siwarex in measuring the Force? If I buy the delta controller the force feedback is going to be ....????

Thanks everybody for their answers and interest

 

[Peter Nachtwey]

The problem seems to be solved now but maybe the PLC answer wont be enough for hydraulic system so I contacted the Delta distributer here anyway.

TDG seems to be more of a integrator. I was told by our sales guy that TDG has done 3 test systems. I have never talk to them myself.

Will PLCs performance be enough or this job? What do you think?

It isn't clear how much mass you are moving, how far it will be moved and what is the accuracy required.

A hydraulic system is more difficult to control than a motor or temperature system. Hydraulic systems can be simply modeled as a mass between two springs. The system is underdamped and the effects of begin underdamped become more and more apparent as the frequency of oscillation approaches the natural frequency. The PID in the S7 is not adequate for controlling underdamped systems because it doesn't have a second derivative gain. The best option is to design the hydraulic system so that the natural frequency is MUCH higher than the 6 Hz you plan to do your testing at. This is expensive.

Or should we start thinking about Delta or something like that?

I think you need to start so you know your options. You can download the software from here
http://www.deltamotion.com/dloads/downloads.php?category=rmc70&subcategory=Software
The manual is on-line.

And also learned know the force feedback is done with a Siwarex FTC which replies at 100Hz so even I get the best sine wave or so the systems sampling point will be 10ms each so if I get another controller what the force feedback will be? What I am trying to ask is there another way faster better than Siwarex in measuring the Force? If I buy the delta controller the force feedback is going to be ....????

That isn't fast enough. At least with our controller you want to have at least 1 KHz or higher response. All these little lags end up being big problems.
At 6 Hz the period is 166 ms and 10 ms is ( 100 hz ) is 21.7 degree of phase lag. That doen't leave much for other phase delays. It take time to pressurize and depressurize oil.

Our customers usually use load cell for the force feed back in a lab type test system. Load cells respond quickly to force changes and only one is required. Normally the there is an amplifier between the controller and the load cell that amplifies the signal to 0-10 volts. Make sure the amplifier is fast too. We just had a case where the customer had a slow amplifier. We told him it was too slow and after wasting 2 months he finally gave in a bought a fast one. Now he is happy. The disadvantage of load cells that they are mounted at the end of the rod and are prone to being damaged unless in a lab environment.

In industrial application the customer usually buy pressure transducers and mount them on either end of the cylinder. It is important that they be mounted where the flow is slow and not in a manifold because of the turbulence there. So many have made that mistake. We then calculate the force by NetForce=PressureA*AreaA-PressureB*AreaB. Two pressure transducers are required. The sampling must be done exactly at the same time or there are errors when the pressure is changing. The RMC can do the scaling so a net force is returned. One pressure transducer it not enough two are required UNLESS the other side is alway ported to the tank but this usually isn't the case in a test system.

Position feedback is usually provided by a MDT rod such as a Balluff or Temposonics rod. These can provide position feedback in the 2-5 micro range very easily.

The RMC has a FPGA that takes care of doing all the sampling at the same time and equal intervals. It is the FPGA that then generates the interrupt for the controller. This eliminates the sample jitter.

If you go back the link I posted above you will see that there are many options for input cards.

BTW, I am President of Delta Computer Systems, Inc.

 

[kamenges]

Originally posted by Peter Nachtwey:

Our customers usually use load cell for the force feed back in a lab type test system.

The Siemens Siwarex system is load cell based. It's the interface module that gets you. It is designed to do much more than just return an amplified voltage.

The moral is you need to get a fast amplifier between the load cells and the PLC or whatever else you will be controlling with. Even if you try to do this with the plc it is probably a good idea to go away from the Siwarex FTC and go with a standard analog input, assuming it has a faster ipdate than the 10msec you get with the FTC. I'm guessing here but I think the FTC will also add some filtering that you don't know about, which will add more phase lag.

Keith

 

[erdemsvri]

TDG seems to be more of a integrator. I was told by our sales guy that TDG has done 3 test systems. I have never talk to them myself.

I talked to them. They are very helpful thanks to them. Just as you told they said they have done 3 test systems and they work well but they are far away from us. Ankara -> Istanbul. Anyway if we need any help about Delta Motion Control Systems. I will contact them.

It isn't clear how much mass you are moving, how far it will be moved and what is the accuracy required.

I dont have any idea about the physical system right now. I will see it in a week or so.

A hydraulic system is more difficult to control than a motor or temperature system. Hydraulic systems can be simply modeled as a mass between two springs. The system is underdamped and the effects of begin underdamped become more and more apparent as the frequency of oscillation approaches the natural frequency. The PID in the S7 is not adequate for controlling underdamped systems because it doesn't have a second derivative gain. The best option is to design the hydraulic system so that the natural frequency is MUCH higher than the 6 Hz you plan to do your testing at. This is expensive.

As I said I dont have any information about physical system right now. I will ask the about the natural frequency of the system if they know.By the way we dont use standart Siemens PID we have written our feed-forward PID on our own and the parameters are on the HMI system so they can be changed any time.

I think you need to start so you know your options. You can download the software from here
http://www.deltamotion.com/dloads/downloads.php?category=rmc70&subcategory=Software
The manual is on-line.

Thanks. Now I know my options and already told them to my boss and she said first I should try to do the job with the configuration given. Time will show us...

That isn't fast enough. At least with our controller you want to have at least 1 KHz or higher response. All these little lags end up being big problems.
At 6 Hz the period is 166 ms and 10 ms is ( 100 hz ) is 21.7 degree of phase lag. That doen't leave much for other phase delays. It take time to pressurize and depressurize oil.

You are absolutely right. That is a big problem. Even 1kHz is not enough for a microcomputer guy like me

Our customers usually use load cell for the force feed back in a lab type test system. Load cells respond quickly to force changes and only one is required. Normally the there is an amplifier between the controller and the load cell that amplifies the signal to 0-10 volts. Make sure the amplifier is fast too. We just had a case where the customer had a slow amplifier. We told him it was too slow and after wasting 2 months he finally gave in a bought a fast one. Now he is happy. The disadvantage of load cells that they are mounted at the end of the rod and are prone to being damaged unless in a lab environment.

I think they use the same style but the feedback of loadcell is directly to the Siwarex not a 0-10V converter in our configuration. I will listen my boss and first try to the Siwarex system which replies each 10ms then if this doesnt work I will try amplification.

Position feedback is usually provided by a MDT rod such as a Balluff or Temposonics rod. These can provide position feedback in the 2-5 micro range very easily.

We use Temposonics too.

The RMC has a FPGA that takes care of doing all the sampling at the same time and equal intervals. It is the FPGA that then generates the interrupt for the controller. This eliminates the sample jitter.

If you go back the link I posted above you will see that there are many options for input cards.

BTW, I am President of Delta Computer Systems, Inc.

I am very happy to meet you and I should say many thanks in your interest and the help you gave to a new engineer like me Peter.

 

[erdemsvri]

The Siemens Siwarex system is load cell based. It's the interface module that gets you. It is designed to do much more than just return an amplified voltage.

The moral is you need to get a fast amplifier between the load cells and the PLC or whatever else you will be controlling with. Even if you try to do this with the plc it is probably a good idea to go away from the Siwarex FTC and go with a standard analog input, assuming it has a faster ipdate than the 10msec you get with the FTC. I'm guessing here but I think the FTC will also add some filtering that you don't know about, which will add more phase lag.

Keith

If you dont want filtering you can disable that in Siwarex we know that and We also dont use Siwarex FTC just to get an amplification. It gives us 32 bits resolution which no analog input card can give. And also Siwarex gives us the chance to calibrate the feedback according to a change in the physical system so I dont need go the test lab at each calibration when loadcell is damaged or so. By the way we know some of the chances we got by using Siwarex and we take advantage of them but it is again slow for us.

Thanks for your help.

 

[Peter Nachtwey]

but they are far away from us. Ankara -> Istanbul.

The world is small. You get connected over Ethernet then tech support can be provided over the interent.

I will ask the about the natural frequency of the system if they know.

If they don't know then you should be alarmed.

 

[erdemsvri]

I have to listen to my boss!! and try to achieve the best with the current configuration but thanks to your help now I know my options and if the system fails we will start looking for a new answer and will talk to you again.

Thanks a lot. Take care