Allen Bradley Hydraulic Servo Motion Control

I have a project that I am getting ready to start that involves two 1756-HYD02 cards. I will be getting LVDT Feedback from three Balluffs and I will be sending position/velocity commands to three Vickers Servo Valves. I have a top ram and two side rams. It is critical that I maintain precise speed control on the two side rams. I was thinking about using MAG in the controllogix motion commands to gear the two axis together. Does anyone have any experience with this instruction or with these cards? I am looking for a way to make both speeds identical and this looked like it would do the job..

Just let me know what would be the best way to handle this..

Thanks,

Define precise.

I would use the MAM command. If you issue the MAM command to both axes at the same time then both axes will move together following identical motion profiles. This way both axes can take advantage of using the feed forwards because the feed forwards are generated from the target motion profile.

If you do need to gear then gear to the target position instead of the actual position. The PLC should be able to generate smooth feed forwards using the master motion profile.

When gearing to another axes' actual position there is a one scan delay communicating the master position to the PLC so it can generate the slave position one scan later.
The slave tends to lag the master any because feed forwards can't be used because it is difficult to determine the master speed and acceleration from millisecond to millisecond.

You didn't say how fast you are moving the axes or how much mass is being moved. If you have a well designed hydraulic system then this should be easy however poor hydraulic design can waste a lot of your time.

Start with the example program.

I have used the MAM instructions before. I guess I was making it more complex with the MAG. Have you used these cards before? If so, please let me know how you set them up in regards to tuning? I was wandering if there was a procedure for tuning this card.

There is an existing Delta Controller running this hydraulic press. The system seems to control fine so I think the hydraulics are performing well. The speed is pretty slow and the cylinder is not pushing anything other than small dies.. So, there is no load at all..

I am curious about the tuning of this card.. I am familiar with how to do the electric motors, hookup and all the Motion Axis Commands. However, when I do electric servos, the default tuning parameters are usually sufficient.. I think this card defaults to zeroes in everything so I was wandering about how to tune this thing..

Do you have any literature that would help a first time user of the 1756-HYD02 card? Have you used it before and what are your thoughts?

plcengineer said:

I have used the MAM instructions before. I guess I was making it more complex with the MAG. Have you used these cards before?

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My company is Delta Computer Systems. I am the President. We make the Delta RMC we sell ourselves and the 1756-HYD02 for Rockwell. I did the programming for the HYD.

If so, please let me know how you set them up in regards to tuning? I was wandering if there was a procedure for tuning this card.

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The tuning works the same as it does for a M02AE.

There is an existing Delta Controller running this hydraulic press. The system seems to control fine so I think the hydraulics are performing well. The speed is pretty slow and the cylinder is not pushing anything other than small dies.. So, there is no load at all..

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If the Delta controller is a RMC then why replace it? You won't get better performance

I am curious about the tuning of this card.. I am familiar with how to do the electric motors, hookup and all the Motion Axis Commands. However, when I do electric servos, the default tuning parameters are usually sufficient.. I think this card defaults to zeroes in everything so I was wandering about how to tune this thing..

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First try the auto tuning that you would use as if you are tuning a M02AE card. A lot of effort was put into keeping the M02AE,HYD02 and M02AS as close to the same as possible. The big difference is that a hydraulic cylinder has a different gain while extending from retracting so hydraulic actuators are tuned in each direction.

Do you have any literature that would help a first time user of the 1756-HYD02 card? Have you used it before and what are your thoughts?

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I have nothing you can't get through through Rockwell

After you get the system wired simply pretend you are programming a M02AE.

When it comes to hydraulics I'm a customer of Peter's so I'll defer to him but there are a couple of things that caught my eye on this thread.

First you mentioned that you are going to be using LVDTs. Why? There are better feedback options, including sensors made by the vendor you mentioned, Balluff. I encourage you to consider other options such as SSI. For really good hydraulic control, it is just as important to know WHEN your cylinder is in a certain postion as it is to know WHAT position it is in.

Second, you mentioned that your system was slow and there was no load at all. I caution you with that assumption, especially if you are hoping to speed things up. F=MA does not get suspended just because its hydraulic; if you accelerate it then there must be a load. Changing the controller alone won't speed the system up. Increased velocities only come with increased accelerations. If you don't do your homework you may find that you don't have enough force available to achieve the needed acceleration, even though all other system components appear to be right, and your system will be slower than predicted. Many an engineer has stumbled over that one.

At this point we don't know the details of your system, but Peter is the right guy to be talking too.

Alaric said:

First you mentioned that you are going to be using LVDTs. Why? There are better feedback options, including sensors made by the vendor you mentioned, Balluff. I encourage you to consider other options such as SSI.

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That is right but the system probably already has the start stop Balluff rods installed. This sounds like an upgrade. SSI does provide higher resolution feedback and better speed estimation.

For really good hydraulic control, it is just as important to know WHEN your cylinder is in a certain postion as it is to know WHAT position it is in.

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That is true and you know I fanatical about it. We have always used external excitation so we know when the positions were valid. This was true even back in the 80s but in the early 90s we started to use FPGA for generating the interrogation pulses and detecting the time between the start and stop pulses. NO sofware is involved in sampling the data, it is all done in the FPGA. No interrupt jitter. The FPGA works at the resolution of the 120Mhz clock.

Changing the controller alone won't speed the system up.

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True

Increased velocities only come with increased accelerations.

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Not always true. It depends on how much distance one has to ramp up and down in.

This should be simple. It still isn't clear why though.

The existing RMC Controller is pretty old and it is tied to a TI PLC. So, the customer is wanting to upgrade the entire control system to a modern platform. I will see that I have used these cards in the past and they are great. This one still performs well but a new control system is desired by the customer.

If I were to use MAM instructions, the one axis wouldn't know if the other one slowed down. I agree that they would have the same profile and should run the same but what about if one hydraulic cylinder/valve combination responded differently. This is why I was looking at MAG to eliminate some of that. Let me know your thoughts because you are definitely the motion guru.

I never really had much luck with auto-tuning the servo drives.. They seemed to never respond properly or were always tuned too hot. Does the auto tuning work better for the hydraulic cylinders? I guess I could manually tune the position command vs actual as well as velocity command vs actual.

I always get confused with the scaling of the transducer or encoder counts in the AB. Do you know of a easy way to explain this? I am talking about in the axis setup with the conversion constant and I don't really understand windup etc..

Thanks,

plcengineer said:

The existing RMC Controller is pretty old and it is tied to a TI PLC. So, the customer is wanting to upgrade the entire control system to a modern platform. I will see that I have used these cards in the past and they are great. This one still performs well but a new control system is desired by the customer.

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OK, the RMC is still good though. It probably talks to the TI over profibus. One could simply replace the Profibus DP card with an Ethernet card though, but that but buying new HYD02s is OK too.

If I were to use MAM instructions, the one axis wouldn't know if the other one slowed down.

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I always hear that . So what happens if you gear the axes and the slave slows down? How does the master know it?

I agree that they would have the same profile and should run the same but what about if one hydraulic cylinder/valve combination responded differently.

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That is a tuning problem. If they are tuned wrong they will not track correctly no matter what method you use. You must be prepared to tune each axis so that when you take a plot of the motion the target position is overlaid by the actual position. You should get a graph from the RMC and see what I mean.

This is why I was looking at MAG to eliminate some of that. Let me know your thoughts because you are definitely the motion guru.

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Simply put, you should NEVER gear axes when you control the all the axes because the the slave axes will use imperfect master actual position instead of the target positions. You gear to external encoders that aren't controlled by the motion controller only.

I never really had much luck with auto-tuning the servo drives.. They seemed to never respond properly or were always tuned too hot.

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That is because most auto tuning programs are there SIMPLY FOR MARKETING REASONS. This allows them to say I can auto tune too. Most servo auto tuning use only 3 positions and times to compute the system model. Any errors is sampling results in a poor model. That RMC you are replacing gathers up to 1024 samples and tries many combinations of of models to find which one statistically fits the 1024 the best. In other words it tries many models in search for the best one.

Too many auto tuning programs assume the world is like a text book.

Does the auto tuning work better for the hydraulic cylinders?

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Not always. In either the statistical or 3 point auto tuning cases the model must match the physical system. Poorly designed hydraulic systems do not auto tune well but that goes for electrical-mechanical systems too.

Actually auto tuning hydraulic is harder than auto tuning servo motors. We know that.

I have seen where the auto tuning does so well that even I am amazed. There are other cases where the auto tuning only gets close. This can be due to non-linear servo valves etc. Manual tuning is then required to finish the tuning.

I guess I could manually tune the position command vs actual as well as velocity command vs actual.

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Yes, I am home now but I can post some plots.

I always get confused with the scaling of the transducer or encoder counts in the AB. Do you know of a easy way to explain this? I am talking about in the axis setup with the conversion constant and I don't really understand windup etc..

Thanks,

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Both the Rockwell and the Delta controllers have scaling wizards. It should be easy just fill in the dialog fields. The big issue is whether or not to use the gradient marked on the Balluff rod or to actually measure the position at two points at either end of travel. At some point one needs to get out the dial indicator.

Integrator windup shouldn't be a problem with either the HYD or RMC. If the integrator winds up then the tuning of the feed forwards is way off. If the feed forwards are tuned up perfectly there is no error and the integrator doesn't change from 0 even while moving. We can plot the integrator's contribution to the total control output. If we can see the integrator's control term vary from 0 we know the feed forwards are not tuned well at that point. Then you must look to see whether the system is accelerating/decelerating or moving at a constant speed to see which feed forward gain needs to be adjusted.

Integrator windup should NEVER be a problem on a well designed motion controller. Usually there is an poorly designed system or tuning issues that are the real problem.

Is it a RMC100? I think, correct me if I am wrong and I may very well be. I think the HYD is based on the RMC100. If that is true I would think the tuning may be the same between the units. If the RMC is doing a good job now, can the tuning variables be copied to the HYD module? I don't know anything about the HYD modules but I am sure Peter does. If the tuning works then the scale settings should as well.

CharlesM said:

Is it a RMC100? I think, correct me if I am wrong and I may very well be. I think the HYD is based on the RMC100. If that is true I would think the tuning may be the same between the units.

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The 1746-QS and the RMC100 are almost the same. The code was written for both at the same time but the RMC100 has been enhanced over the years. Both the 1746-QS and the RMC100 code came from the MMC120 which is a module that fit in the Quantum PLC.

Actually the HYD02 started with M02AE code that was modified to add features for the hydraulics. The M02AS is the same. This was done on purpose so that a ladder written for the M02AE would work on the HYD02. The HYD02 doesn't need a home routine but it will accept the command. This is intentional. It also made it so that the learning curve for both the Rockwell tech support AND the customers was minimized. If you know one you know them all. As some of you know we also made the 1746-QS module for the SLC500 but there wasn't any concept of consistency. There were several hydraulic modules made for the SLC platform and all were very different. This made it hard to move between modules since whole new programs had to be written and each had a new learning curve which is tough for the tech support guys as well as the customers.

What the Rockwell marketing decision maker did like about the RMC is that we can mix and match feed back devices to fit the application. The code and tuning differences between the feedback modules was minimal. This concept of conformity adopted by the Controllogix platform and I think it has been successful.

When we modified the M02AE code we did add special hydraulic features from the RMC100 but not all. For instance the RMC100 can do position/force control. The RMC100 has many specialized command like the quick move.
These commands would not fit into the control logix platform. What the HYD02 provides is a 32 bit floating point interface and it is programmable using the specialized motion blocks. The Control Logix communicates with the RMC100 using MSG blocks or Ethernet/IP IO and most of the real time control is done on the RMC100.

If the RMC is doing a good job now, can the tuning variables be copied to the HYD module? I don't know anything about the HYD modules but I am sure Peter does. If the tuning works then the scale settings should as well.

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The scaling on the RMC100 is different from the HYD02. This is due to the fact that the RMC100 is an integer processor and the HYD02 has a 32 bit DSP. One can simply enter a 0.9 into a HYD module. On a integer controller one enters a 29491. The controller shift that right 15 times ( divides by 32768 ) to get 0.9.



The RMC100 was designed to work with the 16 bit PLC of the 90s. The HYD02 and M0Since they arethe the PLC5 and SLC500 a

plcengineer said:

If I were to use MAM instructions, the one axis wouldn't know if the other one slowed down. I agree that they would have the same profile and should run the same but what about if one hydraulic cylinder/valve combination responded differently. This is why I was looking at MAG to eliminate some of that. Let me know your thoughts because you are definitely the motion guru.

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When you do this (and I would recommend it as Peter does) you need to have a watchdog routine looking at all position feedbacks and throwing a fault if the position differences get outside of your acceptable window. Again, if a hose breaks or there is a leak in the system, it doesn't matter which instruction is driving things, it matters that you are watching over the process and reacting properly if the critical parameters go out of tolerance.

Good catch Norm, I forgot to mention the cure

You can look at the position feedbacks but it is easier to look at the following error bit for the two axes. If either following error bit is set you ramp down both actuators using the same command using the same parameters at the same time.

Peter,

I think I am going to go ahead with the two MAM instructions and launch them at the same time. My friend has similar application and was wandering what your thoughts were about using a virtual axis along with MAG commands. I was telling him to just use MAM like you recommended. What are your thoughts with doing a virtual axis coupled with MAG?

The results should be the same. EXCEPT...

plcengineer said:

Peter,

I think I am going to go ahead with the two MAM instructions and launch them at the same time. My friend has similar application and was wandering what your thoughts were about using a virtual axis along with MAG commands. I was telling him to just use MAM like you recommended. What are your thoughts with doing a virtual axis coupled with MAG?

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With the MAM you simply issue a command to go to the same place. Normally when starting you tell both physical axes to go an average point between the two axes just incase they are screwed. You want to make a motion that will unbind the axes just in case. After that you can issue the same MAM to both axes and they will track.

Gearing is a relative type of motion. First you must move both axes together like above in the MAM example. Then you most set the virtual axis' target and actual position to the same location as the physical axes target position. Now you issue the MAG command to gear the two physical axes to the virtual axis. The virtual axis and the two physical axes should be at the same position. Now you can move the virtual axis and the slave axes will follow.

The only advantage the MAG method has is that one only need to issue one command after getting geared but the cost is extra rungs to get geared. In either case you still need to monitor the error bits of each of the slave axes.

Peter,

First of all, I really appreciate all the great advice that you gave me on setting up my motion application with the 1756-HYD02. I have the panel constructed and I am going through some checks and I had a few questions that I would like to ask. I was a little confused with the velocity scaling for the output of the servo in the RSLOGIX5000. I couldn’t get the servo card to output voltage until I set that to 100. Now, that I did that, I can see the +10VDC for forward and -10VDC for reverse. How do I know what to set that to.. I was confused when I read the help file.. I did read that the autotune would set that up so would you autotune the servo card first, then tune it manually the rest of the way..

Also, I gave the servo a MAM command using motion direct commands.. Once I issued the command for a forward absolute position, the signal went to +10VDC. I moved the balluff magnet to simulate motion and once I made it to the setpoint of the MAM, the voltage didn’t go to zero or start a decel signal. Do you know what may be causing this???

Also, is there a way to actually run some simulations with my system without being hooked to a servo valve and cylinder. Basically, we have everything hooked up in the panel shop so we can move the balluffs and see the position etc.. I was wandering if I could do some simulations without having the servo valve hooked up. I am trending the position command/position feedback, velocity command/feedback but I never saw the commands change when I issued a MAM. Now, when I homed the axis, I could see the command signal change(I know it was called command but it wasn't position command).. I would like to do as much simulation as I can so when I put this thing in, I am pretty confident I have it pretty close with the exception of tuning a bit.. Please let me know what I can do..

Thanks for the help!!!!!