Die casting machine servo valve shot control

russrmartin

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Join Date
Aug 2002
Location
Eastman, Wisconsin
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Hi all. We've got a client who has a die casting machine that they are planning to upgrade. The proposed solution is AB compactlogix, the existing control is an old Bosch system, with a Moog servo valve and controller. The Mook equipment will all stay, the AB will just replace all of the discrete control, and will feed the signals to the Moog controller. They are extremely concerned with controlling the actual shot. The typical stroke follows a velocity profile, until a target pressure is achieved, signaling that the cavity is full. At that point, the velocity profile yields to a pressure profile during the solidification of the metal. The signals provided back are a quadrature type signal on the ram itself, and actual pressure at the ram head. As I look at this, I don't see controlling the shot being a problem, but I have no experience with this type of machine. Are there any potential major issues that would keep one from simply using two separate simple PID loops for each respective profile? Any potential landmine issues that you may have experienced, I am game to hear. Thanks.

Russ
 
Do you realize that if your die cast machine moves at 200 inches per second the position will change by 0.2 inches every millisecond? The point is that scan time is critical, the faster the better. How fast are the analog inputs and outputs used for control? An analog input with a time constant in the 100 micro second range is required. Again, the shorter the time constant the better.

You really should consider using a fast motion controller designed to do the job. A controller with 0.5 millisecond deterministic scan or faster would be a good start.

It is very difficult/impossible to avoid the pressure spike when the die is full. Even if the controller has a scan time of 0.5 milliseconds the pressure will increase quite a bit during that time.

See this thread
http://www.plctalk.net/qanda/showthread.php?t=42776&highlight=hydraulic
The thread shows how quickly pressure will rise as a function of flow. The pressure in a die cast machine will increase even faster due to the fact that there is a lot of kinetic energy that must be dissapated by using force x distance and because metal does decrease much the force/pressure will be high. Controlling the speed at the time the die is full is crtical and being able to detect when the pressure starts to increase is critical. This simply requires high sample rates.

By the way Nathan aka surferb was the only one to get the answer right on all three forums where this question was posted. The correct answer for the pressure increasing from 1000 psi to 2000 psi was just over 2 milliseconds.
 
Thanks, more info, and questions.

Peter,

Thanks for the response. You validated my concerns, but I'd like to quantify the suspicions to prove that even a dedicated processor would not handle the job. The specifics of this unit are that position/velocity feedback is in the form of quadrature pulses, with a resolution of .25mm. Maximum stroke velocity is 7 m/s. The client has determined a steady state acceptable error of 1 mm. I'd like to identify what the minimum sample rate I can use for acceptable control would be, thus, mathematically eliminating this as a potential solution. What the aforementioned data means to me is that I can expect at most to see a pulse at max speed of 2800 per second, or .000357 seconds per pulse. Because of the pressure issues you mentioned, missing a pulse is highly undesireable, so my scan time would need to be faster than 357uS at all times. Can you validate that I am on the right track here? If so, I think I can confirm scantimes that are possible with the given controller and determine whether this is a maybe solution, or a not possible solution. Thanks for the input.

Russ
 
The client has determined a steady state acceptable error of 1 mm.
He is asking for a lot even with the best controller and mechanical and hydraulic design. I see this all the time. Some sanity calculations are required.

The best velocity resolution is 0.00025m/0.0005s = 0.5 m/s during the scan time of 0.0005s the actuator position may be 0.5m/s*0.0005s=0.00025m. This is one term in finding the best error that is achievable. This looks fine on paper if everything else worked instantly. Hydraulic valves take time to shift. Fast one can be fully opened in 8 milliseconds but that is assuming they are given a 90% to 100% control step signal. That will not happen if you want to smooth the ramps. A fast servo valve can cause more hydraulic shock than a bang-bang valve because the servo valve can open faster or close faster than a bang-bang valve. Oil also takes time to pressurize so that a force can build up. This takes only milliseconds but your actuator may be a long way over the 1mm error limit.

I'd like to identify what the minimum sample rate I can use for acceptable control would be, thus, mathematically eliminating this as a potential solution. What the aforementioned data means to me is that I can expect at most to see a pulse at max speed of 2800 per second, or .000357 seconds per pulse.
That is slow, in fact it is too slow. From my calculations above you can see it is better to have much finer resolution. It would be best to have feedback counts in 1,000,000 counts/second range. However, there is a practical problem with this. The high resolution feed back devices that are ideal for velocity control may not stand up to the shock. Both factors must be taken into consideration.

Because of the pressure issues you mentioned, missing a pulse is highly undesireable, so my scan time would need to be faster than 357uS at all times.
From what you said above 0.00025(m/c)/7(m/s) = 0.0000357(s/c). You lost a decimal point and you really need to count the number of pulses you get per scan and not the time between pulses. You get about 14 counts per 0.5 millisecond scan. Some times you will get 13 counts, most of the time you will get 14 counts and sometime you will get 15 counts but the difference between 13 and 14 or 15 and 14 is about 7 per% or about 0.5 m/s in velocity error. It may be possible to get down to 0.00025s scans but then the velocity resolution rises to 1 m/s. Decreasing the scan time without increasing the feedback resolution doesn't always get much improvement. Getting good speed control will not be easier with faster scans unless the resolution is better.

Can you validate that I am on the right track here?
I think your concerns about speed are valid. I think the specifications are unrealistic in the real world no matter how good the controller, the valve and machinery is. The attitude of the customer needs to change from I want 1mm resolution to I am willing to pay for the attempt to get 1mm resolution. If not you will be on 'mission impossible' aka project from hell. Sometimes I have been surprised.

If so, I think I can confirm scantimes that are possible with the given controller and determine whether this is a maybe solution, or a not possible solution. Thanks for the input.
Russ
Note, to everyone else. I had sent a PM to Russ with a link to videos that shows how we ensure our scan times are synchronous and deterministic at 0.5 milliseconds

This is also why sweating the code speed details like avoiding divisions and mod operators is important. Processing speed is important. See Alaric's quiz thread.
 

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