S7-1200 and hydraulic pressure/force control

Hi,

I am looking into a project to build a control system for a hydraulic test machine. Working with hydraulics is not something I have done before with an S7-1200.

My hydraulic test machine will have a load cell in it, capable of reading the load being applied by the hydraulic components. For my control system I would like to be able to stop at pre-defined loads, for example if I program to say 5.00tonnes, I would like to be able to stop my machine to within +/- 0.01tonne.

I think I need to use servo control, however all of the Siemens videos I have watched talk about position control (in mm). Rather than what I would assume I need, which is pressure control.

I have watched this video:
TIA Portal Tutorial #09: Control step- and servomotors with Motion Control

I cannot see how you could sensibly translate position into force, because the test specimen in the machine may vary, physically, but also elongation under load would vary. I expect I need PID control with my load cell as my feedback.

Is anybody able to suggest some reading material for this type of application?

I'm quite set on using Siemens PLCs as I know them, and I don't want to learn a new PLC and application at the same time.

Thanks

Here's something to read...


https://support.industry.siemens.co...r-motor-or-hydraulic-cylinder-?dti=0&lc=en-WW

DLMUK said:

I'm quite set on using Siemens PLCs as I know them, and I don't want to learn a new PLC and application at the same time.

Click to expand...

This is usually a mistake. You will waste a lot of time and still not achieve what you want to do. A lot of people insist on using a PLC and eventually give up. Some are stubborn and live with their mistake. We see this all the time.



Pressures/forces can change quite rapidly. Often more rapidly than the PLCs PID can respond. It is best to have millisecond or faster scan times. The best way to do this is to use a RMC75E. It is designed to do position/pressure/force control in testing applications.
https://deltamotion.com/peter/Videos/SyncPosForce.mp4
https://deltamotion.com/peter/pdf/a...er Accelerates Gear Testing - Tech Briefs.pdf
Look at the speed and precision.


You can use NI for a front end if you want or you can roll your own using our RMCLink .net assembly.



We have a regional sales manager in the Edinburgh, Scotland.
Tim Gessner, [email protected]

what is your application?
what is the speed of operation?
what are your various loads?
I have done as you have requested, but I need some details on what your application is before I can give any assistance.
james

Peter Nachtwey said:

This is usually a mistake. You will waste a lot of time and still not achieve what you want to do. A lot of people insist on using a PLC and eventually give up. Some are stubborn and live with their mistake. We see this all the time.

Click to expand...

After setting up a system with a dedicated hydraulic control (I think it was Moog though) and faffing about controlling via a PLC to never actually have good control, I definitely agree with this.

I've done really really slow hydraulic lifts before (moving 2 feet in 60 seconds) "successfully" with a PLC (1512). The hydraulic system was designed so poorly that any issues with the PLC being slow were dwarfed by the physics issues. It only worked because the required speed was so slow that it ended up mostly not mattering.



I've heard of more typical high speed hydraulic motion being done with really fast 1500s (1517), via a special hydraulic library you need to ask for.


Never been involved with force based control with hydraulics.



Advice from a guy who works with Siemens PLCs all day and not much else: You'd buy a servo controller to control your servos (the drive); it's perfectly reasonable to buy a hydraulic controller to control your hydraulics. You would spend a lot of time on a science experiment trying to get it to work, and it might or might not in the end. The cost of your time might quickly outpace the cost of the specialized hydraulic controller.

DLMUK said:

Rather than what I would assume I need, which is pressure control.

Click to expand...

Is the load cell going to see a number that is directly proportional to the secondary pressure in the hydraulics system?


What is the actual element the control system (PLC or otherwise) is going to control? Is it a setpoint of a hydraulic pressure control valve, or is it the open/closed position of the the pressure control valve?


As noted in previous posts, the time response of the system is going to determine if a PLC can do the job or not. There are at least two operating regimes:

• one is move/setup, i.e. before the pressure and load cell reading are correlated i.e. when moving the pieces to the point where they "touch" or whatever is the case, in this case pressure and load are both zero, so the PID would not be involved
• the other is after the pieces touch, where the pressure and load cell readings are correlated. This is where the control system actually starts operating. Whether a PLC PID can do the job depends on the dynamics of the system. If the pieces involved are stiff then it will be a stiff system, and deadtimes from speed of sound would be in the ms range, both in the actual force applied and in pressure measurements. I'm not surprised many PLC-based systems fail here. If the design behavior of the system is slow, then maybe a PLC could be used, but until you know the dynamics you won't be able to guess at that.

As for reading materials, I would suggest PID theory relating sampling period times and the dynamics of the system. If the system exists and you can make manual adjustments, maybe you can map out the dynamics and get an idea of what is required.


If the system is slow enough, a simple linear scale from load to pressure could be calibrated, with maybe a trim on load. But again, it all depends on the dynamics, and there is ample warning about that in previous posts.


That said, it seems to me giving up on the PLC and purchasing a system designed to do this, equivalent to a servo controller for position control, will cost far less than the time tilting at the windmill of controlling a fast, stiff system with a slow PLC.

As others have said, alot of this depends on required bandwidth. But if all you are worried about is pressure and it doesn't need to be particularly quick then you could probably do this with a proportional pressure reducing valve controlled out of the plc. This basically shifts the dynamics of the fluid control into the hydraulic system. You can set a base pressure command based on your system design and then trim the command based on load cell feedback.

Again, this assumes you have no need for position control whatsoever; all you are concerned with is force.

Keith

Pressure regulators don't work because they only regulate the pressure on one side. There is usually opposing pressure on the other side of the piston.
The load cell is the most accurate way to measure applied force. Load cells are good for testing environments. The problem most people have is that they buy a slow amplifier so the load cell is too slow.


Using two pressure transducers can also be used to measure the applied force using the pressure and areas on both sides of the piston to calculate a differential force. The problem with this method is that it ignores seal friction. Also, people tend to mount the pressure transducers in the wrong spot and tend to use pressure transducers that are too slow.

I think the big unstated quantity here is required bandwidth. It sounds like the load cell will be there regardless of the fluid control method. If this can happen relatively slowly (a second or two to settle at pressure as opposed to 10-20 msec) I think you could use proportional pressure control from a plc to do this using the load cell to trim the command. Actually I know it can be done; I've done it. But it really can't be done quickly this way. High accuracy with high speed requires specialized tools.

Keith

You haven't actually stated what way the force will be applied, I was once involved in a shear beam testing rig, I wrote the software but had no input into the design, this had two modes, non-destructive & destructive.
The tests involved a beam placed across two supports with a hydraulic ram pressing down on the concrete re-enforced beam, this was done with loadcells & simple hydraulics so a force was applied, when the force reached the setpoint the hydraulic ram was stopped and held for x minutes, there was also a transducer to measure the flex of the beam, this was a re-hash of an existing system due to obsolete hardware, there was no need for PID or any other special control, it was the maths to produce the reports that became a little complex, however, as it was an existing system just used the original formulas.
This rig was to a BS specification, I have no idea of your particular application however, just a thought.
Forgot to mention, the destructive test was to monitor the applied force and the eventual destruction.