New Controls Engineer First Project Startup

So I have currently been at my first controls job for about 6 months and am starting up a new project for one of our vendors. This project is an axle press and I will be setting up the controls to apply a pressure to an axle bushing with a hydraulic system (2 hydraulic pumps, 2 Double Solenoid Spring Centered Directional Valves). Pretty simple setup but I have no experience with controlling hydraulic systems. From the research I have done online I don't think I will need a motion controller (because this system doesn't have to move very quickly or extremely precisely). Wondering if anyone has any advice as far as what I will need in a controller (currently was just looking at a CompactLogix controller) or any advice as to whether or not I will need to do much PID things to control the speed/pressure. Thank you in advance!

TonysControls said:

So I have currently been at my first controls job for about 6 months and am starting up a new project for one of our vendors. This project is an axle press and I will be setting up the controls to apply a pressure to an axle bushing with a hydraulic system (2 hydraulic pumps, 2 Double Solenoid Spring Centered Directional Valves). Pretty simple setup but I have no experience with controlling hydraulic systems. From the research I have done online I don't think I will need a motion controller (because this system doesn't have to move very quickly or extremely precisely). Wondering if anyone has any advice as far as what I will need in a controller (currently was just looking at a CompactLogix controller) or any advice as to whether or not I will need to do much PID things to control the speed/pressure. Thank you in advance!

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This is your first project? (Is this a school assignment?)

Are the valves digital or analog? Also, what feedback do you have if any, if none a PID won't really do anything.

Hey there, no this isn't a school project. I've never been involved in the initial setup of a PLC (nor a hydraulic system) so far only worked in making changes to the control systems we have in house and maintaining.

As far as feedback in the system there is a pressure sensor for each of the valves. Thank you for the reply!

[The hardware in question:]
IFM PN7671 - Pressure Sensor
Rexroth 4WE6G6X/EW110N9K4/62 - Directional Valve

Still need more information. The pressure sensor unless you have an IO-Link deal (honestly not used and didnt read much about it) seems to only have two relay outputs that could be used with preset limits or a AB PLC. So if you do not have the IO-link seems fairly useless other than to display pressure and manually set the manifold where you want it, and a couple relay outputs. The valves will just allow you to move an actuator, possible this press could be built without a PLC but lacking info on this design.

*Side note I do not do much hydrallics either, just trying to point some stuff out. Also that sensor has a fairly high max pressure range, becareful.

Oh yea sorry to have left out some of the details. So what I have right now is a hydraulic system that we outsourced (Pump, Reservoir, Manifold, Valves, Pressure sensor) and I am trying to figure out what non-hydraulic components are gonna be needed to adequately control it.

For the pressure sensor I will be using IO-Link to communicate with the PLC.

I think I should be able to do this with just a simple PID loop with the control variable being the valves and the process variable being the pressure on the system (read by the pressure sensor through the IO-Link). But I wanted to get some more seasoned opinions before I finish this purchase proposal.

TonysControls said:

I think I should be able to do this with just a simple PID loop with the control variable being the valves and the process variable being the pressure on the system (read by the pressure sensor through the IO-Link).

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PID assumes an analog-ish output.


Those are solenoid valves, so isn't any control variable signal to those valves digital i.e. not analog i.e. either a "0" or a "1?" Are you planning on using some kind of PWM? If yes, is the valve suitable for that?


What kind of time constants do the various pieces of the system have?

TonysControls said:

I think I should be able to do this with just a simple PID loop with the control variable being the valves and the process variable being the pressure on the system (read by the pressure sensor through the IO-Link). But I wanted to get some more seasoned opinions before I finish this purchase proposal.

Click to expand...

By valves are you talking the directional rexroth you listed above or do you have some kind of analog flow control valve? As I take the rexroth valves to besimple directional valves to extend and retract an actuator. Unless I am missing something no way to control the pressure with them, the spool moves and whatever pressure is their goes out that port to the actuator.

Depends on the hydraulic system, some (not many) have proportional "dump" valves.
The dump valve sends the oil back to the sump. Open the solenoids, and close dump valve until desired setpoint is obtained.

Ken Moore said:

Depends on the hydraulic system, some (not many) have proportional "dump" valves.
The dump valve sends the oil back to the sump. Open the solenoids, and close dump valve until desired setpoint is obtained.

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Ooh, cool.

Non-linear gain, I would guess, so maybe a problem for PID. Or maybe not.

But very cool.

PLC would be operating in kHz range, but hydraulics operate at speed of sound in a fluid; adding a low-pass filter to the system (i.e. capacitance i.e. volume) might fix that.

With any luck @Peter Nachtwey will step in soon with the right answer.

what is the press cycle time?
If i am not mistaken, pid controls are for extended periods of time, not press cycles.
you need to explain what you have in more detail.
i have done a similar type of machine before, but i need a few more details.
what type of valves, single or double sol. blocked center or open center, proportional?
what is the hydraulic pressure you are working with.
what is the material you are pressing into the axle.
what is the customers pressure specs?
stroke length?

other devices to consider.
door safety switches, light curtains.
operator controls - automatic and manual.
display for showing the specs and actual data?

regards,
james

also,
is there anything that tells you where the stroke is such as an lvdt.
are there load cells?
james

If you are using straight directional valves , 1 or 0. You may want to add pressure reducers and flow controls to the manifold so that you can somewhat adjust the pressure and flow.

TonysControls said:

... I will be setting up the controls to apply a pressure to an axle bushing ....

Click to expand...

TonysControls said:

[The hardware in question:]
IFM PN7671 - Pressure Sensor
Rexroth 4WE6G6X/EW110N9K4/62 - Directional Valve

Click to expand...

Details needed.

Does the pressure sensor measure the pressure being applied to the axle, or does it measure the hydraulic line pressure? How much pressure are you trying to apply to the axle, for how long, and how do you know you succeeded ?
As others have pointed out, this "pressure sensor" is merely a switch, providing "I see|don't see pressure", but not indicating how much more or less than its setting is at. There is no way to make a PID loop work with this kind of set-up.

The valve, when energized, puts hydraulic fluid from the pump into the hydraulic line (presumably extending some sort of shaft which is what is applying pressure to the axle). When de-energized, opens the hydraulic line back to the sump, relieving whatever hydraulic pressure is in the line. Will that cause the shaft to retract, or is there a second valve that will be opened to use the hydraulic pump to drive the shaft in the reverse direction ?


If you read deeply into the kinds of questions we're asking, the mistake you're making is you're trying to answer "How do I control this?" rather than "What am I controlling?".

Forget about the "how" for the moment: PID, PWM, sequence-of-operations. None of that matters until you have a clear picture of the pieces that you have to play with, and what "success" looks like.

The hydraulic subcontractor will do all the hard work for you. But you need from them an I/O list so you can then spec out the appropriate PLC. You'll also need some P&IDs (Process and Instrumentation Diagrams if you didn't know, being new; not to be confused with PID - proportional, integral, derivative control) that shows loosely what the physical layout of the system you're trying to control.

Once you have those, then you can start working on

TonysControls said:

trying to figure out what non-hydraulic components are gonna be needed to adequately control it.

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Questions there may include:

• How does the axle get to where it needs to be? How does the PLC know that its there?
• How does the axle leave, and how do we prevent it from leaving before the pressure-thing is done? What is the next axle doing while we're squashing the current one? How does the PLC know that?
• What kind of safety system is needed to halt the pressure-applying thing in case there's someone's arm instead of the axle that's having pressure applied?
• Are you designing the electrical panel/enclosure for the PLC? What kind of power distribution do you need?

Visualize the process in your minds eye first. Watch it work mentally. THEN figure out how to control it.

How would a super-strong human apply pressure to the axle?
Unlike a human, the PLC has no eyes, ears, nor touch. So how does the PLC know what's going on (inputs)?
Unlike a human, the PLC has no muscles. So what are it's "muscles" -- the valves and pumps (outputs)?

There are always two questions to ask:
How does the PLC know?
What should it do if something goes wrong? (then see the first question to ask, "How does the PLC know something is wrong?")


Good luck!

This was EXTREMLY helpful I can't thank you all enough for replying (seems like a forum community that I am going to love interacting with). Now on to your questions and advise.

Aardwizz said:

Does the pressure sensor measure the pressure being applied to the axle, or does it measure the hydraulic line pressure? How much pressure are you trying to apply to the axle, for how long, and how do you know you succeeded ?

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• The pressure sensor will be measuring the pressure of the hydraulic line
• These are tractor axle and the pressure applied to each side will be 2150 PSI
• The pressure will need to be applied for about 10 seconds is what I was told
• The PLC will know it succeeded when;
  1. The cylinder has been driven down entirely
  2. The pressure from the second side of the axle has been applied for the time required (~10 sec)

Aardwizz said:

As others have pointed out, this "pressure sensor" is merely a switch, providing "I see|don't see pressure", but not indicating how much more or less than its setting is at. There is no way to make a PID loop work with this kind of set-up.

The valve, when energized, puts hydraulic fluid from the pump into the hydraulic line (presumably extending some sort of shaft which is what is applying pressure to the axle). When de-energized, opens the hydraulic line back to the sump, relieving whatever hydraulic pressure is in the line. Will that cause the shaft to retract, or is there a second valve that will be opened to use the hydraulic pump to drive the shaft in the reverse direction ?

Click to expand...

That makes perfect sense with the pressure switch not being a sensor, I have just emailed the company we outsourced to about trying to get a new quote that includes an actual digital pressure sensor that the PLC can interface with as well as asking about what sorts of control valves they offer.

Aardwizz said:

If you read deeply into the kinds of questions we're asking, the mistake you're making is you're trying to answer "How do I control this?" rather than "What am I controlling?".

Forget about the "how" for the moment: PID, PWM, sequence-of-operations. None of that matters until you have a clear picture of the pieces that you have to play with, and what "success" looks like.

Click to expand...

I really really want to thank you for this advice. I actually am the only controls engineer we have here and my predecessor left before I even had orientation so the actual "philosophy" of controls I don't know much about. I am learning most of the practical knowledge on the fly.

Aardwizz said:

The hydraulic subcontractor will do all the hard work for you. But you need from them an I/O list so you can then spec out the appropriate PLC. You'll also need some P&IDs (Process and Instrumentation Diagrams if you didn't know, being new; not to be confused with PID - proportional, integral, derivative control) that shows loosely what the physical layout of the system you're trying to control.

Click to expand...

I also requested an I/O list to help me spec the PLC like you suggested, I have read P&IDs before but have yet to create one so looking forward to the new challenge (is CAD standard for P&ID creation or should I be looking for other software?)

Aardwizz said:

Visualize the process in your minds eye first. Watch it work mentally. THEN figure out how to control it.

How would a super-strong human apply pressure to the axle?
Unlike a human, the PLC has no eyes, ears, nor touch. So how does the PLC know what's going on (inputs)?
Unlike a human, the PLC has no muscles. So what are it's "muscles" -- the valves and pumps (outputs)?

There are always two questions to ask:
How does the PLC know?
What should it do if something goes wrong? (then see the first question to ask, "How does the PLC know something is wrong?")

Click to expand...

Again can't thank you enough for the wise words. These ways of thinking about the controls process will do me well and I hadn't ever really been asked to think about the controls process like this before, greatly appreciate all the people taking time out of their day to help the new guy!

TonysControls said:

That makes perfect sense with the pressure switch not being a sensor, I have just emailed the company we outsourced to about trying to get a new quote that includes an actual digital pressure sensor that the PLC can interface with as well as asking about what sorts of control valves they offer.

Click to expand...

I think the IFM sensor will work fine for pressure feedback with the use of IO-Link based off the little skimming I did, I did not read anything in real depth though. IFM also had a sample compactlogix program I came across. So as long as you have the master or whatever is needed to use that sensor with IO-Link I do not "think" you would need another analog sensor for feedback unless you were looking for feedback in another area of the system.

Also if the process is to drive the cylinder down entirely as stated some reed switches or the likes on the cylinders extend and retract could be handy for feedback to your plc.