Problem with Hydraulic Positioner

This is on an edger. It runs fine in forward. In reverse, it runs normal for a couple seconds, then sudddenly stops, then starts again if it hasn't passed its destination. It happens in both auto and jog mode.

The assembly is positioned with two cylinders, one on either side. They are each set up as an axis in a 1771-QB module. All behavior seems to be identical in both axes.

The commanded velocity holds steady during the unwanted stops.

The output voltages out of the QB module seem to dip unexpectedly at the same time as the unwanted stops, but holds mostly steady otherwise.

We put in a new QB module and it did not help.

Any help or troubleshooting ideas would be greatly appreciated!

Legacy product CIRCA late 1980's

J E said:

This is on an edger. It runs fine in forward. In reverse, it runs normal for a couple seconds, then sudddenly stops, then starts again if it hasn't passed its destination. It happens in both auto and jog mode.

The assembly is positioned with two cylinders, one on either side. They are each set up as an axis in a 1771-QB module. All behavior seems to be identical in both axes.

The commanded velocity holds steady during the unwanted stops.

The output voltages out of the QB module seem to dip unexpectedly at the same time as the unwanted stops, but holds mostly steady otherwise.

We put in a new QB module and it did not help.

Any help or troubleshooting ideas would be greatly appreciated!

Click to expand...

The 1771-QB nor the PLC may not be the problem.. You have a machine with an EXPERT hydraulic positioning/pressure control module that may responding to a degraded environmental conditions that are now present.

The machine was launched in the late 80's or early 90's. You have run the machine flawlessly for about 25 years without problem, or any maintenance. I would seriously look at the health of the hydraulic system, and/or the sensors. IF you want to maintain obsolete technology.

If this machine is part of your revenue chain.... Do not spend a nickel on obsolete controls... Update the electronics to off-the-shelf hydraulic control devices of the year 2018.

Rockwell Automation has modules for hydraulic control, but there are other vendors that are hydraulics position/pressure controls that play with with Rockwell PLCs. Delta Motion RMC 75 or Bosch Rexroth HACD... Others..


Obsolete controls harkens in the back of my mind a quote from Red (Shawshank Redemption) .. "Get busy living (buy new PLC), or Get busy dying (spend money on stuff from e-bay to limp one more year).

What is your employers long-term business model for this machine?

Regards

Plastic

I am a systems integrator that was called in to help. We encourage customers to upgrade plc5 to clx, though this system needs help on the mechanical side as well. In the meantime, we still need to try and diagnose the issue as the upgrade would be a long term project. I agree the hydraulics or magnetic position sensors are likely faulty but I’m not sure how to determine which it is. There are other axes on the system that are behaving correctly.

Wow! You have some ancient hardware.
I doubt the problem is with the MDT rods because the system appears to run OK in the extend direction. Servo hydraulic actuators tend to go faster in the extend direction than in the retract direction because there is more area to push on when extending. There is also more force to overcome obstructions.

I doubt the problem is with the QB module.
I would check the accumulator and make sure it is charged properly. If the supply pressure drops there may not be enough force to overcome any resistance while retracting. I would also check the error status. It is possible the actuator gets a following error and stops.

If there was some way to graph the target and actual positions and control output, that would be handy. I know that wasn't built into the PLC/5 or QB module.

I am familiar with sawmill systems. I use to work for USNR back in the 1980s. My company, Delta Computer Systems, makes the 1756-HYD02 and 1756-M02AS for Rockwell as well as our own controllers.

Your positioning system is strange for an edger. Usually there are 4 to 5 set points that position the board on a feed chain. Then rolls come down and the board is fed into the edger as the set pins retract. Usually the set pins have very little mass to move when retracting so I don't see how there could be a problem unless there is some sort of damage.

Hey Peter, many thanks for responding.

There are actually three set points. The system moves in reverse to one of those set points based on the size and shape of the wood. It is during that motion that the sudden stops occur. Then it grabs the wood and moves forward and drops it off on to rollers.

The issue also occurs during a jog, even if the jog speed is set extremely low. It does take a lot longer for the stopping issue to occur at the low speed.

I was able to “graph” the commanded velocity vs actual in that jog mode. I just set up data tables and grabbed the values once every .05 seconds. Basically the desired velocity stays completely constant while the actual velocity ramps up to it properly, then suddenly drops to almost nothing, then ramps back up again. I can try getting commanded position vs actual as well. I can also tell it to display error codes to see if I am getting any.

It is interesting that both axes seem to do exactly the same thing, and it would make sense that problems with pressure might cause that. I will have them check the accumulator. They didn’t think pressure was an issue because the other cylinders on the machine weren’t showing any problems.

I still wonder why the QB module output voltage goes to near zero during the false stops. For testing purposes is it possible on these systems to just send a reference voltage of say -5V from a meter to the two axes for a couple seconds to test or would that potentially cause problems?

Again, check the error status when the actuators make a 'false stop'

Error was "First block after powerup must be a parameter block". Both QB modules were randomly rebooting, always at the same time. Long story short, an analog output cable going to one of the valves was bad. I guess the short only caused issues when we sent a negative reference and not a positive reference as only reverse motion seemed to be problematic.

How did you spot that the cable was the issue?

After I saw the error, I told them i suspected a power issue. I was thinking chassis problems, but then they told me that a conductor in one of the output cables had gone bad a few weeks ago and that they had swapped to a spare conductor in the cable. We decided to change the cable right away and it fixed the issue.

Apparently they had discussed changing the cable several days ago but decided it wouldn’t help because both axes were acting up identically.