Troubleshooting Challenge Thinkstock

Tube Bender Has Drift Problem

Can you figure out why this control isn't maintaining position accuracy?

Voss Valves fabricates stainless-steel aircraft tube assemblies. Lightning struck one of their buildings, destroying the control panel of an old 1985 automated tube bender that had been updated around 2000.

A new, more modern control was installed, but it did not maintain the needed position accuracy on three proportionally controlled hydraulic motors. A closed-loop PID position-control loop used a rotary encoder on each motor, and the output from each was sent to the existing amplifier cards. The cards received the PID command signal from the new controller and drove the proportional valves. There wasn’t any adjustable function on the cards that could condition or adjust the signal to the valve. The size D03 proportional valves had standard overlapped spools (properly sized for the flow) but without spool position feedback.

The owner was technically sharp and felt that the control technician wasn’t tuning the PID loop correctly. The technician, on the other hand, felt the rotary encoder was causing the problem. However, installing a new one had no effect on the problem.

I looked at the application, and spoke to the operator. I found he knew the machine well and that the old controller also had a similar drifting problem. However, the operator knew how to compensate for the problem, and most of his parts would pass inspection. The new drifting problems were somewhat different, and the owner wanted different operators to be able to use the machine. The basic circuit for one of the motor circuits is shown.

What do you think would solve the problem?

Find the Solution

Think you know the answer to this month’s problem? Submit your solution via e-mail. All correct answers submitted by Nov. 24, 2017, will be entered into a random drawing for a $50 gift card. The winner will be notified, and his or her name will be printed in a future issue. Only one gift card will be awarded to any participant within a calendar year.

Congratulations to Doug Hite, of Charlotte, N.C., whose name was chosen from those who correctly solved last month’s challenge.

Solution to Last Month’s Challenge:

Telescopic Cylinder Drops Unexpectedly

When telescopic cylinders extend, all the moving stages should go out together. When the outer (largest) stage extends to its limit, the remaining stages should continue to extend until the next outer stage bottoms out. Only then should the final (smallest) stage extend. When retracting, the sequence occurs in the reverse order.

At the engine plant, adjusting one of the cylinder’s head nuts corrected the improper staging. The head nut on the non-moving stage holding the seal packing in place against the outer moving stage needed to be tightened to increase friction on the large outer sleeve. Another fix would be to loosen the head nut on the outer sleeve to decrease friction where it contacts the inner moving sleeve.

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