KerryP
Member
I work in an auto parts plant that has an electroplating line. This plating line has seven hoists that lift racks of parts out of tanks, move along rails, and lower the racks into the tanks. The racks are supported by copper bars that rest in v-blocks on the tanks. The twelve year old controls use a PLC5, VFDs, proximity switches on the hoists and a row of flags mounted along the rails. There are four proximity switches on each hoist – forward decelerate, at position, reverse decelerate and a home prox that acts like a zero reference for the position counter program. These proximity switches and flags are a source of significant downtime and several of our sister plants have replaced the hoist position hardware with laser rangefinders. I have project approval to upgrade to ControlLogix and purchase laser range finders.
My limited experience with motion or position control has involved projects in which the PLC sent basic commands to servo controllers that then took care of everything else. I don’t see an opportunity here to incorporate rotary encoders or servo motors because there is no hard mechanical connection between the drive components and the rail position. The hoists ride on metal wheels on a metal rail where some slippage is inherent.
Therefore it looks like I have to program a position control system that uses a linear displacement measurement and standard VFDs. The good news is that speed is a minor issue and the required position accuracy is around +/- 0.25”, pretty loose. However, I have observed the plating hoists in our other plants and I was not impressed. Lots of PID loops and they don’t work very well. They tend to overshoot or undershoot and then spend a couple of seconds hunting for the target position.
I am hoping to get some general guidance here. Any suggestions?
My limited experience with motion or position control has involved projects in which the PLC sent basic commands to servo controllers that then took care of everything else. I don’t see an opportunity here to incorporate rotary encoders or servo motors because there is no hard mechanical connection between the drive components and the rail position. The hoists ride on metal wheels on a metal rail where some slippage is inherent.
Therefore it looks like I have to program a position control system that uses a linear displacement measurement and standard VFDs. The good news is that speed is a minor issue and the required position accuracy is around +/- 0.25”, pretty loose. However, I have observed the plating hoists in our other plants and I was not impressed. Lots of PID loops and they don’t work very well. They tend to overshoot or undershoot and then spend a couple of seconds hunting for the target position.
I am hoping to get some general guidance here. Any suggestions?