Bringing stations back into step

Located in a large automotive stamping plant is a six station transfer press with fully integrated transfer robots between stations. The presses are all clutch/brake type and fitted with TR Electronic programmable encoders. These are configured with parallel, 24V DC push pull drivers and programmed to 360 steps per revolution by one revolution, with Gray code output. The encoders are coupled so that one revolution of the encoder’s shaft is equivalent to one complete down and up cycle of the press. The parallel output signal from the encoders is split and goes to two separate controllers: one for the main press control and the other for the transfer robot control. Unfortunately, both controllers were reporting erroneous positions, which occurred at the top of the stroke where the encoder output changed from 359 to 0, and all six presses were exhibiting the exact same symptoms. Every item was repeatedly checked and nothing was found out of order. Eventually it was discovered that the problem was with the bit transitions from steps 359 to 0.

If the PLC read the data lines before all the bits had zeroed, it interpreted that as a different number, and shut down the press thinking it had jumped backwards. Then the press had to be reset virtually every cycle.

A latching technique was used overcome the problem. The solution used the qualities of Gray code, whose basic premise is that only one bit can change state from one stop to the next. To have 360 steps with Gray code, a system called Shifted-Gray code is used. This entails taking the number of steps that are wanted and shifting them within the code table to a place where the transition from highest count to lowest has only one bit changing. This is done by taking the next highest 2n number, and centre the 360 steps in the middle of the 512 steps. In this case it will shift the counts by 76. When the encoder is at 0, it will actually give a value of 76 and when it is at 359 the value will be 435.

The encoder used is the fully programmable CE-65 series. All that needed to be done to implement the solution was to select Shifted-Gray as the output. The internal microprocessor shifted the output by 76 steps.