PROBLEM:  Client is a machine manufacturer with a well-established product line. The trend in the industry has been toward fully automated machines. They must also provide this class of machine at a competitive level. The portion of the machine to be automated involves large grinding/polishing wheels that remove a very thin layer of material or tarnish from steel sheets.

SOLUTION: The machine already requires PLC control, so the solution was relatively simple. Rather than utilizing expensive servo systems to control the wheel positions. We specified LVDT's (Linear Voltage Displacement Transducer) be connected directly to the adjusting arm of the grinding wheel. Standard DC gear motors were then applied to the adjusting screws. The combination of a zero backlash screw, and the LVDT, yielded a highly accurate positioning system. Driving the DC gear motor with a regenerative DC drive creates a powerful "poor man's servo system".  The PLC system simply runs a PI algorithm to position the wheel within +/- 0.001" of the required location.

BENEFITS:  This system is thousands of dollars cheaper per axis than an intelligent servo system and hundreds less than an open loop servo system. The ½ HP DC motors used are far more powerful than those that would have been used in a servo system. The additional power available from the DC motors gives the system flexibility, reliability, and speed. The DC drive itself is small and uses 1/5^th the panel space required by a servo amp/control. The positioning accuracy is well within the requirements for this type of machine.