I have talked about a number of aspects of motion control in my blogs so far, speed, inertia, torque and so forth, but generally speaking they have all been time focussed.

After a recent trip to help a client in Italy I have position, or gear based controls, top of mind. The beauty of a gear based relationship is that time is irrelevant. There is a direct relationship between input and output. The ratio between one part and another is still the same even when nothing is happening!

Historically processes would have been linked by a series of mechanical shafts and timing belts that ensured that specific actions took place in a set, exactly repeatable, sequence. This is fine as long as what is required of the process doesn’t need to be changed – for a different product or product size for instance. Another downside is that mechanical linkages force layout choices and space utilization is not ideal.

Today’s solution is to implement the ‘electrical line shaft’ to maintain fixed relationships regardless of speed or time. An encoder on the ‘shaft’ of the drive motors provides precise position information via cable to the controller. That then maintains the required gearing ratio between drive shafts. It is then a simple matter of altering controller settings to change ratios according to production demands.

Take for example a line set up to wrap 6 inch chocolate bars. There will be, say, three operations to keep in phase – a 6 inch move for the chocolate, wrapping and sealing. If the product is changed to a 10-inch bar with an electrical line shaft it is a simple matter of setting a new ratio, testing and running. In the mechanical version the line would be out for hours or more as timing chains are reset, tested and reset again until the line is performing as required.

Time may not stand still but gear ratios are gear ratios forever!

   
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