Sensing for servo Repairs

Reliability and robustness were the key requirements when Wyko Industrial Services (now Eriks UK) set about building a test and calibration rig for its servomotor repair business in Aintree near Liverpool (GB). They based their design on a non-contact torque sensor that could effectively be isolated from damaging shock loads.

“We knew the rig would be in virtually constant use, so wanted to design out breakdowns as far as possible,” said Vic Harris Wyko’s Director for Electronic Developments. “Unexpected stoppages would be disruptive to us, but potentially disastrous to our customers who often run 24 hours a day and therefore need to manage their spares and repairs with military precision.”
Wyko’s clients include BAE, transmission giants Ford Getrag and several of the food processing majors, alongside many smaller companies.

Basically the rig runs the test motors against a 22kW DC spindle motor controlled by a Sprint Electric four quadrant DC drive which is switchable between ‘speed’ and ‘torque’ modes to give both back driving and load testing capabilities. Back driving is very important because it lets Wyko capture the set up data for each servo motor before it is dismantled, which in turn makes recalibration quick and simple.

Harris says he selected a TorqSense non-contact sensor, made by Sensor Technology, because it was the most advanced solution he could find and its non-contact characteristic protects it from many forms of physical damage that could result from sudden load changes.
“The torque sensor is rated at 20Nm and can take 400 per cent overload, so it can cope with just about anything it is ever likely to experience on our rig. To be extra safe we have mounted it using Fenner spider coupling on both shafts, with the rubber inserts selected to take the sting out of any unforeseen loads or sudden pressures.”

TorqSense monitors the shaft under test using a RF transmission head, which both supplies power to and picks up signals from two tiny piezo ceramic combs attached to the shaft. Rotational torque in the shaft contracts or stretches the combs, altering their RF emission signals in proportion to the torque experienced.

In fact this arrangement utilises Raleigh Waves, or Surface Effect Waves, physical phenomena identified by experimental physicist Lord Raleigh over 100 years ago when he was trying to simulate and analyse earthquakes.

“Sensor Technology also supplied us with a software suite, TorqView2, which interprets the sensor’s output signal in real time and presents it in ease-to-understand graphic form. It also logs and analyses the data, so we can print out a pass certificate for each tested motor.”