No tape, no glass, no sweat

Dave Wilson saw a new type of linear servo actuator at the NDES show in Chicago

In the past, users of linear motor positioning technology have had to resign themselves to the fact that such systems used glass scales and metal or magnetic tapes as encoder feedback positioning mechanisms. The result was an expensive form of motion control. At this year’s NDES show, US-based Trilogy Systems revealed a less expensive alternative that, the company claims, will be used in numerous applications where linear motors in the past were just too expensive.

Unfortunately, glass scales and magnetic tapes are totally separate components from the linear motor. They have their own alignment, mounting, interfacing and cleanliness requirements. And, they are expensive. A typical linear encoder has from five to eight times greater cost than a rotary encoder. Alignment is often difficult too.

What is more, linear motor stages and tables are usually larger than their rotary motor driven counterparts, in part because of the linear scale. Often, it is difficult to justify the increased performance linear motors offer against the price and `risk’ of using a new technology. In the past, this has relegated linear motors to very high performance applications most commonly found in the semiconductor and electronic assembly industries where the cost could be justified. Now, with the introduction of its patented Linear Encoder Module LEM incorporated within its linear servo actuators, Trilogy hopes that all that will change.

The LEM, together with a linear motor coil, comprise the linear equivalent to the rotary motor/encoder combination. In operation, the LEM is a linear encoder that uses the same magnetic field as the linear motor to provide the position feedback. There is no scale, no tape, and no glass. The LEM itself consists of a small head that butts directly against the motor coil, and a decoder box that accepts and conditions the head signals.

The LEM requires no special or precision alignment. Each magnet track does, however, contain characteristic errors that must be compensated for the LEM to be accurate. These errors are caused by slightly varying magnet strength, small air gap width variations, and the LEM head alignment. To compensate for these errors, a characteristic magnet track error or signature is measured. A decoder board contains an EPROM memory that is calibrated or error-mapped to compensate for these characteristic errors. Each actuator is factory calibrated using a precision linear scale to achieve a similar accuracy. Actuators may also be calibrated in the field. Upon power-up, the LEM must be sent `home’ where the home sensor signals that the head is at the first pair of N-S magnets.

The total measuring length limit of the LEM is approximately 300in to 420in with a 5 micro m repeatability depending upon the type of motor used. The LEM has a peak velocity of 200in/sec.

The base and carriage material used is ground steel or ground aluminium. For the bearings, Trilogy have chosen to use high speed THK self lubricating SSR bearings.

Actuators can also be customised for special mounting holes. Base and carriage thickness can be modified to accommodate special requirements. Accessories such as bellows, spacer plates and XY transition plates are also available. In addition to single axis or XY applications, gantries can be assembled using several actuators and accessory plates.

Needless to say, Trilogy Systems believes that its LEM technology will enable it to produce smaller actuators that are more competitively priced against traditional screw and belt drive actuators.

Trilogy Systems Tel: +1 281 338 2739