Blog: Direct Drive Linear and Coordinate Stepping motors

The development of the Sawyer linear motors, both single axis and xy type, is briefly presented.
The theory of operation is presented for the benefit of those who may be
unfamiliar with these devices. This article presents the basic operating parameters and the characteristics of the devices.
The control and drive technology for these motors is discussed. Since the motor actually consists of two distinct parts,
the motor (or forcer) and the platen (or stationary part), the motor system is critically dependent on the platen design.
The article discusses the design and characteristics of several types of platens. The critical system parameters which control the application of these motors are discussed.
 
In the late 1960's a linear stepper motor, known as a Sawyer motor, was developed. Both a single axis and an xy version of these motors were produced.
By utilizing micro-stepping, high resolution is achieved; by the use of accelerometer feedback and sophisticated drive electronics, excellent transient response was achieved.
At the same time special microcomputer software and systems and specialized digital output hardware were developed which allowed these motors to be utilized on very
high performance plotters. A small family of these motors and drive electronics was developed and the motors were applied to a number of proprietary products such as plotters, wire harness building machines, and wafer probers and steppers.

For a number of years there was no serious effort to exploit the general potential of these motors as system components beyond the applications noted above.
One exception was the application of some fairly large motors to laser fabric cutters.
The Sawyer motor comes in a variety of sizes and characteristics, and by selection of the drive electronics it is possible to utilize a given motor in a variety of niches in a cost performance trade-off matrix.
As an example, a given motor may be capable of linear forces of twenty pounds at zero velocity and ten pounds when moving at 30"/sec.,
while producing final positioning accuracies of .005" and repeatability of .0005", with a minimum cost electronic driving system. The same motor driven with more elaborate drive electronics might achieve a
ten pound force at 60"/sec., or with a still different set of drive electronics it might produce an absolute accuracy of .0005" and a repeatability of .0002".
The unique characteristics of these motors require an understanding of how to optimize the systems because at the present time the motors are
in the top end of the price range of linear positioning systems and, in the easier applications better known techniques such as rotary motors and lead screws
may have the edge where cost is concerned. But where one finds an application which requires their unique combinations of advantages or
where one requires extreme reliability and life expectancy, the Sawyer motors rapidly become competitive.
The motors are particularly well-suited to large systems which require high motion speed while maintaining good positioning accuracy.
It is quite easy to position several motors on a single axis and in one version of the motor (the two-axis motor), motors and their loads pass behind each other so that second motors can actually serve the same points in the positioner space. Indeed, in many applications there isn't anything else like them.
The motors are based on the so-called Sawyer principle. The name comes from the inventor named on the patents.