Robot Mechanisms and Mechanical Devices Illustrated - Profe Saul

Chapter 1 Motor and Motion Control Systems 35
In sinusoidal commutation, the linear encoder that provides position
feedback in the servosystem is also used to commutate the motor. A
process called “phase finding” is required when the motor is turned on,
and the motor phases are then incrementally advanced with each encoder
pulse. This produces extremely smooth motion. In HED commutation a
circuit board containing Hall-effect ICs is embedded in the coil assembly.
The HED sensors detect the polarity change in the magnet track and
switch the motor phases every 60º.
Sinusoidal commutation is more efficient than HED commutation
because the coil windings in motors designed for this commutation
method are configured to provide a sinusoidally shaped back EMF waveform.
As a result, the motors produce a constant force output when the
driving voltage on each phase matches the characteristic back EMF
waveform.
Installation of Linear Motors
In a typical linear motor application the coil assembly is attached to the
moving member of the host machine and the magnet assembly is
mounted on the nonmoving base or frame. These motors can be mounted
vertically, but if they are they typically require a counterbalance system
to prevent the load from dropping if power temporarily fails or is routinely
shut off. The counterbalance system, typically formed from pulleys
and weights, springs, or air cylinders, supports the load against the
force of gravity.
If power is lost, servo control is interrupted. Stages in motion tend to
stay in motion while those at rest tend to stay at rest. The stopping time
and distance depend on the stage’s initial velocity and system friction.
The motor’s back EMF can provide dynamic braking, and friction brakes
can be used to attenuate motion rapidly. However, positive stops and
travel limits can be built into the motion stage to prevent damage in situations
where power or feedback might be lost or the controller or servo
driver fail.
Linear servomotors are supplied to the customer in kit form for
mounting on the host machine. The host machine structure must include
bearings capable of supporting the mass of the motor parts while maintaining
the specified air gap between the assemblies and also resisting
the normal force of any residual magnetic attraction.
Linear servomotors must be used in closed loop positioning systems
because they do not include built-in means for position sensing.
Feedback is typically supplied by such sensors as linear encoders, laser
interferometers, LVDTs, or linear Inductosyns.