Page 105 - Electric Drives and Electromechanical Systems

98 Electric Drives and Electromechanical Systems

A linear motor has significant advantages over ball and leadscrew drives due to the
removal of errors causes by wear on the nut and screw and friction, which are
common when the drive has a high duty cycle. Even with the use of a high perfor-
mance ballscrew the wear may become significant for certain application over time
due to decreasing accuracy and repeatability and certain process (e.g., pharmaceu-
ticals and optoelectronics) may be compromised from dust as a result of wear.
As the length of a leadscrew or ballscrew is increased, so its maximum operating
speed is limited, due to the flexibility of the shaft leading to vibration, particularly
if a resonant frequency is encountered - this is magnified as the length of the shaft
increases as given by Eq. (3.40). While the speed of the shaft can be decreased, by
increasing the pitch, the system’s resolution is compromised.
While the linear motor does provide a suitable solution for many applications, it is not
inherently suitable for vertical operation, largely due to the problems associated with
providing a fail-safe brake. In addition, it is more difficult to seal against environmental
problems compared with a rotary system, leading to restrictions when the environment
is particularly hostile, for example when there is excessive abrasive dust or liquid
present. Even with these issues, linear motors are widely used is many applications,
including high speed robotics and other high-performance positioning systems.

3.8 Review of motor-drive sizing

This chapter has so far discussed the power transmission elements of a drive system,
while Chapter 2, Analysing a drive system, looked at issues related to the determination
of a drive’s requirements. This concluding section provides an overview of how this
information is brought together, and the size of the motor and its associated drive are
identified. The objective of the sizing procedure is to determine the required output
speed and torque of the motor and hence to allow a required system to be selected. The
process is normally started once the mechanical transmission system has been fully
identified and quantified.
The main constraints that have to be considered during the sizing procedure when a
conventional motor is being used can be summarised as follows:

The peak torque required by the application must be less than both the peak stall
torque of the motor and the motor’s peak torque using the selected drive.
Does the drive provide enough torque at standstill to allow the system to be star-
ted? This important check is required as the starting friction may be considerable
higher that the running friction, particularly when retrofitting machine tools and
mobile robotic applications.
The root-mean-square (r.m.s.) torque required by the application must be less than
both the continuous torque rating of the motor and the continuous torque which
can be delivered by the motor with the specified drive system.