When considering motion control systems for point-to-point positioning,
many engineers automatically think of stepper motors. In doing so, says
Darren Briggs, they are missing out on the important benefits offered by
modern, easy-to-use digital servo drives
In designing point-to-point positioning systems, engineers have a choice
of using stepper motors or servo drives. Although both alternatives, at
least on the face of it, can achieve similar results, they do it in very
different ways. Before discussing the benefits and limitations of each
approach, it is, therefore, useful to compare the basics of stepper and
servo operation.
Stepper motors, as their name suggests, move in discrete angular steps,
the size of which is determined by the physical construction of the
motor. Stepper motor drive circuitry usually accepts a pulse input, with
the motor moving by one step for each pulse received. More sophisticated
arrangements are possible but the fundamental operation remains the same
- the input signal tells the stepper motor to perform a number of steps
corresponding to the required motion and, if all is well, the motor
carries out the command.
This all sounds very sure and simple, but there is one very important
proviso - stepper motor control is essentially an open-loop process.
There is an assumption that if the stepper drive circuitry tells the
motor to perform a certain number of steps, the correct number of steps
will be executed, even if, for example, the mechanical load on the motor
or the operating speed of the machine is altered. In practice, this
assumption is not always justified.
Servomotors, unlike stepper motors, provide continuous rotary motion -
their design is not based on a stepping pattern. For use in position
control applications, a servomotor must, therefore, be provided with a
position feedback device, which can sense the angular position of the
shaft. Typically, this is an optical encoder contained within the
servomotor body.
The basic mode of operation is again simple - the servo drive receives
information about the required motion, and then controls the motor to
achieve the required position, as indicated by the encoder. The system
then holds this position until the next move is initiated. This
closed-loop mode of operation has an obvious advantage, in that the drive
always knows whether the required position has been reached, and can
automatically take corrective action if it has not. In practice, this
means that the accuracy of a servo-based positioning system is less
likely to be affected by fluctuating motor load than a similar system
which uses a stepper motor. This is important in modern manufacturing
applications where a single machine is often required to handle many
different products, each of which may impose a different mechanical load.
Their closed-loop operation also allows servo drive systems to offer
consistent performance over a wide speed range. This is an important
advantage, not only when the machine needs to run at different speeds to
handle different products, but also when the machine is required to run
at various speeds during commissioning. These are decisive benefits, but
servo drives have even more to offer. Today, it is very easy to produce
encoder technology offering very high resolution and, therefore, precise
control, based on mechanical designs for the motor, which have been tried
and tested over many years of successful usage.
Servo drives are also capable of producing substantially higher peak
torques than stepper motors. This again means that numerous positioning
operations can be optimised on the same machine, and valuable time can be
shaved from machine operating cycles. In today's market conditions, just
a few milliseconds saved in applying a label or sealing a pouch
translates into a significant competitive advantage for the end user.
So, if