Page 105 - Servo Motors and Industrial Control Theory -

98 5 Stepping Servo Motors

Fig. 5.7 Typical speed-
torque characteristic of step-
ping motors
Pull-out torque vs.
switching rate
Torque Response
range
Slew
range

Pull-in torque vs.
switching rate
Maximum Maximum
pull-in rate pull-out rate

encoder and the information about the output position is fed back to the controller.
The required position is compared with output position and with respect to the error
the switching rate is applied. In this mode, the loss of steps is avoided. This is shown
in block diagram form in Fig. 5.8.
Although the input and output positions are in digital form, they can be written
in analogue form. The controller output may be written as

V: K(θθ− ) (5.13)
i
o
V is the output voltage which determines the switching rates. K is the gain and θ ,
i
θ are the demand and actual output position. When the stepping motor is energized
o
at a specific switching rate then the voltage equation assuming with inductance L
may be written as
V: =RI+LsI (5.14)

It should be noted that for stepping motors there is no internal velocity feedback.
This means that the damping ratio in stepping motors are less than that of DC servo

External
Torque

output position
+ controller stepping motor Load
Input Pulses
-
position encoder

Fig. 5.8 Stepping motors in closed loop position control