PERFORMANCE
27
MICROSTEP ACCURACY
PERFORMANCE
MOTOR TOLERANCES
Most step motors are specified as having a
±5% non-accumulative step tolerance. This
implies that a 200 step per revolution motor will have an absolute accuracy of 1 part out
of 2000.
If the motor is run open-loop (as most step motors are) the user cannot expect to
position a motor accurately at anything greater than a 10 microstep resolution.
Consequently higher step resolutions, in open-loop applications, only contribute to
motor smoothness.
MOTOR LINEARITY
For every motor there is a function that relates the angle of rotation to the electrical
angle of the winding currents. If it were directly proportional, then sine-cosine varying
currents would cause a uniform rate of rotation, resulting in uniformly spaced
microsteps.
For most motors this function resembles
Figure 9. The motor current profile must
be distorted from the sine-cosine profile
to compensate for non-linearity. The
effectiveness of this will determine the
microstep position accuracy.
The electrical to mechanical angle
function is dependent on motor current.
By varying the value of the current set
resistor, it may be possible to trim out
residual positional error. Should this be
inadequate, Centent can generate a
compensation profile for motors of a like
model number or type. This custom
profile is 'programmed' into the
customer's CN0162. See CURRENT
PROFILE OPTION (page 25) for details.
0
ELEC
T
R
IC
A
L
AN
G
L
E
-9
0°
+9
0°
-1.8°
+1.8°
MECHANICAL ANGLE
Figure 9: Motor linearity
