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MPS
/M
OTOR
/E
NCODER
4-19
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Closed-Loop Step Motors
PCI controllers can control step motors with encoder feedback. Closed-loop steps are con-
trolled by a PID algorithm running on the DSP in real time. The controller’s accept TTL-level
(0V to 5V, 40mA max) encoder input from either differential or single-ended encoders. Dif-
ferential encoders are preferred due to their excellent noise immunity.
Encoder signals are read in quadrature. Every line on the encoder produce a rising edge and a
falling edge on channels A+ and B+ which is interpreted by the PCI controller as 4 encoder
counts.
Connecting closed-loop step motors to the controller is similar to servo motors, except that the
step and direction lines are connected instead of the analog signal. The minimum connections
are Step+ (or Step-), Di (or Direction-), Signal Ground, Encoder A+ and B+ lines, and
+5V.
Note that when only
Step+
or
Step-
is used, it may be necessary to
jumper unused terminals on
the step drive.
Before connecting
Step+
or
Step-,
consult your step drive’s manual.
In general, use
Step+
for drives with active high logic, and use
Step-
for drives with active low
logic. Both
Step+
and
Step-
lines can be connected to drives with differential inputs. If in
doubt, fax the drive’s pinouts to
Motion Engineering
along with any questions.
Figure 4-16
Typical Closed-Loop Step Motor Connections (PCI)
Warning!
For the best performance, ensure that the ratio between the
encoder resolution
(counts
per revolution) and the
step resolution
(steps per microsteps per revolution) is 1:4.
Lower ratios (1:1, 1:2) will be difficult to tune and will have poor static stability.
Higher ratios (1:6, 1:8) will have poor constant velocity stability.
Note!
For drives that trigger on the rising edge of the pulse input, use
Step+
.
For drives that trigger on the falling edge of the pulse input, use
Step-
.
GND
+5 volts
Encoder A+
Encoder A-
Encoder B+
Encoder B-
Encoder Index+
Step +
GND
Step
Drive
Motor
Encoder
Dir
3
16
15
41
3
4
39
38
5
6
Encoder Index- 40
T
O
C
LOSED
-
LOOP
S
TEP
M
OTOR
PCI
STC-136
Axis 0
Axis 0
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