HCTL-1100 Datasheet, PDF (28/40 Pages) Agilent(Hewlett-Packard) – General Purpose Motion Control ICs

Commutator Constraints and Use
When choosing a three-channel
encoder to use with a DC brush-
less or stepper motor, the user
should keep in mind that the
number of quadrature encoder
counts (4x the number of slots in
the encoder’s codewheel) must be
an integer multiple (1x, 2x, 3x,
4x, 5x, etc.) of the number of
pole pairs in the DC brushless
motor or steps in a stepper
motor. To take full advantage of
the commutator’s overlap feature,
the number of quadrature counts
should be at least 3 times the
number of pole pairs in the DC
brushless motor or steps in the
stepper motor. For example, a
1.8°, (200 step/revolution)
stepper motor should employ at
least a 150 slot codewheel = 600
quadrature counts/revolution = 3
x 200 steps/revolution).
There are several numerical
constraints the user should be
aware of to use the Commutator.
The parameters of Ring, X, Y, and
Max Advance must be positive
numbers (00H to 7FH).
Additionally, the following
equation must be satisfied:
(-128D) 80H ≤ 3/2 Ring
+ Offset ± Max Advance
≤ 7FH (127D)
[8]
In order to utilize the greatest
flexibility of the Commutator, it
must be realized that the
Commutator works on a circular
ring counter principle, whose
range is defined by the Ring
register (R18H). This means that
for a ring of 96 counts and a
needed offset of 10 counts,
numerically the Offset register
can be programmed as 0AH
(10D) or AAH (-86D), the latter
satisfying Equation 8.
If bit #2 in the Status register is
set to allow the commutator to
count in full counts, a higher
resolution codewheel may be
chosen for precise motor control
without violating the commutator
constraints equation (Equation 8).
Example: Suppose you want to
commutate a 3-phase 15 deg/step
Variable Reluctance Motor
attached to a 192 count encoder.
1. Select 3-phase and quadrature
mode for commutator by
writing 0 to R07H.
2. With a 3-phase 15 degree/step
Variable Reluctance motor the
torque cycle repeats every 45
degrees or 8 times/revolution.
3. Ring register
(4)(192) counts/revolution
= ––––––––––––––––––––––
8/revolution
= 96 quadrature counts
= 1 commutation cycle
4. By measuring the motor torque
curve in both directions, it is
determined that an offset of 3
mechanical degrees, and a
phase overlap of 2 mechanical
degrees is needed.
(4) (192)
Offset = 3°––––––––
360°
≅ 6 quadrature counts
Figure 11. Phase Advance vs. Motor Velocity.
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