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DMC-1700/1800 Chapter 2 Getting Started • 33
this voltage may need to be increased and for systems with very small motors, this value
should be decreased.
For example,
BZ -2
will drive the X axis to zero, using a 2V signal. The controller will then leave the motor
enabled. For systems that have external forces working against the motor, such as
gravity, the BZ argument must provide a torque 10x the external force. If the torque is
not sufficient, the commutation zero may not be accurate.
If Hall Sensors are Available:
The estimated value of the commutation phase is good to within 30°. This estimate can
be used to drive the motor but a more accurate estimate is needed for efficient motor
operation. There are 3 possible methods for commutation phase initialization:
Method 1. Use the BZ command as described above.
Method 2. Drive the motor close to commutation phase of zero and then use BZ command.
This method decreases the amount of system jerk by moving the motor close to zero
commutation phase before executing the BZ command. The controller makes an estimate
for the number of encoder counts between the current position and the position of zero
commutation phase. This value is stored in the operand _BZx. Using this operand the
controller can be commanded to move the motor. The BZ command is then issued as
described above. For example, to initialize the X axis motor upon power or reset, the
following commands may be given:
SHX ;Enable X axis motor
PRX=-1*(_BZX) ;Move X motor close to zero commutation phase
BGX ;Begin motion on X axis
AMX ;Wait for motion to complete on X axis
BZX=-1 ;Drive motor to commutation phase zero and leave
;motor on
Method 3. Use the command, BC. This command uses the hall transitions to determine the
commutation phase. Ideally, the hall sensor transitions will be separated by exactly 60°
and any deviation from 60° will affect the accuracy of this method. If the hall sensors are
accurate, this method is recommended. The BC command monitors the hall sensors
during a move and monitors the Hall sensors for a transition point. When that occurs, the
controller computes the commutation phase and sets it. For example, to initialize the X
axis motor upon power or reset, the following commands may be given:
SHX ;Enable X axis motor
BCX ;Enable the brushless calibration command
PRX=50000 ;Command a relative position movement on X axis
BGX ;Begin motion on X axis. When the hall sensors
detect a phase transition, the commutation phase is re-set.
Step 8C. Connect Step Motors
In Stepper Motor operation, the pulse output signal has a 50% duty cycle. Step motors operate open loop and do not
require encoder feedback. When a stepper is used, the auxiliary encoder for the corresponding axis is unavailable