Gripper Support
PreciseFlex™ DDR Collaborative Robots
P/N: PFD0-DI-00010, Rev 5.0.0, April 9, 2022
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Copyright © 2022, Brooks Automation, Inc.
Note that in order to home the gripper must open all the way its maximum hard stop. The spring force at
this point is about 10N. So the motor current should not be set below about 12N/18NX1.26A or 0.8A for
the simple method of controlling gripper squeeze, giving a range of about 18N minimum to 24N maximum
squeeze for the 23N gripper.
Gripper Squeeze (Asymmetric Method)
There may be cases where 18N of squeeze is too much. In this case there is a more sophisticated
method to control squeeze.
There are two parameters in the database, 10351 and 10352 that can be used to limit the torque from the
PID loop in the positive and negative directions. These parameters were developed to limit the
downwards force of a robot running with dynamic feedforward, where the dynamic feedforward
compensates for the gravity torque of the robot. The feedforward torque is NOT limited by these
parameters, only the PID torque. So for a perfectly balanced robot, setting these parameters to a low
value for a gravity loaded axis limits the maximum force the axis can apply from any position error. So if
the axis crashes into a hard stop, the downwards or upwards force can be limited to a small value.
These same parameters can be used to limit the gripper squeeze in an asymmetric manner. Parameter
10352 can be set to a negative value of torque counts (tcnts) to limit the torque from the PID loop in the
controller in the negative direction only. Parameter 10351 can similarly be set to limit tcnts from the PID
loop in the positive direction. Since the spring compensation in the gripper is treated as a feedforward
torque, these parameters do not affect the spring compensation torque.
For this case it is more exact to know the exact number of tcnts to oppose the spring at various
openings. For the portrait mode opening of 83mm it takes 1600 tcnts to oppose the spring. For the
landscape mode opening of 123mm it takes 2200 tcnts to oppose the spring.
If the rated torque of the 23N gripper motor has been set to its maximum value of 1.26A, the formula for
setting parameter 10352 is (Spring force at position) +( (-<Contents of 10352>-<tcnts to oppose spring
force>)/4378)X18N, where 4378 is the number of tcnts corresponding to 1.26A or the rated torque of the
motor. For example, for portrait mode the spring force is about 6N, and if the contents of 10352 are -
3200, this value will be 6N + (3200-1600)/4378)X18N or about 12.5N. If the value of 10352 is -1600, the
squeeze will be 6N which is the spring force only.
In a similar manner parameter 10351 can be used to limit the gripper opening force. In this case the
value for the opening force is ( (<Contents of 10351>-<tcnts to oppose spring force>)/4378)X18N –
(Spring force at position). For example, in landscape mode the spring force is about 8N, and if the
contents of 10351 are 5200, this value will be (5200-2200)/4378X18N -8N or 4.3N. Note that 5200 is
about as low a value as you would want to use in landscape mode for parameter 10351, to ensure there
is enough force to oppose the spring and open the gripper all the way to the homing position. For many
cases, 10351 can be left at its default value of 0, in which case it is disabled.
End of Travel Sensor
The Precise 23N EGripper includes a sensor to detect when the gripper is closed to hard stop position.
The spring will return the gripper to this position if power is off and there is no plate in the gripper. This
sensor is wired to Digital Input 2 on the Gripper Controller Board which can be read at Digital Input
