6. Software Reference (Undefined variable: MyVariables.ProductName)
Controlling the PreciseFlex Servo Grip-
pers
Part Number: PF40-DI-00010 Rev. A
For the PF400 23 N gripper the formula for determining the approximate gripper squeeze is 7 N +(
Rated Current/1.26 Amps)X18 N for squeeze and (Rated Current/1.26Amps)X18N –9 N for gripper
opening force.
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 10 N. So the motor current should not be set below about 12 N/18 NX1.26
A or 0.8 A for the simple method of controlling gripper squeeze, giving a range of about 18 N
minimum to 24 N maximum squeeze for the 23 N gripper.
Gripper Squeeze (Asymmetric Method)
There may be cases where 18 N 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. 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. 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 83 mm it takes 1600 tcnts to oppose the spring. For the
landscape mode opening of 123 mm it takes 2200 tcnts to oppose the spring.
If the rated torque of the motor has been set to its maximum value of 1.26 A, the formula for setting
parameter 10352 is (Spring force at position) +((-<Contents of 10352>-<tcnts to oppose spring
force>)/4378)X18 N, where 4378 is the number of tcnts corresponding to 1.26 A or the rated torque
of the motor. For example, for portrait mode the spring force is about 6 N, and if the contents of
10352 are -3200, this value will be 6 N + (3200-1600)/4378)X18 N or about 12.5 N. If the value of
10352 is -1600, the squeeze will be 6 N 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)X18 N –
(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)/4378X18 N -8 N or 4.3 N. Note that 5200
is about as low a value as anyone 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.
85
Copyright © 2023, Brooks Automation
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