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selected to allow enough motor torque to overcome the spring and still provide reasonable opening force
for inside grips.
The motor for the 23N gripper can apply about 18N of force at its rated current of 1.26A. When closing
the fingers the motor adds its force to the spring force, so a maximum closing force of about 24-26N is
possible, depending on portrait or landscape gripping. When opening, the motor must oppose the spring
force, so a maximum opening force of about 8- 12N is possible, depending on the opening of the fingers.
The motor squeeze force can be limited by modifying the rated current of the motor. This can be done by
writing into the 5
th
field in Parameter Data Base # 10611. The motor current can be set once and saved
into flash or modified dynamically by a GPL program using the Controller.PDbNum instruction.
For the 23N gripper the formula for determining the approximate gripper squeeze is 7N +( Rated
Current/1.26Amps)X18N for squeeze and ( Rated Current/1.26Amps)X18N –9N for gripper opening force.
For the 60N gripper the formula for determining the approximate gripper squeeze is 7N +( Rated
Current/2.0Amps)X53N for squeeze and ( Rated Current/2.0Amps)X53N –9N 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 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 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.
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