5. Software Reference PreciseFlex 100 Robot
Controlling the PreciseFlex Servo Gripper Part Number: 603988 Rev. A
Gripper Squeeze (Simple Method)
The spring applies a closing force of approximately 7 Newtons at a finger opening of 103 mm, which
is halfway between a portrait titer plate grip at 83 mm and a landscape titer plate grip at 123 mm.
The force is closer to 6 N in portrait mode and 8 N in landscape mode and 9-10N at the full open
homing position. These closing forces appear adequate to prevent dropping titer plates weighing up
to 200 gms and are selected to allow enough motor torque to overcome the spring and still provide
reasonable opening force for inside grips.
The motor can apply about 18 N of force at its rated current of 1.26 A. When closing the fingers, the
motor adds its force to the spring force, so a maximum closing force of about 24-26 N 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- 12 N 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.
The formula for determining the approximate gripper squeeze is 7 N +(Rated Current/1.26 Amps)
X18 N for squeeze and (Rated Current/1.26 Amps) X18 N –9 N for gripper opening force.
NOTE: 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. Therefore, 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.
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.
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Copyright © 2023 Brooks Automation, Inc.
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