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Counterbalance Mechanism

A counterbalance mechanism (CBM) or passive gravity compensator can minimize the torque required to maintain the posture of a robot by appropriately compensating for the gravitational torque generated by both the weight of the robot and payload.
When applied to collaborative robots, the CBM enables a 1.5 to 2 times increase in payload while using the same motors.
1 DOF counterbalancing is mainly performed by installing the CBM only at joint 2, where the largest gravitational torque occurs, but the maximum gravity compensation effect can be obtained by 2 DOF counterbalancing with CBMs installed at both joints 2 and 3.

It is possible to implement a 25 kg (30 kg) payload collaborative robot with the medium-sized motors used for a 15 kg (20 kg) payload collaborative robot.
For the same payload, a low-capacity motor can be used, resulting in energy savings (30 to 35%) and relatively safer in case of malfunction. Hence, it is suitable for mobile robot arms. Hence, it is suitable for mobile robot arms.
Compatible with the existing collaborative robot controller with minor mechanical modifications.

Unlike industrial robots, which can increase payload simply by using higher-power motors, collaborative robots operating in the same space as humans cannot use high-capacity motors that require high voltage (e.g., 96 V, 200 V) for safety reasons. According to IEC62386, machines working with humans are limited to DC 60 V or less for safety. Therefore, CBMs are necessary to increase payload or reach without compromising safety or using high-voltage motors.

Link-type CBM (LCBM): Delivered in a long rod form using a coil spring. It is installed in the empty space of link 2 and must be dimensioned and mounted according to the robot’s geometry. Connected in parallel with the robot, it has no impact on robot stiffness.
Modular CBM (MCBM): Provided in the form of a module (similar to a harmonic drive). Inserted between link 2 and joint 2 requiring minimal mechanical modification. Connected in series, it slightly reduces the robot stiffness.
External CBM (ECBM): Mounted externally via clamping, requiring no modifications to the robot’s mechanical structure. Connected in parallel, it has no impact on robot stiffness.

Model	LCBM	MCBM	ECBM
Shape
Spring type	Coil spring	Gas spring	Gas spring
Mounting location	Inside link 2	Between joint 2 and link 2	Outside link2
Mounting feature	Link 2 needs to be slightly redesigned.	The mounting interface at joint 2 needs to be modified.	Mounted directly onto the existing robot
Connection	Parallel connection	Series connection	Series connection
Robot stiffness	No change	10% decrease at joint 2	No change
Compensation torque	100 – 650 Nm (1 spring), 1300 Nm (2 springs)	100-300 Nm	100-300 Nm
CBM hysteresis	Almost none	Relatively large hysteresis	Relatively large hysteresis
Control performance	Applying CBMs has no impact on the control performance