Electric Gripper Stall Detection Using Back-EMF Compliance
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Solution Overview
Problem
Existing electric gripper systems are costly and complex, requiring sophisticated controllers to operate, which is not cost-competitive with pneumatic grippers and lacks simplicity in controlling stepper motors for unpredictable loads and positions.
Innovation Solution
A compliant stepper motor drive system with a stall-detecting controller and compliance device that uses back electromagnetic field voltage monitoring to generate predictable force without encoders or sensors, allowing the stepper motor to operate at varying torque and speed, and power down to maintain grip mechanically.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a complex controller is used to operate the electric gripper, then the gripping force can be maintained reliably, but the cost and complexity of the system increases
Solution Approach 1:
The system uses the stepper motor's own back-EMF signal to detect compression and determine when gripping force should be applied. The compliance mechanism itself provides the feedback needed for control, eliminating the need for external sensors or complex controllers. The motor's electrical characteristics are used to sense mechanical compression, allowing the system to self-regulate.
Solution Approach 2:
The patent replaces complex mechanical control systems with an electrical sensing approach. Instead of using mechanical switches, position sensors, or complex feedback mechanisms, the system uses the electrical back-EMF of the stepper motor to detect mechanical compression and trigger the gripping force application.
2Reliability
If the actuator is continually energized to maintain grip force, then the part is securely held, but energy consumption increases
Solution Approach 1:
The system applies high torque only periodically when needed - specifically when compression is detected and during the gripping phase. Once the grip is established, the motor is de-energized and the compliance mechanism maintains the force passively. This periodic application of energy replaces continuous energization.
Solution Approach 2:
The compliance mechanism serves as a passive force maintenance element after the motor applies the initial gripping force. The mechanical compliance element (spring or elastomer) stores and maintains the gripping force without requiring continuous electrical energy input from the motor.
3Speed
If the stepper motor operates at high speed, then the gripper closes quickly, but the torque available for gripping is reduced
Solution Approach 1:
The system dynamically changes the motor's operating characteristics based on the compression detection. During the approach phase, the motor operates at high speed with low torque. When compression is detected, the system automatically transitions to high torque mode by applying additional voltage pulses, allowing the motor to deliver maximum force when needed.
Solution Approach 2:
The control system changes the electrical parameters applied to the motor based on compression detection. When the back-EMF indicates compression, the controller increases the voltage and current to the motor windings, changing the motor's operational state from high-speed/low-torque to low-speed/high-torque mode.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
Enables a cost-effective, simple, and energy-efficient electric gripper system that can grip objects with consistent force without continuous power, reducing energy consumption and temperature buildup.
Implementation Method 1
The compliance device enables movement of the motor with respect to the body. The deflection of the compliance member develops a force on the part according to its modulus of elasticity.
Implementation Method 2
The controller may detect when the stepper motor is approaching its stall torque without using an encoder or sensor by monitoring the back electromagnetic field voltage (EMF) of the motor.
Implementation Method 3
The controller can provide further braking by shorting the motor coils, thus creating a back-EMF brake.
Data Source
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AI summary
A gripping device has a body with a stepper motor positioned within the body. A lead screw is driven by the stepper motor. A gripper is coupled with the lead screw for gripping and releasing a part. A compliance enables movement of the motor with respect to the body. A controller is electrically coupled with the motor to provide motion control and maintain a set of learned positions based on sensing when the motor is approaching its stall torque.