Stepper Motor Stall Detection Using Back-EMF for Grippers
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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 unknown 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 (EMF) monitoring to generate predictable force without sensors, allowing the stepper motor to adapt torque and speed for gripping and releasing objects, and powers down to save energy.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If electric gripper systems use sophisticated controllers to operate, then the gripping control capability is improved, but the device complexity and cost increase
Solution Approach 1:
The system uses the stepper motor's own back-EMF signal for stall detection and gripping force control, eliminating the need for external sensors and complex controllers. The motor's electrical characteristics are exploited to provide self-diagnostic capability, making the system simpler and more cost-effective while maintaining operational capability
Solution Approach 2:
The patent replaces mechanical sensing systems (such as force sensors or position sensors) with electrical sensing based on back-EMF monitoring. This substitution reduces hardware complexity while achieving the same control objectives through electrical field measurements rather than mechanical ones
2Reliability
If the stepper motor operates continuously to maintain grip force, then the gripping reliability is improved, but the energy consumption increases
Solution Approach 1:
Instead of continuous operation, the system uses periodic pulse signals to the stepper motor to maintain gripping force. The motor is activated only when needed to counteract gravitational force, reducing energy consumption while maintaining reliability through intermittent control actions based on back-EMF feedback
3Productivity
If the stepper motor operates at high speed during gripping, then the productivity is improved, but the torque output decreases
Solution Approach 1:
The system dynamically adjusts the stepper motor's operating parameters based on the gripping phase. During the approach phase, high speed is used for efficiency, while during the gripping phase, speed is reduced and torque is increased to ensure reliable contact and force application, optimizing both productivity and force requirements
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 predictable force at unknown locations, maintaining grip without continuous power and providing a reliable braking mechanism.
Implementation Method 1
The compliance device (16) deflects and, like a spring develops a force proportional to its deflection. The deflection force is transmitted to the gripper (20) generating a gripping force on the part (22).
Implementation Method 2
The controller (70) monitors the back electromagnetic field voltage (back EMF) of the motor coils. This back EMF may be sensed through current shunts or another suitable type of back EMF sensor.
Implementation Method 3
The controller can provide further braking by shorting the motor coils, thus creating a back-EMF brake.
Data Source
Figure 1~2
Figure 3~4
Figure 5~7
AI summary
A stepper motor control system includes: a stepper motor configured to actuate an actuator to engage an object; a power control module configured to: during a first period before compression of a compliance mechanism occurs, apply electrical pulses to stator coils of the stepper motor in a predetermined order and based on a first predetermined torque and a first predetermined speed, where compression of the compliance mechanism occurs when the actuator contacts the object; and during a second period after the compression of the compliance mechanism occurs, apply a predetermined number of extra pulses to the stator coils of the stepper motor in the predetermined order and based on a second predetermined torque and a second predetermined speed.