Screwdriver Torque Control via Motor Current Limit and Kinetic Braking
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Solution Overview
Problem
Existing screwdrivers face challenges in achieving high repeat accuracy for a predetermined tightening torque, leading to inaccuracies in screwing applications, particularly with small screws or sensitive workpieces, due to the need for extensive monitoring of multiple parameters, which increases computing effort and can result in either reduced accuracy or prolonged screwing time.
Innovation Solution
A screwdriver with a control device that calculates a motor current limit based on the predetermined torque, switching to a waiting state when exceeded, and initiates braking based on kinetic energy, with the motor energized only partially or not at all during this phase, to ensure precise torque control and efficient screwing.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If multiple parameters are constantly monitored to achieve high repeat accuracy for tightening torque, then manufacturing precision is improved, but device complexity and computing effort increase
Solution Approach 1:
The patent extracts and monitors only the most critical parameter - motor current - rather than continuously monitoring multiple parameters. The control device calculates a limit value for motor current based on the predetermined tightening torque and compares the actual motor current against this limit. This extraction of the essential parameter reduces computing effort while maintaining high repeat accuracy for tightening torque.
Solution Approach 2:
The control device performs preliminary calculation of the motor current limit value based on the predetermined tightening torque before the screwing operation begins. This pre-calculated limit value is then used during the operation, eliminating the need for complex real-time calculations and reducing computing effort while ensuring accurate torque control.
2Productivity
If the drive is braked actively using a rotating field in the opposite direction, then productivity is improved by reducing screwing time, but manufacturing precision deteriorates due to potential damage from excessive braking
Solution Approach 1:
The patent employs feedback control by continuously monitoring the motor current and comparing it against the pre-calculated limit value. When the motor current exceeds the limit, the control device initiates braking. This feedback mechanism ensures that braking is applied only when necessary and at the appropriate moment, preventing excessive braking that could damage the workpiece while still achieving quick cessation of the screwing operation for high productivity.
Solution Approach 2:
The control device dynamically changes the motor's operational state by switching from energized to braked state based on the motor current parameter. When the motor current exceeds the calculated limit value, the control device activates the braking mechanism, creating a dynamic response that maintains precision while improving productivity through rapid operation completion.
3Manufacturing precision
If the motor remains fully energized during the waiting state, then manufacturing precision is maintained through continuous torque generation, but use of energy increases
Solution Approach 1:
The patent implements periodic action by controlling the motor to be energized only during specific phases of the screwing operation - specifically during the acceleration phase and the braking phase - rather than maintaining continuous energization. The motor is energized to generate torque during acceleration, then de-energized during the waiting state when the screw is being tightened, and re-energized only when braking is required. This periodic energization maintains torque control accuracy when needed while significantly reducing overall energy consumption.
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
This approach achieves high repeat accuracy for multiple screw connections by dividing the tightening torque into components generated before, during, and after the waiting state, allowing for precise control and reduced torque deviation, thus improving the reliability of screwing operations.
Implementation Method 1
an electric motor (14) that is coupled to a drive shaft (20) for driving a tool (22) for tightening a screw connection
Implementation Method 2
actuates a braking device for actively braking the drive motor (14) by means of a rotating field in the opposite direction to the respective direction of rotation of the drive motor
Data Source
Figure 1~2
Figure 3
Figure 4~5
AI summary
A screwdriver is described with a drive (12) comprising an electric motor (14) coupled to a drive shaft (20) for driving a tool (22) for tightening a screw connection with a predetermined tightening torque, a current sensor (38) for detecting a motor current (IM), and a control unit (36) for controlling the drive (12), configured to decelerate the drive upon reaching a braking criterion. The control unit (36) is configured to determine the switch-off criterion based on the predetermined tightening torque for calculating a limit value for the motor current, upon exceeding which the drive (12) is placed in a standby state. The control unit (36) is also configured to determine the braking criterion based on the predetermined tightening torque for calculating an activation time or angle of rotation, upon exceeding which the braking process is initiated.at least in dependence on the kinetic energy of the drive at the time the limit value for the motor current is exceeded, wherein the electric motor (14) continues to be energized during the standby state until the braking process is initiated, is not energized at all or is only partially energized (Fig. 1).