Cordless Power Tool Dynamic Motor Control Circuit
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Solution Overview
Problem
Cordless power tools face challenges in accurately measuring and controlling torque due to low voltage, high current requirements, signal interference, and thermal management, particularly in precision applications where robust and reliable dynamic motor control is necessary.
Innovation Solution
A cordless power tool design featuring a four-wire Kelvin resistor for current measurement, a dynamic motor control circuit with high gain differential current amplifiers, and a closed-loop control system that includes a torque transducer and digital hall switches to provide precise torque control and motor position data, along with a power circuit board with integral heat sinks and optimized cooling to manage heat and interference.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If high current is used to provide sufficient power in low voltage cordless tools, then power output is improved, but signal interference and measurement precision deteriorate
Solution Approach 1:
The current measurement function is segmented from the high current path using a four-wire Kelvin resistor configuration. Two wires carry the high current while the other two wires separately measure the voltage drop, isolating the measurement circuit from the high current switching noise and enabling precise torque measurement despite high power operation.
Solution Approach 2:
A dedicated torque transducer serves as an intermediary device between the motor output and the mechanical load. This transducer directly measures torque without being affected by electrical interference from the high current motor operation, providing accurate torque data for closed-loop control.
2Extent of automation
If high current switching is implemented for dynamic motor control, then motor control capability is improved, but thermal management and circuit reliability worsen
Solution Approach 1:
The power circuit board incorporates localized thermal management features including integral heat sinks positioned directly at the switching device locations. This localized approach efficiently dissipates heat generated by high current switching while maintaining compact form factor and preventing thermal damage to surrounding circuitry.
Solution Approach 2:
The circuit design includes protective measures such as snubber circuits and proper grounding schemes implemented beforehand to cushion against voltage spikes and electromagnetic interference generated by high current switching. These protective elements prevent circuit damage before it can occur during dynamic motor control operation.
3Ease of operation
If compact housing is used for handheld tool, then ease of operation is improved, but thermal management and signal interference worsen
Solution Approach 1:
The power circuit board integrates multiple functions including motor control switching devices, heat dissipation structures, and signal processing circuits into a single compact unit. The integral heat sinks are directly formed on the circuit board, merging thermal management functionality with the electrical circuit structure, enabling efficient heat dissipation in a compact handheld form factor.
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
The solution enables precise torque control with a resolution of 0.1 A across a wide current range, reducing signal interference and thermal issues, thus enhancing the reliability and precision of cordless power tools for fastening applications.
Implementation Method 1
a four wire Kelvin resistor for reliable current measurement
Implementation Method 2
a torque transducer on board the tool in communication with the output shaft
Implementation Method 3
digital hall switches in communication with the motor for measuring motor speed
Implementation Method 4
at least one high gain differential current amplifier
Implementation Method 5
a plurality of spaced apart integral heat sinks board residing on both primary surfaces of the power circuit board
Implementation Method 6
optimized cooling to manage heat
Data Source
AI summary
Cordless power tools include a pistol housing having an upper portion that merges into a downwardly extending handle, a DC motor residing in the upper portion of the housing, the DC motor having a rotor that drives an output shaft; a torque transducer on board the tool in communication with the output shaft; and a dynamic motor control circuit residing in the housing in communication with the motor and torque transducer. The dynamic motor control circuit includes a Kelvin resistor in communication with the motor for measuring motor current and digital hall switches in communication with the motor for measuring motor speed. The motor current can vary by at least 100 A during operation.