Microcontroller Pin Multiplexing for FET Current and Temperature Sensing
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Solution Overview
Problem
Cordless power tool trigger switches face challenges in efficiently integrating a microcontroller with limited pin count to monitor FET current, FET temperature, and battery voltage while maintaining compact size and cost-effectiveness.
Innovation Solution
A control system utilizing an 8-pin microcontroller with multiplexing to combine FET current and temperature sensing into one analog I/O pin, employing a thermistor and capacitor setup, and using a differential gain stage to accurately measure these parameters, allowing for efficient operation and reduced component count.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If multiple sensing functionalities are integrated into a single analog I/O pin, then the pin count and device complexity are reduced, but the measurement precision and reliability may be affected due to signal interference and multiplexing overhead
Solution Approach 1:
The system implements periodic multiplexing by sequentially switching the analog I/O pin between different measurement functions (FET temperature sensing and current sensing) with controlled timing. The pin alternates between high-impedance input mode for temperature measurement and active driving mode for current measurement, ensuring that each function receives dedicated measurement time slots while sharing the same physical pin resource.
Solution Approach 2:
Before each temperature measurement, the system pre-charges the coupling capacitor through the analog I/O pin by setting it to high output for a predetermined time period. This preliminary charging action ensures that the capacitor is properly conditioned before the actual temperature measurement begins, eliminating capacitive coupling effects that would otherwise interfere with the thermistor reading.
2Adaptability or versatility
If a capacitor is coupled between the analog I/O pin and ground to block DC current, then FET current sensing is enabled, but the pin cannot simultaneously measure temperature due to capacitive coupling interference
Solution Approach 1:
The system uses periodic switching to alternate between two measurement modes: during temperature measurement intervals, the pin is set to high-impedance input mode allowing the thermistor to be read accurately; during current measurement intervals, the pin drives the FET gate and the capacitor blocks DC current. This time-division multiplexing allows both functionalities to coexist on the same pin without mutual interference.
Solution Approach 2:
Before taking a temperature reading, the system performs a preliminary charging action by setting the pin to high output for a predetermined time, allowing the capacitor to charge to the appropriate voltage level. This preliminary action prepares the capacitive coupling network for subsequent temperature measurement by establishing proper initial conditions and eliminating residual capacitive effects.
3Adaptability or versatility
If the analog I/O pin is set to high output for a predetermined time to charge the capacitor, then FET current measurement is enabled, but power consumption increases during the charging phase
Solution Approach 1:
The system implements periodic charging cycles where the analog I/O pin is set to high output only during brief predetermined time intervals sufficient to charge the capacitor to the required voltage level. After charging, the pin immediately transitions to high-impedance input mode for temperature measurement or to active driving mode for current control, minimizing the duration of high-power consumption states while maintaining both sensing capabilities.
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 cost-effective and space-efficient integration of multiple functionalities in a cordless power tool trigger switch, allowing for accurate monitoring and control of FET current, temperature, and battery voltage, while reducing power consumption and assembly complexity.
Implementation Method 1
with a capacitor connected in parallel with the thermistor
Implementation Method 2
waiting a predetermined amount of time for the capacitor to charge
Implementation Method 3
A temperature sensor connected to the analog I/O pin is comprised of a thermistor disposed proximate to a heat source to be measured
Data Source
Figure 1
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AI summary
A control system for use with a cordless power tool having a trigger switch includes a microcontroller having an analog I/O pin and a pin I/O driver driving the pin to high output and low output, and setting the pin to high impedance input. A temperature sensor connected to the analog I/O pin is comprised of a voltage divider circuit and a thermistor disposed proximate to a heat source to be measured. A control module of the microcontroller measures and records temperature of the heat source by first setting the analog I/O pin high, waiting a predetermined amount of time for a capacitor of the voltage divider circuit to discharge, setting the analog I/O pin to high impedance input, taking the temperature reading, and storing the temperature reading in a computer readable medium.