Control circuit and method for manipulating a power tool
Patent No. US8841871 (titled "Control circuit and method for manipulating a power tool") on Jun 7, 2011. The application was issued on Sep 23, 2014.
What is this patent about?
’871 is related to the field of power tool control systems, specifically focusing on the accurate monitoring and display of battery capacity. In traditional cordless tools, battery level indicators often rely on simple voltage measurements that fluctuate under load or change with temperature, leading to inaccurate readings that do not reflect the actual energy remaining for the motor.
The underlying idea behind ’871 is to derive the true open circuit voltage of a battery while it is actively discharging by mathematically compensating for internal and external losses. Instead of just reading the terminal voltage, the system accounts for the voltage drops caused by the battery’s own internal resistance and the resistance of the tool's wiring and switches, while also adjusting for how these resistances change as the battery heats up.
The claims of ’871 focus on a control circuit that utilizes a specific mathematical formula to determine battery capacity: V = V’ + I·Rc + I·Rb·Ct. This architecture requires a detection circuit for terminal voltage and temperature, a current measuring circuit, and a controller that integrates these inputs to calculate a result that includes the voltage consumed both internally by the cells and externally by the discharge loop.
In practice, the invention functions by continuously monitoring the current flow and temperature during tool operation. By applying a temperature compensation factor, the controller can accurately estimate the battery's internal resistance at any given moment. This allows the tool to display a stable and reliable capacity level on both the battery pack and the tool body, ensuring the user is not misled by voltage sags that occur during heavy cutting or drilling tasks.
This approach differs from prior solutions by maintaining active monitoring throughout the entire discharge cycle rather than just at startup. By incorporating the discharge loop internal resistance into the calculation, the system provides a high-fidelity representation of the battery's state of charge that remains consistent regardless of the electrical load or the thermal state of the power tool's components.
How does this patent fit in bigger picture?
Technical Landscape
In the early 2010s when ’871 was filed, power tool battery monitoring was typically implemented using direct voltage measurements across battery terminals to estimate remaining charge. At a time when systems commonly relied on static voltage readings taken while the tool was in an idle state, hardware and software constraints made it non-trivial to provide accurate capacity feedback once the motor was engaged and drawing current. Engineering practices often treated battery gauging and motor control as separate functional blocks, where the internal resistance fluctuations caused by thermal changes during discharge were frequently ignored, leading to inaccurate capacity displays and a lack of real-time safety monitoring during active tool operation.
Prosecution Position
The disclosed invention represents a technical advancement through the integration of dynamic thermal and current compensation into the battery capacity calculation architecture. By measuring the battery temperature and the current flowing through the motor, the system calculates an open-circuit voltage that accounts for internal resistance and discharge loop losses, overcoming the constraint of voltage sag during operation. This architectural shift enables a continuous monitoring loop that updates the capacity display while the motor is active, rather than only at startup. The technical effect is a more accurate representation of energy reserves and enhanced operational safety through real-time parameter judging that can trigger motor shutdown in response to over-current or over-temperature conditions.
Claims
The patent contains a total of 6 claims, with claim 1 serving as the sole independent claim. This independent claim focuses on a control circuit for a power tool designed to calculate the open circuit voltage of a battery pack by integrating measured voltage, current, internal resistance, and temperature factors into a specific mathematical formula. The dependent claims serve to further define the hardware components and parameters of the system, such as specifying the nature of the temperature factor, identifying the processing units used for calculations, and detailing the visual indicators used to display battery capacity.
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US8841871
- Application Number
- US13154549A
- Filing Date
- Jun 7, 2011
- Publication Date
- Sep 23, 2014
- External Links
- Slate, USPTO , Google Patents