The traditional approach to preventing injuries from saws is strictly mechanical and uses blade guards. However, the guards can often be removed or misused, resulting in the loss of fingers and even toes. Engineers at SawStop took advantage of the digital signal processing (DSP) technology used to control the motors in higher-end saw systems to add protection for the user. By measuring the difference in the electrical properties of wood versus a finger, the electronic system protects users from extensive injury when accidentally contacting the saw's blade. The unique approach has resulted in dozens of patent applications for woodworking equipment and it may meet other sensing requirements.
The Sawstop system uses Texas Instruments' TMS320LF2403A DSP-based controller in the motor control to detect a change in the induced voltage on the blade caused by the capacitance of the human body and force a brake into the teeth of the blade within 5 msec. To sense the presence of a finger instead of wood, a 500 kHz drive signal is applied to a Class C amplifier to generate a 500 kHz sine wave that is applied to the saw blade. The amplitude can be adjusted between about 3 and 25V peak to peak. Since the blade is a good conductor, the high frequency signal is uniform across the entire saw blade. An integrator generates an absolute value of the signal sensed from the blade over a 6-µsec period. At the end of each 6 µsec, the amplitude of the integral is sampled by the DSP and the sampling capacitor is reset for the next period. The integrator is used because it provides a measurement of the amplitude of the signal on the blade that is relatively immune from noise perturbations. Brake-to-blade circuitry measures the amplitude of the signal detected from the blade by a brake pawl, which is proportional to the distance between them. The system can detect an 8-inch blade on the saw with a brake cartridge that is designed for a 10-inch blade. If this occurs, the DSP will flash an error code on the control panel and the motor cannot be started until the error is corrected.
The patent rights for the SawStop approach are available for licensing on a non-exclusive basis to tool manufacturers.
Sensing When to Brake The computed peak-to-peak amplitude of the 500 kHz signal on the blade in volts as a function of time. Eash dip is the tooth contacting and passing through the finger. When a finger comes into contact with the blade, the maximum drop is about 40 percent total attenuation of the signal.
The DSP-Based Control System The TMS320LF2403A DSP controls the saw motor through the motor relay drive to ensure that the user cannot turn the saw on if there is a problem that would precent the system from detecting contact or firing the brake. If a finger is detected, the firing circuit triggers the discharge of the high-voltage capacitor to burn a fuse wire and release the spring that pushes the brake into the blade.
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