Austin, TX – The traditional approach to preventing the loss of a digit or two to a saw blade is through mechanical means. But even that isn’t foolproof, as guards can be removed or misused. You could make an argument about evolution in action right now. But engineers at SawStop took a different tack by employing digital signal processing (DSP) technology and advanced controls to stop a saw blade in its tracks.
Taking advantage of the fact that the human body has a large electrical capacitance and wood has a low capacitance, the patented system induces a sine wave signal onto the blade using a 500 kHz drive signal applied to a Class C amplifier to generate a 500 kHZ wave. The system then monitors the blade at high speeds to detect any changes in the wave’s amplitude. (Since it has a low electrical capacitance, wood barely affects the sine wave.)
Should a finger come into contact with the blade and the motor controls detect a change in the induced voltage on the blade, an aluminum brake is forced into the teeth of the blade within 5 msec. In the on-stage demo, a volunteer hot dog sustained only a nick.
All of the advanced I/O and data and signal processing are clearly a cut about the capabilities of a traditional plc, explained Brian Ruhmann, NI CompactRIO Software Engineer. “But with the ability to do the high-speed timing, digital signal processing, and triggering directly on the CompactRIO FPGA, it took us less than two weeks to design, prototype, and deploy this type of control system.”
Shawn Slockers, NI Hardware Engineer, holds up the hot dog that survived the demo nearly unscathed. It sustained only minor injuries (a nick) at the hands of a whirring saw blade, thanks to a DSP-based control system.
On-board decision making: 40 mHz
PID and motion control: 200 kHz
Analog I/O: 800 kHz
Digital I/O: 30 mHz