Turning the handle is multiplied into drill revolutions

2 min read

Tom Shelley reports on a novel approach to speed and position control suitable for low cost consumer products and car accessories

A very low cost drive allows a hand turned handle to deliver multiplied torque and speed to a power drill, and, if required, to additionally deliver force feedback. When the handle is no longer turned, the drill soft stops. Although developed for DIY and professional power tools, it is also being actively considered for car window winders and seat adjusters as well as kitchen food mixers. Inventor Craig Webster is Group Leader of the Power Products team at Cambridge Consultants. He explained to Eureka that, "The challenge was to manage the interaction between the machine and the person. Designing a system that would be tolerant of different people using it was particularly difficult. We realised that user perception of performance was very important. For example, the drill has to stop quickly when the user stops turning the handle, but not so quickly that the drill jumps in his hand." The solution adopted was to attach the handle on the side of the drill to a small DC motor. Turning the handle makes the motor act as a generator, producing a small voltage. Had the developers wanted, they could also have fed current back to the motor to provide a feeling of variable resistance to being turned. "We could have used almost any device to do these things, but small DC motors are the cheapest sensors we could get." In order to ensure manufacturability at minimum cost, the design is based on a standard power drill technology, with standard sensing and power electronics. The main drive motor is a brushless permanent magnet DC motor bought in the Far East, driven by pulse width modulated electronics that Webster says is "Easily 95% efficient. We have developed and own intellectual property in sensorless drives and have a fairly novel algorithm to reduce noise in brushless DC motors. Conventional block commutation results in torque ripple. We have developed a system based on signals from Hall sensors that are already present in the DC brushless motor and use this information to estimate motor position at any instant in time based on a computer model. Our model can handle transients in the motor load whereas previous phase locked loop strategies tended to fall over." Trying the drill out demonstrated its controllability as compared to conventional power drills that either run or stop in response to a trigger switch. Applied to car window winders, it would improve their ability to be stopped wherever wanted, as well as avoiding possible accidents arising from trapped fingers. The same applies to car seat adjusters, whereas food mixers would allow precise operator control of the process in the same way as the drill. Cambridge Consultants Craig Webster at Cambridge Consultants Pointers * Control is by turning a handle attached to a small motor which acts as a generator, producing a small voltage according to how fast it is turned, also offering the possibility of producing force feedback if partially energised * The analogue voltage produced by the handle turned small motor is used to control the speed of the main motor. If the handle is no longer turned, the main motor soft stops