Miniature drills are used in medical applications but they have very particular requirements.
They must not produce carbon dust or electrical sparks. But more importantly they need to be controlled very accurately.
Design firm Escatec has developed a control algorithm for a sensorless brushless DC motor to be used as a surgical drill with the control precision for medical procedures.
The precision motor control was achieved by applying a specially written, speed-dependent, rotor-position detection algorithm. Combining this position detection, including an additional algorithm to detect the rotor position at standstill, with different pulse width modulation (PWM) control schemes enables the motor to be controlled independently of the applied mechanical load, inclusive of stand-still during direction change.
The controller is in open-loop mode only during the first phase of the ramp-up. Afterwards, the position-detection is used to calculate the power to reach the requested speed.
The firm, which has ISO 13485 accreditation for the design and manufacture of medical devices and develops medical device software according to IEC 62304, has also designed battery protection circuitry which it says extends battery lifetime.
Combining this software-controlled protection circuit with an external charging device enables the battery to be safely charged to its full capacity, and the capacity calculated and stored into the motor controller’s memory as a battery condition indication.
“We had to design a means of being extremely accurate with the control of the drill motor,” said Christian Dünki, a specialist in medical applications at Escatec in Switzerland.
“Also we had to create a solution to allow starting the motor through load ranges from free run up to high torque, including stall detection and active stop, plus an oscillation mode with very low angle drift.”
The three graphs how the back- EMF (electro-motive force) signals change during the control process. The lower left figure shows the back-EMF signals during the beginning of the ramp-up.
During this phase, space vector pulse width modulation (SVPWM) is used to start the motor’s rotation.
The figure in the middle shows the back-EMF signals where the back-EMF voltage is integrated and the result is used to distinguish the commutation point.
The right figure shows the back-EMF signals where zero-cross-detection is used to measure the position. The back-EMF voltages are measured and analysed by detecting the zero-cross event of one motor-phase at a time.