Patent ID: 6981402

Claim:
In a gas monitoring instrument having an electrically powered motor for driving a pump to bring a sample of gas from a location being tested to a sensor for determining the presence of contaminants in the gas, a motor speed control comprising: a) a pulse modulation control and drive operatively connected between a source of voltage and the motor for controlling application of voltage drive pulses to the motor; b) means operatively associated with the motor for sampling motor back-e.m.f. during intervals between motor drive pulses to provide an output indicative of actual motor speed; c) means operatively connected to the means for sampling back-e.m.f. to provide a digital signal output containing information on the actual motor speed; d) a microprocessor connected to the output of the means to provide a digital signal and connected in controlling relation to the pulse modulation control and drive for comparing the actual motor speed to a desired motor speed for controlling the operation of the pulse modulation control; and e) means for connecting the microprocessor to the means for sampling back-e.m.f. for determining the intervals between modulated motor drive pulses during which motor back-e.m.f. is sampled.