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A system and method for measuring fluid flow rate in a system where fluid is pumped, such as a gas monitoring instrument where gas is pumped from a space such as a room or enclosure through a conduit to a sensor. The flow of gas through a pump is determined by measuring motor back-e.m.f. which is pr

A system and method for measuring fluid flow rate in a system where fluid is pumped, such as a gas monitoring instrument where gas is pumped from a space such as a room or enclosure through a conduit to a sensor. The flow of gas through a pump is determined by measuring motor back-e.m.f. which is proportional to motor speed. In a system where motor speed is regulated by pulse width modulation of the motor drive voltage, the back-e.m.f. is sampled during intervals between the drive pulses applied to the motor, and in a further aspect the sampling is done at selected, spaced-apart or infrequent intervals such as once for every ten or once for every hundred motor drive pulses. Advantageously, in an instrument that uses a microprocessor and analog-to-digital converter to measure gas-concentration, the same microprocessor and converter can provide the PWM control of the pump, in response to the back-e.m.f. generated by the pump motor between the drive pulses. The microprocessor compares the output of the analog-to-digital converter, corresponding to the back-e.m.f., which in turn corresponds to the pump motor speed, to a set-point value, representative of the desired pump motor speed. The processor then adjusts the PWM to control the pump motor to achieve and maintain the desired speed. As a result, in a gas monitor, the gas-flow rate may be maintained close to a desired gas flow rate regardless of the voltage supplied by the battery or other voltage source, regardless of the degree to which the gas is filtered, and under a wide range of operating conditions.

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The invention claimed is: 1. 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 cont

The invention claimed is: 1. 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. 2. The motor speed control according to claim 1, wherein the means for connecting the microprocessor to the means for sampling back-e. m.f. causes sampling of motor back-e.m.f. at a rate lower than the pulse rate of the modulated motor drive pulses. 3. 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) 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 which is indicative of pump flow rate; and c) a control operatively connected to the back-e.m.f. sampling means for determining the intervals between motor drive pulses during which motor back-e.m.f. is sampled to provide an indication of pump flow rate. 4. The apparatus according to claim 3, wherein the control causes sampling of motor back-e.m.f. at a rate lower than the pulse rate of motor drive pulses. 5. A method for monitoring the flow of gas in a gas monitoring instrument wherein a sample of gas is caused to flow from a location being tested to a sensor for determining the presence of contaminants in the gas, the method comprising: a) providing pulse modulation of a motor driving a pump which causes the flow of gas in the instrument; b) sampling motor back-e.m.f. during intervals between motor drive pulses to provide an indication of motor speed to determine the rate of flow of gas through the instrument; and c) selecting the intervals between motor drive pulses during which motor back-e.m.f. is sampled. 6. The method according to claim 5, wherein motor back e.m.f. is sampled at a rate lower than the pulse rate of motor drive pulses. 7. In a system having an electrically powered motor for driving a pump to bring a quantity of fluid from one location to another, 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. 8. The motor speed control according to claim 7, wherein the means for connecting the microprocessor to the means for sampling back-e. m.f. causes sampling of motor back-e.m.f. at a rate lower than the pulse rate of motor drive pulses. 9. In a system having an electrically powered motor for driving a pump to bring a quantity of fluid from one location to another: 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 which is indicative of pump flow rate; and c) a control operatively connected to the back-e.m.f. sampling means for determining the intervals between motor drive pulses during which motor back-e.m.f. is sampled to provide an indication of pump flow rate. 10. The apparatus according to claim 9, wherein the control causes sampling of motor back-e.m.f. at a rate lower than the pulse rate motor drive pulses. 11. A method for monitoring the flow of fluid in a fluid monitoring system wherein a quantity of fluid is caused to flow from one location to another, the method comprising: a) providing pulse modulation of a motor driving a pump which causes the flow of gas in the system; b) sampling motor back-e.m.f. during intervals between motor drive pulses to provide an indication of motor speed to determine the rate of flow of gas through the system; and c) selecting the intervals between motor drive pulses during which motor back-e.m.f. is sampled. 12. The method according to claim 11, wherein motor back-e.m. f. is sampled at a rate lower than the pulse rate of motor drive pulses. 13. A motor speed control comprising: a) a pulse modulation control and drive for connection between a source of voltage and a motor for controlling application of motor 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 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. 14. The motor speed control according to claim 13, wherein the means for connecting the microprocessor to the means for sampling back-e.m.f. causes sampling of motor back-e.m.f. at a rate lower than the pulse rate of the modulated motor drive pulses. 15. A method for controlling motor speed comprising: a) providing pulse modulation control of motor voltage drive pulses applied to the motor; b) sampling motor back-e.m.f. during intervals between motor drive pulses; c) comparing the sampled back-e.m.f. indicative of actual motor speed to a signal indicative of desired motor speed for controlling the pulse modulation; and d) determining the intervals between modulated motor drive pulses during which motor back e.m.f. is sampled. 16. The method according to claim 15, wherein motor back-e.m. f. is sampled at a rate lower than the pulse rate of the modulated motor drive pulses. 17. 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; 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; and f) wherein the microprocessor extends the interval between motor drive pulses during which back e.m.f. is sampled to provide full recovery of the back-e.m.f. and sufficient sampling time. 18. 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) 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 which is indicative of pump flow rate; c) a control operatively connected to the back-e.m.f. sampling means for determining the intervals between motor drive pulses during which motor back-e.m.f. is sample to provide an indication of pump flow rate; and d) wherein the control extends the interval between motor drive pulses during which back-e.m.f. is sampled to provide full recovery of the back-e.m.f. and sufficient sampling time. 19. A method for monitoring the flow of gas in a gas monitoring instrument wherein a sample of gas is caused to flow from a location being tested to a sensor for determining the presence of contaminants in the gas, the method comprising: a) proving pulse modulation control of motor driving a pump which causes the flow of gas in the instrument; b) sampling motor back-e.m.f. during intervals between motor drive pulses to provide an indication of motor speed to determine the rate of flow of gas through the instrument; c) selecting the intervals between motor drive pulses during which motor back-e.m.f. is sampled; and d) wherein the intervals between motor drive pulses during which back-e.m.f. is sampled is extended to provide full recovery of the back-e.m.f. and sufficient sampling time. 20. A method for monitoring the flow of gas in a gas monitoring instrument wherein a sample of gas is caused to flow from a location being tested to a sensor for determining the presence of contaminants in the gas, the method comprising: a) proving pulse modulation control of motor driving a pump which causes the flow of gas in the instrument; b) sampling motor back-e.m.f. during intervals between motor drive pulses to provide an indication of motor speed to determine the rate of flow of gas through the instrument; c) selecting the intervals between motor drive pulses during which motor back-e.m.f. is sampled; and d) wherein the indication of motor speed obtained from sampling motor back-e.m.f. is utilized to control the pulse modulation to control the motor speed and the rate of gas flow through the instrument. 21. The method according to claim 20, wherein the indication of motor speed is compared to a desired motor speed to provide a difference signal to control the pulse modulation. 22. In a system having an electrically powered motor for driving a pump to bring a quantity of fluid from one location to another, 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; 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; and f) wherein the microprocessor extends the interval between motor drive pulses during which back e.m.f. is sampled to provide full recovery of the back-e.m.f. and sufficient sampling time. 23. In a system having an electrically powered motor for driving a pump to bring a quantity of fluid from one location to another: 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 which is indicative of pump flow rate; c) a control operatively connected to the back-e.m.f. sampling means for determining the intervals between motor drive pulses during which motor back-e.m.f. is sampled to provide an indication of pump flow rate; and d) wherein the control extends the interval between motor drive pulses during which back-e.m.f. is sampled to provide full recovery of the back-e.m.f. and sufficient sampling time. 24. A method for monitoring the flow of fluid in a fluid monitoring system wherein a quantity of fluid is caused to flow from one location to another, the method comprising: a) providing pulse modulation of a motor driving a pump which causes the flow of gas in the system; b) sampling motor back-e.m.f. during intervals between motor drive pulses to provide an indication of motor speed to determine the rate of flow of gas through the system; c) selecting the intervals between motor drive pulses during which motor back-e.m.f. is sampled; and d) wherein the interval between motor drive pulses during which back-e.m.f. is sampled is extended to provide full recovery of the back-e.m.f. and sufficient sampling time. 25. A method for monitoring the flow of fluid in a fluid monitoring system wherein a quantity of fluid is caused to flow from one location to another, the method comprising: a) providing pulse modulation of a motor driving a pump which causes the flow of gas in the system; b) sampling motor back-e.m.f. during intervals between motor drive pulses to provide an indication of motor speed to determine the rate of flow of gas through the system; c) selecting the intervals between motor drive pulses during which motor back-e.m.f. is sampled; and d) wherein the indication of motor speed obtained from sampling motor back-e.m.f. is utilized to control the pulse modulation to control the motor speed and the rate of fluid flow through the system. 26. The method according to claim 25, wherein the indication of motor speed is compared to a desired motor speed to provide a difference signal to control the pulse modulation. 27. A motor speed control comprising: a) a pulse modulation control and drive for connection between a source of voltage and a motor for controlling application of motor 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 for comparing the actual motor speed to a desired motor speed for controlling the operation of the pulse modulation control; 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; and f) wherein the microprocessor extends the interval between motor drive pulses during which back-e.m.f. is sampled to provide full recovery of the back-e.m.f. and sufficient sampling time. 28. A method for controlling motor speed comprising: a) proving pulse modulation control of motor voltage drive pulses applied to the motor; b) sampling motor back-e.m.f. during intervals between motor drive pulses; c) comparing the sampled back-e.m.f. indicative of actual motor speed to a signal indicative of desired motor speed for controlling the pulse modulation; d) determining the intervals between modulated motor drive pulses during which motor back e.m.f. is sampled; and e) wherein the interval between motor drive pulses during which back-e.m.f. is sampled is extended to provide full recovery of the back-e.m.f. and sufficient sampling time. 29. A gas monitoring instrument comprising: a) a sensor for determining the presence of contaminants in the gas; b) an inlet for receiving a sample of gas from a location being tested; c) a pump to bring the sample of gas to the sensor; d) an electronically powered motor for driving the pump; e) 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; f) 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 which is indicative of pump flow rate; and g) a control operatively connected to the back-e.m.f. sampling means for determining the intervals between motor drive pulses during which motor back-e.m.f. is sampled to provide an indication of pump flow rate. 30. The instrument according to claim 29, wherein the control causes sampling of motor back-e.m.f. at a rate lower than the pulse rate of motor drive pulses. 31. The instrument according to claim 29, wherein the control extends the interval between motor drive pulses during which back-e.m.f. is sampled to provide full recovery of the back-e.m.f. and sufficient sampling time. 32. A gas monitoring instrument comprising: a) a sensor for determining the presence of contaminants in the gas; b) an inlet for receiving a sample of gas from a location being tested; c) a pump to bring., the sample of gas to the sensor; d) an electrically powered motor for driving the pump; e) 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; f) 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; g) 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; h) 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 i) 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. 33. The motor speed control according to claim 1, wherein the means for connecting the microprocessor to the means for sampling back-e. m.f. causes sampling of motor back-e.m.f. at a rate lower than the pulse rate of the modulated motor drive pulses. 34. The motor speed control according to claim 1, wherein the microprocessor extends the interval between motor drive pulses during which back-e.m.f. is sampled to provide full recovery of the back-e.m.f. and sufficient sampling time.