Apparatus and method for controlling the speed of an induction motor in a closed-loop system

Claim 1

In combination with: (a) an A.C. induction motor; (b) adjustable-frequency power converter means operatively connected with said induction motor for providing a variable frequency power signal to energize said induction motor, said frequency-converter being operable to provide an A.C. output signal having frequency determined by a received control signal; (c) means operated by said induction motor for performing a function which has an effect upon at least one external measurable parameter; and (d) sensor means disposed to sense said one measurable parameter for periodically providing a measurement signal responsive thereto, said measurement signal being accurate within a measurement error band based on the measurement accuracy of said sensor means; wherein the invention comprises: control means suitable for operative connection to receive said periodic measurement signals from said sensor means and to provide a control signal to said adjustable-frequency power converter means, for generating said control signal responsive to cummulative changes in said measurement signals received over a predetermined period of time, said control signal being of a nature whereby when applied to said power convertor, said power convertor energizes said motor so as to regulate said measurable parameter to an accuracy greater than the measurement accuracy of said sensor means.

Claim 8

A method of controlling the value of a measured parameter of an environment in a closed-loop control system to an accuracy greater than that of the means measuring said parameter, to maintain the value of said measured parameter within the maximum accuracy tolerance levels of the means measuring said parameter, said closed-loop system being characterized by: sensing means for measuring said one parameter and for providing a measurement signal responsive thereto, said sensing means having upper and lower measurement accuracy tolerance levels defining a measurement accuracy dead-band about the measurement signal; control means connected to receive said measurement signal for producing a control signal responsive thereto; and a prime mover connected to receive the control signal and being responsive thereto to act upon said environment, which action affects the value of said measured parameter; wherein the method comprises the steps of: (a) determining a desired value at which said one parameter is to be maintained; (b) causing the prime mover to act upon said environment so as to place the value of said one parameter within the measurement accuracy dead-band tolerance levels of said sensing means; (c) periodically causing said sensing means to measure said one parameter, to periodically produce parameter measurement signals; (d) causing said periodically produced measurement signals to be transferred to the control unit; (e) determining the respective changes in values between successive ones of said periodically produced and transferred measurement signals and producing parameter change signals representative respectively of each said determined change in value; (f) storing in a memory said respective parameter change signals which represent a predetermined successive number of said measurement signals last transferred in time to said control unit; (g) determining a cumulative value of said stored parameter change signals; (h) generating a control signal for changing the speed of the prime mover responsive to said determined cumulative value; and (i) causing said prime mover to be energized in response to said generated control signal whereby said prime mover acts upon said environment to maintain the value of said measured parameter at a value within the measurement accuracy dead-band tolerances of said sensing means, thereby attaining a control accuracy for said one parameter which is more accurate than the accuracy tolerance levels of the sensing means.

Claim 9

A method of controlling the value of a measured parameter of an environment in a closed-loop control system to an accuracy greater than that of the means measuring said parameter, to maintain the value of said measured parameter within the maximum accuracy tolerance levels of the means measuring said parameter, said closed-loop system being characterized by: sensing means for measuring said one parameter and for providing a measurement signal responsive thereto, said sensing means having upper and lower measurement accuracy tolerance levels defining a measurement accuracy dead-band about the measurement signal; control means connected to receive said measurement signal for producing a control signal responsive thereto; and a prime mover connected to receive the control signal and being responsive thereto to act upon said environment, which action affects the value of said measured parameter; wherein the method comprises the steps of: (a) determining a desired value at which said one parameter is to be maintained; (b) causing the prime mover to act upon said environment so as to place the value of said one parameter within the measurement accuracy dead-band tolerance levels of said sensing means; (c) periodically causing said sensing means to measure said one parameter, to periodically produce parameter measurement signals; (d) causing said periodically produced measurement signals to be transferred to the control unit; (e) determining the respective changes in values between successive ones of said periodically produced and transferred measurement signals and producing parameter change signals representative respectively of each said determined change in value; (f) storing in a memory said respective parameter change signals which represent a predetermined successive number of said measurement signals last transferred in time to said control unit; (g) determining a cumulative value of said stored parameter change signals; (h) generating a control signal for changing the speed of the prime mover responsive to said determined cumulative value; and (i) causing said prime mover to be energized in response to said generated control signal whereby said prime mover acts upon said environment to maintain the value of said measured parameter at a value within the measurement accuracy dead-band tolerances of said sensing means, thereby attaining a control accuracy for said one parameter which is more accurate than the accuracy tolerance levels of the sensing means.