Speed detector and indicator for DC motors

• US 3,942,114 A
• Filed: 10/15/1974
• Issued: 03/02/1976
• Est. Priority Date: 10/15/1974
• Status: Expired due to Term

• Current Assignee: William E. Keeling
• Original Assignee: William E. Keeling
• Inventors: Keeling, William E.
• Primary Examiner(s): Smith, Alfred E.
• Assistant Examiner(s): Hille, Rolf
• Application Number:
  US05/514,694
• Time in Patent Office: 504 Days
• Field of Search: 324/177, 318/317, 318/309, 318/312, 318/315, 318/490, 246/182 R, 317/5
• US Class Current: 324/177
• CPC Class Codes: G01P 3/46 by measuring amplitude of generated current or voltage in general G01R19/00
• CPC Class Codes: Y10S 388/901 Sample and hold

Abstract

A speed detector for a direct current motor is coupled substantially in parallel with the motor such that only the motor input voltage and a function of the input current are inputs to the speed detector/indicator system. Analog speed detection is provided by using either operational amplifiers or single stage transistor circuits to sample the current and voltage to generate a voltage proportional to speed of the motor.

Claims

1. In a motor control system having a motor for variable speed operation and a voltage throttle circuit for driving the motor, the improvement comprising:

first and second amplifiers each having an input and an output, the output of said first amplifier being coupled to the input of said second amplifier, a first resistive means coupled between the output and the input of said second amplifier for providing adjustable feedback, an indicator circuit coupled to the output of said second amplifier for responding to any output signal therefrom, the inputs of said first and second amplifiers being coupled to said motor throttle circuit for receiving the input signals therefrom for providing an output signal indicative of motor speed, and a second resistive means coupled between the input of said second amplifier and said motor throttle circuit for providing an adjustable signal thereto.

2. A speed detector system as set forth in claim 1 wherein the input of said first amplifier is coupled to said motor throttle circuit for receiving an input voltage signal therefrom, and the input of said second amplifier being coupled to receive a portion of said motor drive signal which is a function of motor current wherein said second resistive means is a first potentiometer connected between the input of said second amplifier and said motor throttle circuit.

3. A speed detector system as set forth in claim 2 wherein said amplifiers are operational amplifiers, said first amplifier being an operational amplifier with feedback, serving as an inverter and said second amplifier being an operational amplifier serving as an adder, said first resistive means is a second potentiometer, and said output signal from said amplifiers being indicative of motor speed as defined by the equation S = K_A (V_T - K_B I) where

5. A speed detector system as set forth in claim 3 and further comprising a current sensing resistor connected between one side of said motor and said voltage throttle circuit, said input of said second amplifier being coupled through said first potentiometer between said current sensing resistor and said motor for receiving said portion of said motor drive signal, and said input of said first amplifier being coupled directly to the other motor input.

6. A speed detector system as set forth in claim 3 and further comprising a current sensing resistor connected between one side of said motor and said voltage throttle circuit, said input of said first amplifier being coupled between said current sensing resistor and said motor for receiving said portion of said motor drive signal, and said input of said second amplifier being coupled through said first potentiometer directly to the other motor input.

7. A speed indicator system as set forth in claim 1 wherein said first and second amplifiers are discrete transistor amplifiers having the emitters thereof coupled in common to a system ground, the base of said second transistor being coupled through said first resistor to the collector output of said first transistor, the collector of said second transistor being the output connected to said indicator circuit, the base of said first transistor being the input coupled to receive said motor driving signal input;

and further comprising a direct current voltage source coupled to said transistors for providing power thereto, said first resistive means is a second resistor coupled between the collector and base of said second transistor, and said second resistive means is a third resistor coupled between the base of said second transistor and said motor throttle circuit to receive said portion of said motor drive signal for providing an output signal indicative of motor speed as defined by the equation S = K_A (V_T - K_B I), where

S is speed,

K_A is a constant in units of speed per volt provided by said second resistor,

K_B is a constant in ohms provided by said third resistor,

V_T is input terminal voltage, and

I is input terminal current.

8. A speed indicator system as set forth in claim 6 wherein said indicating circuit is a calibrated voltmeter having one side coupled to said second transistor collector and the other side coupled to ground;

and further comprising an adjustable, offset voltage compensation network coupled to said voltmeter for cancelling offset signals thereto.

4. S is speed,

K_A is a constant in speed per volt provided by said second potentiometer,

K_B is a constant in ohms provided by said first potentiometer,

V_T is input terminal voltage, and

I is input terminal current.