Digital control system for electric motors in power tools and the like

• US 4,307,325 A
• Filed: 03/24/1980
• Issued: 12/22/1981
• Est. Priority Date: 01/28/1980
• Status: Expired due to Term

• Current Assignee: Black & Decker Incorporated (Stanley Black & Decker, Inc.)
• Original Assignee: Black & Decker Incorporated (Stanley Black & Decker, Inc.)
• Inventors: Saar, David A.
• Primary Examiner(s): Smith, Jr., David
• Application Number:
  US06/133,463
• Time in Patent Office: 638 Days
• Field of Search: 318/345 E, 318/461, 318/463, 318/465, 318/464, 318/456, 318/341, 318/490, 318/334, 318/471, 318/472, 318/473, 318/4, 361/240, 361/242, 361/25, 361/27, 364/557, 324/158 MG, 340/588, 340/589, 340/648, 73/362 SC
• US Class Current: 388/809
• CPC Class Codes: B23B 2260/128 Sensors
• CPC Class Codes: B23B 45/02 driven by electric power
• CPC Class Codes: H02H 6/005 using digital thermal images
• CPC Class Codes: H02H 7/0854 responsive to rate of change of current, couple or speed, e.g. anti-kickback protection H02H7/0851 takes precedence
• CPC Class Codes: H02P 23/0077 Characterised by the use of a particular software algorithm
• CPC Class Codes: H02P 25/145 whereby the speed is regulated by measuring the motor speed and comparing it with a given physical value, speed feedback
• CPC Class Codes: Y10S 388/907 Specific control circuit element or device
• CPC Class Codes: Y10S 388/934 Thermal condition
• CPC Class Codes: Y10S 388/937 Hand tool

Abstract

A digital control system for electric motors in power tools of the portable or stationary type includes a microcomputer controlled speed regulator connected to the electric motor for maintaining a preselected speed of the motor substantially constant between no load and full load operation of the motor. The preselected speed is inputted to the microcomputer by means of a rotary switch or similar input switching arrangement. When the tool is just starting or the operator changes the speed switch, the microcomputer controls the speed of the motor to slowly approach the desired speed. The microcomputer is programmed to detect an overload condition and activate an alarm, and an impending kickback condition and stop the electric motor.

Claims

1. The method of monitoring the operating temperature of a motor including the steps of:

storing a matrix of heating values representing the relative heating condition of the motor when operated at various speeds and loading conditions with one heating value being assigned to each designated operating region of the motor, periodically determining in which of the designated operating regions the motor is currently operating based upon the speed of the motor and the voltage applied to the motor and adding the corresponding heating value assigned to that region to an accumulated total, and

determining whenever the accumulated total exceeds a predetermined amount.

2. A control circuit for monitoring the operating temperature of an electric motor, comprising:

a speed sensor for sensing the speed of the motor and producing a corresponding speed signal,

first digital storage means for storing a plurality of heating values representing the relative steady-state heat generating condition of the motor when operated at various speeds and loading conditions,

second digital storage means for storing an accumulated total of past heating values, and

means for periodically selecting from said first digital storage means the appropriate heating value corresponding to the present operating condition of the motor as determined in accordance with said speed signal and the voltage being applied to the motor and adding said appropriate heating value to the accumlated total in said second digital storage means.

3. The control circuit of claim 2 further including means for determining whenever the accumulated total in said second digital storage means exceeds a predetermined amount.

4. The control circuit of claim 2 wherein said heating values are empirically derived.

5. In a power tool having an electric motor and means for applying power to the motor, a control circuit for monitoring the operating temperature of the motor comprising:

a speed sensor for sensing the speed of the motor and producing a corresponding speed signal; and

microcomputer means for monitoring the temperature of the motor in accordance with information derived from said speed signal and the power applied to the motor including first memory means for storing a plurality of heating values representing the relative steady-state heat generating condition of the motor when operated in a corresponding plurality of designated regions each defined by a predetermined range of speed and torque values, storage means for storing an accumulated total of past heating values, and processor means for periodically determining from said speed signal and the power applied to the motor the designated region within which the motor is operating, selecting in accordance therewith the corresponding heating value stored in said first memory means, and adding said heating value to the contents of said storage means.

6. The control circuit of claim 5 wherein said processor means is programmed to determined the level of power applied to the motor in accordance with the firing angle of a semiconductor control device connected to the motor.

7. A method of monitoring the temperature of a motor including the steps of:

determining the cooling effect of the motor in terms of its speed,

determining the heating effect of the motor in terms of applied power,

determining a heating value, which is a function of both the determined cooling effect and the determined heating effect, from a set of heating values established over a predetermined motor operating range,

adding the heating value periodically to an accumulated total of past heating values, and

signalling the occurrence of the accumulated total exceeding a predetermined amount.