Amperage control for protection of battery over current in power tools
View Patent ↗Amperage control of a power tool motor is provided by pulse width modulation of current from a power supply. The pulse width modulation may be varied according to the determined motor current and measured power supply voltage. The power supply preferably includes a battery, such a lithium ion or nickel cadmium.
1. A method for extending the life of a power supply that supplies power to a motor in a power tool, comprising:
supplying voltage pulses of an initial width to the motor using pulse width modulation;
comparing a motor current generated due to the supplied voltage pulses to at least one of three different thresholds; and
changing the pulse width from the initial width to one of zero, a first alternative pulse width and a second alternative pulse width based on the comparison to a given threshold.
2. The method of claim 1 , wherein the alternative width is shortened.
3. The method of claim 1 , wherein the alternative width is lengthened.
4. A power tool, comprising:
a housing;
a power supply disposed within the housing and including a plurality of lithium ion batteries;
a motor disposed within the housing and providing torque to the power tool; and
a pulse width modulation controller within the housing for varying the pulse width of voltage pulses supplied to the motor based on a comparison of the motor current to one of at least three different thresholds.
5. The power tool of claim 4 , wherein the lithium ion batteries are rechargeable.
6. The power tool of claim 4 , wherein the pulse modulation controller includes a switch that supplies current to the motor when the switch is turned on.
7. The power tool of claim 6 , wherein the pulse modulation controller further includes a microprocessor that turns the switch on and off.
8. The power tool of claim 7 , wherein the pulse modulation controller further includes voltage measuring circuitry to determine a turn on time for the switch.
9. The power tool of claim 8 , wherein the voltage measuring circuitry measures battery voltage and motor voltage.
10. The power tool of claim 9 , wherein power tool further includes circuitry for speed control.
11. The power tool of claim 10 , wherein the circuitry for speed control includes a potentiometer.
12. The power tool of claim 11 , wherein the voltage measuring circuitry further measures a voltage across the potentiometer.
13. The power tool of claim 6 , wherein the pulse width modulation circuit further includes a ramp generator that provides a ramp signal.
14. The power tool of claim 13 , wherein the pulse width modulation circuit further includes a comparator that compares the ramp signal to a measured motor voltage to determine the pulse width modulation waveform.
15. An apparatus for limiting current to a load, comprising:
means for supplying power;
means for providing torque to a load;
means for switchably supplying power to the means for providing torque from the means for supplying power, the means for switchably supplying power being actuated according to a pulse width modulation technique; and
means for controlling the means for switchably supplying power based on a comparison of current generated in the apparatus to one of at least three different thresholds.
16. The apparatus of claim 15 , wherein the means for supplying power includes lithium ion batteries.
17. The apparatus of claim 16 , wherein the means for supplying power provides a direct voltage between 30 and 40 volts.