MAIN FUNCTIONAL REQUIREMENT: Provide portable method for drilling holes and driving fasteners
DESIGN PARAMETER: Cordless drill
EXPLANATION OF HOW IT WORKS/ IS USED:
This signal appears "time averaged" as the shaft speed. In a low duty cycle the motor is getting a very short pulse. It is turning on, then off quickly and there is a relatively long time before it turns on again. This on/off switching appears as a constant slow speed. At higher duty cycles the voltage of the pulse is still constant but the length of the pulse per unit time is greater. Since the motor is on for a longer percentage of time it spins faster. The duty cycle is a measure of the relative width of the pulse in time and determines the speed of the output shaft.
There are two main advantages to a PWM controller. The first is the size and relative ease with which it can be implemented. The circuit consists of a timer and a transistor. The trigger controls the timer. The second advantage is its efficiency. The transistor acts as a switch and is therefore either on or off. Theoretically this will dissipate no power. Minimizing the loss in the control circuitry means longer runtime for the drill and that a greater percentage of the power is transferred to the motor.
This drill uses a DC (direct current) motor with replaceable brushes. It is capable of a nearly constant torque output.
Power is stored chemically in the battery.
Pin=V*I (power input to drill from battery)
Power is lost in the system primarily through resistance (I2*R) in the electrical elements other than the motor and friction in the gears and motor. The PWM controller circuitry uses relatively no power. In addition, the transistor is not ideal and thus has a slight resistance of its own when in the on state.
Ploss=I2*Relectrical paths other than motor+f(friction)
Power goes out as torque and rotation out of the chuck.
Pout=T*w - Ploss
The runtime of the tool is limited by the amount of energy stored in the battery.
Size may also be limiting. Larger batteries and motors are capable of longer runtimes and more torque and speed. However, this drill will run under heavy use for longer than it takes to charge a second battery and provides more torque than is necessary for driving the average fastener.
Power in the motor is I2*Rmotor. Since there is a linear relationship between I and R, minimizing resistance in the motor yields proportionally higher current which is then squared as power thus increasing the efficiency of the motor.
Conversation with Stephen Ludwick, January 28 1999, 9:30 AM, MIT 35-030
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