Question

a) Describe how each of the following can be achieved
i. Controlling a DC motor with pulse width modulator.
ii. Controlling a stepper motor that has a built-in controller.

Answer 1

Question

i)   Controlling a DC motor with pulse width modulator.

Answer : Controlling a DC motor with pulse width modulator.

We use DC motors in many day to day applications like in our computers and in many toys . As, we know DC motors are operated by  DC power source and in most of the cases it is required to adjust the speed of the motor as per our requirement .We are having many methods to control the speed of DC motor, most common one is variable resistance method but it has its own limitations and cannot be applied on digital equipment .Therefore , for the digital equipment Pulse width modulation is best method to control the speed of the DC motor .

Given circuit is used to produce pulse width modulating signal at fixed frequency output.The capacitor " C"  is timing capacitor . Which gets charged and discharged when current is flowing through R_A and R_B (Timing network).

As this circuit is based on 555 timer circuit and also familiar with it .Therefore the voltage at the pin 3 is equal to the supply voltage , which switch the transistor to fully ON . This voltage is also output voltage . There fore the voltage at Pin 3 is output voltage .

As R_A and R_B is timing network the timing of charging and discharging of capacitor C depends on R_A and R_B. Capacitor charges through R_A and diverted around  R_B through Diode D ₁. Immediately after charging the capacitor discharges through diode D ₂ and R_B into Pin 7 .

During Discharging the output at Pin 3 is zero . Which means transistor is switched  OFF.

Time taken by capacitor to charge discharge depends totally on R_A ,R_B and C. One complete cycle of charging and discharging is denoted by T.

T_H = when transistor is ON

T_L= when transistor is OFF

T= T_H + T_L

f = 1 / T

By this way we can adjust frequency and maintain it to be constant .

ii. Controlling a stepper motor that has a built-in controller.

Answer : Controlling a stepper motor that has a built-in controller.

Stepper motor is nothing but a brush less DC motor which divides the full rotation into the equal number of steps.It will specify the incremental distance it rotates per step .executed number of steps controls the degree of rotation of motor Shaft .

As we know Stepper motor has inherit ability of controlling the position of motor , as it has built in position steps which will control the motor accurately that how far and fast the motor will rotate .Rate at which the the stepper motor rotate a distance is directly proportional to the number and frequency of pulse commands.

In stepper motor we have multiple coils organised in groups called phases .If one phase is energize with sequence, motor will move one step at a time . With this we can achieve precise positioning and speed control by applying computer stepping.