Barba M., Bricchi M.,Carlos A., Martinez L., Marron E.
Abstract— In this paper, we will explain how we develop a device which could control velocity and the turnover from a Direct Current (DC) motor.
C motors are widely used in applications of all kind across the industry.
DC motors are inexpensive, easy to
drive,and are available in all sizes and shapes.
This paper will discuss how a DC motor
works, how to drive a DC motor in order to control the velocity and turnover.
II. H BRIDGE
Basically an H-bridge is an electronic circuit which enables DC electric motors to be run forwards or backward.
We will use the diagram above to show how the H Bridge works.
Fig 1. H-Bridge with Switches.
TheH-Bridge has 4 transistors that works like switches. Opening and closing certain switches will make the motor to move forward or backward.
For example, if switch S1and S4 are closed and S2 and S3 are opened, the flow across the motor will make it to move forward. If S2 and S3 are closed and S1 and S4 are opened. The flow will make the motor to run backwards.
In the following table we will show thedifferent states that the H-Bridge will use in our design.
S1 | S2 | S3 | S4 | Results |
1 | 0 | 0 | 1 | Forward |
0 | 1 | 1 | 0 | Backwards |
0 | 0 | 0 | 0 | Free |
Table 1. States of H-Bridge.
III. PULSE WIDTH MODULATION (PWM).
Pulse Width Modulation (PWM) is a way of digitally encoding analog signal levels. With a PWM, the duty cycle of a square wave is modulated to a specificfrequency. The PWM signal is digital because the full DC supply is either fully on or fully off. Therefore, the voltage or current source is supplied by repeating series of on and off pulses. The on-time is the time during the DC supply is applied to the load, and the off-time is the period during the DC supply is switched off.
Pulse Width Modulation (PWM) is a common technique for speed control becauseby modulating the width of the pulse the average power can be varied, and thus the motor speed. Also, by controlling analog circuits digitally, system costs and power consumption can be drastically reduced.
An advantage of a PWM is that the output transistor is either on or off, not partly “on” as with normal regulation, so less power is wasted as heat. In addition, with a suitable circuitthere is very small voltage loss across the output transistor.
Fig 2. PWM Pulse.
The figure 2. explains the way in which the PWM operates. In each case, the signal has maximum and minimum voltages of 12V and 0V.
* In the first figure, the signal has a mark-space ratio of 1:1. With the signal at 12V for 50% of the time, the average voltage is 6V, so the motor runs at half its maximumspeed.
* In the second figure, the signal has a mark-space ratio of 3:1, which means that the output is at 12V for 75% of the time. This clearly gives an average output voltage of 9V, so the motor runs at 3/4 of its maximum speed.
* In the third figure, the signal has a mark-space ratio is 1:3, giving an output signal that is 12V for just 25% of the time. The average output voltage of thissignal is just 3V, so the motor runs at 1/4 of its maximum speed.
By varying the mark-space ratio of the signal over the full range, it is possible to obtain any desired average output voltage from 0V to 12V.
Our design consists of two circuits, the one for the H-Bridge and one with the PWM.
First we will analyze the H-Bridge design in the next figure.
Fig 3. Design of the HBridge.
Our H-Bridge Consists of 4 Darlington Transistors. 2 PNP’s and 2 NPN’s that acts like switches that we explained in the part II of this paper.
We use the Darlington transistors because they deliver more current than normal transistors and because they have a diode that protects the circuit when the DC motor is disconnected.
The package with the Darlington Transistor is shown in the figure...