Experiment (2) DC Motor Control (Direction and Speed)

Transcription

1 Introduction Experiment (2) DC Motor Control (Direction and Speed) Controlling direction and speed of DC motor is very essential in many applications like: 1- Robotic application to change direction and speed of moving robot. 2- Industrial application to change direction and speed of rotating machinery. 3- Domestic application to vary speed of battery (DC) operated portable fan. 4- Defense application to rotate radar, automatic gun, tank gun in either direction 5- Communication application rotate dish antenna upward downward or clockwise anticlockwise. In this experiment, we are going to build a bridge to control the direction of a DC motor, then we will explain how to use L298N H-bridge module and Arduino to control its speed and direction. Objectives This experiment is to be performed in order to: 1- Learn how to build an H-bridge from discrete components. 2- Familiarize the students with the function of H-bridge module. 3- Control both the direction and speed of DC motors. 4- Illustrate the principle of pulse width modulation (PWM). Theory To control a DC motor in direction and speed it is required to use special drive circuits such as H-bridge, which is an electronic circuit that enables a voltage to be applied across a load in either direction. Figure 1 shows a simplified structure for H-Bridge connections. Figure 1: H-bridge structure H-bridges can be built from discrete components; transistors or relays, figure 2 shows an H- bridge built from npn transistors and freewheeling diodes. Also, H-bridges are available as integrated circuits (as L298N driver module). Eng. Malek Alkayyali Page 12

2 Figure 2: An H-bridge built from 4 npn transistors L298N is a high voltage, high current dual- full bridge driver designed to drive various loads such as DC motors, see figure 3. Note: Refer to L298N datasheet for more details. Figure 3: L298N dual motor controller module The following list illustrates the function of each terminal in figure 3: 1. DC motor 1 "+", (connection A+). Eng. Malek Alkayyali Page 13

3 2. DC motor 1 "-", (connection A-) V jumper: remove this if using a supply voltage greater than 12V DC. This enables power to the onboard 5V regulator. 4. Connect your motor supply voltage here, maximum of 35V DC. Remove 12V jumper if >12V DC. 5. Ground (GND). 6. 5V output if 12V jumper in place, ideal for powering your Arduino (etc.). 7. DC motor 1 enable jumper. Connect to PWM output for DC motor speed control. 8. IN1 9. IN2 10. IN3 11. IN4 12. DC motor 2 enable jumper. Connect to PWM output for DC motor speed control. 13. DC motor 2 "+", (connection B+). 14. DC motor 2 "-", (connection B-). Pulse-width modulation (PWM) technique that controls the width (in time) of an electrical pulse, formally the pulse's duration, based on modulator signal information. It is used when a digital system needs to control a system that expects an analog signal of varying amplitude. A typical example is a 24V motor, the speed of the rotor can be controlled by changing the voltage from low (0V) to high (24V). The idea from PWM is to pass the rotor always 24V but in discrete pulses as shown in figure 4. Figure 4: A PWM signal with three different duty cycles The advantage of controlling a motor with PWM instead of instead of a real analog signal is that the full torque of the motor can be used. In DC motors, there is a linear relationship between the voltage supplied and the torque obtained from the motor (the higher the voltage, the higher the torque). If small motor is used for moving a loaded car and a small analog voltage is used, the motor will not move, because of the load and friction between the wheels and the floor. If a PWM signal is used, it will give a full kick to the motor for a short time and the motor will rotate Eng. Malek Alkayyali Page 14

4 with its full torque. Speed is controlled by spacing as needed (by adjusting the duty cycle of the signal). It is important here to be noticed that the PWM signal is not used to directly drive the load (motor), it is used to control a switch, for example a power transistor that delivers the needed power to the load. Equipment 1. DC Power Supplies. 2. L298N driver. 3. Wires. 4. Arduino. 5. DC motor. 6. TIP120 and TIP 125 transistors. 7. Diodes. Procedures Part 1: DC motor direction control a- Using 4 transistors: 1) Build an H-bridge using 4 transistors. 2) Calculate the suitable value for the base resistance (i.e. R1, R2, R3, and R4). 3) Connect the DC motor as shown in figure 5. Figure 5: H-Bridge circuit using 4 transistors 4) Connect a 5 V DC power supply to I1 and connect I2 to the ground. Notice the direction of the motor. Eng. Malek Alkayyali Page 15

5 5) Switch off the 5 V power supply and reverse the connection in step 4 and notice the direction of the motor. Part 2: DC motor speed control 1) Connect the circuit shown in figure 6. Function generator 2) Generate a PWM signal with frequency of 1 khz and 10% duty cycle using the function generator. 3) Increase the duty cycle and notice the speed of the motor. Discussion and Conclusions 1- What is the component that can be used instead of transistor in part 1-b? 2- What are diodes (D1, D2, D3 and D4) in the circuit shown in figure 5 called? Why do we use these diodes? 3- Illustrate the principle operation of an H-bridge to control the direction of DC motors? Hint: Use a truth table to show the possible state combinations for the switches. Eng. Malek Alkayyali Page 16