Arduino Application: Speed control of small DC Motors

Transcription

1 Arduino Application: Speed control of small DC Motors ME 120 Mechanical and Materials Engineering Portland State University

2 Learning Objectives Be able to describe the use of PWM for motor speed control Be able to explain the role of a snubber diode Be able to implement PWM speed control of a DC motor Be able to use the H-Bridge with the Sparkfun Tinker Kit to control a small DC Motor Additional references: Experiment 9 in the Sparkfun Tinker Kit experiment guide ME 120 course notes on PWM 2

3 Using a transistor as a high speed switch 3

4 Transistor as a switching device Each Arduino output channel has a 40 ma limit Only current to power a very small DC motor Arduino is not designed as a power supply Maximum current draw for an Arduino is 200 ma Use the Arduino as the brain Let another switching element be the brawn 4

5 Use an NPN Transistor as a switch 2N4401 MMBT4401 C 2N4401 / MMBT4401 E C B E TO-92 SOT-23 Mark: 2X B NPN General Pupose Amplifier This device is designed for use as a medium power amplifier and switch requiring collector currents up to 500 ma. Absolute Maximum Ratings* TA = 25 C unless otherwise noted This device is designed for use as a medium power amplifier and switch requiring collector currents up to 500 ma Symbol Parameter Value Units VCEO Collector-Emitter Voltage 40 V VCBO Collector-Base Voltage 60 V VEBO Emitter-Base Voltage 6.0 V IC Collector Current - Continuous 600 ma TJ, T stg Operating and Storage Junction Temperature Range -55 to +150 C *These ratings are limiting values above which the serviceability of any semiconductor device may be impaired. NOTES: 1) These ratings are based on a maximum junction temperature of 150 degrees C. 2) These are steady state limits. The factory should be consulted on applications involving pulsed or low duty cycle operations. Thermal Characteristics TA = 25 C unless otherwise noted Symbol Characteristic Max Units 2N4401 *MMBT4401 PD Total Device Dissipation Derate above 25 C mw mw/ C RqJC Thermal Resistance, Junction to Case 83.3 C/W RqJA Thermal Resistance, Junction to Ambient C/W *Device mounted on FR-4 PCB 1.6" X 1.6" X 0.06." 5 ã 2001 Fairchild Semiconductor Corporation 2N4401/MMBT4401, Rev A

6 Use Digital I/O pin to switch LED on/off Digital I/O pin LED Digital output

7 Code to control brightness of an LED int LED_pin = 11; // PWM pin LED or motor control void setup() { pinmode( LED_pin, OUTPUT ); } void loop() { int duty, pot_pin=0, reading; } reading = analogread(pot_pin); // read potentiometer duty = map(reading,0,1023,0,255); // rescale to 8-bit duty = constrain(duty,0,255); // be safe analogwrite(led_pin,duty); // set duty cycle In the following examples, the Arduino code does not need to change when the electrical circuit is changed. The Arduino code only needs to used a single digital output pin, which in this code is LED_pin.

8 Use a Transistor to switch LED on/off Digital I/O pin Transistor LED 5V Digital output

9 NPN Transistors as Switches Transistors can be used as switches: By applying relatively small voltage to the base, electrical current will flow from the collector to the emitter. B C NPN E C is the collector B is the base E is the emitter

10 NPN Transistors as Switches When used as a switch, I CE, the current from the collector to the emitter is large compare to I BE, the current from the base to the emitter. B C NPN E C is the collector B is the base E is the emitter I BE I CE

11 What is a snubber diode, and why should I care? 11

12 Simplest DC Motor Circuit Connect the motor to a DC power supply +5V +5V I Switch closed 12

13 Current continues after the switch is opened Opening the switch does not immediately stop current from flowing in the motor windings +5V I + Inductive behavior of the motor causes current to continue to flow when the switch is opened suddenly. Charge builds up on what was the negative terminal of the motor. 13

14 Reverse current Charge build-up can cause damage +5V I Current surge through the voltage supply + Arc across the switch 14

15 Motor Model Simple model of a DC motor: Windings have inductance and resistance Electrical energy is stored in the windings the inductance effect We need a way to safely dissipate electrical energy when the switch is opened after the motor has been running +5V +5V I 15

16 Flyback diode or snubber diode Adding a diode in parallel with the motor provides a path for the dissipation of stored energy when the switch is opened +5V + The flyback diode allows charge to dissipate without arcing across the switch, or without flowing back to ground through the +5V voltage supply. 16

17 DC motor speed control circuit The circuit for DC motor speed control uses the idea from the LED brightness control circuit. Replace the LED circuit with the DC motor and snubber diode +5V motorpin 330Ω 17

18 Motor control with an H-Bridge 18

19 H-Bridges simplifies motor control and enable features not possible with a single transistor An H-bridge Uses logic-level signals to switch higher current power to the motor just like a transistor Allows polarity of motor power, and hence direction of rotation, to be reversed, Includes fly-back diodes (snubbers) Is available in a single IC package Image of L293DNE quadruple half-h motor driver from Sparkfun.com 19

20 Low-current digital I/O signals from an Arduino control the behavior of the H-bridge High current Power Supply +V GND Motor Controller + or + or DC Motor GND Logic power PWM Logic line 1 Logic line 2 Logic line... Low current H-bridge motor controller chip Arduino (or equivalent) 20

21 Basic H-Bridge has four switches S 1 S 3 + S 2 Motor S 4 Note that there are many ways to implement this concept. For example, 24 different designs are shown at 21

22 Closing two switches supplies power that causes motor to turn one way S 1 S S 2 Motor S 4 22

23 Closing two other switches supplies power that causes motor to turn the opposite way S 1 S S 2 Motor S 4 23

24 Using the H-bridge from the Tinker kit Motor + Motor 24

25 Using the H-bridge from the Tinker kit 25

26 Using the H-bridge from the Tinker kit //define the two direction logic pins and the speed / PWM pin const int DIR_A = 5; const int DIR_B = 4; const int PWM_pin = 6; void setup(){ // -- set all motor control pins as output pinmode(dir_a, OUTPUT); pinmode(dir_b, OUTPUT); pinmode(pwm_pin, OUTPUT); } Serial.begin(9600); // Used to display motor speed values // Code continued on next slide 26

27 Using the H-bridge from the Tinker kit // Code continued from previous slide void loop(){ int motorspeed, potpin=3, potval; // -- Set control lines to drive forward digitalwrite(dir_a, HIGH); digitalwrite(dir_b, LOW); // -- Read potentiometer to set PWM for motor speed potval = analogread(potpin); motorspeed = map( potval, 0, 1023, 0, 255); motorspeed = constrain( motorspeed, 0, 255); analogwrite(pwm_pin, motorspeed); } Serial.print(potval); Serial.print(" "); Serial.println(motorSpeed); 27

28 Next Steps After getting the basic code to work Find the minimum PWM setting that will make the motor spin Use the minimum PWM setting in the constrain() command Make a servo sweep back and forth while the DC motor is running Add a button to change direction of the motor 28