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 a transistor as a high speed switch Be able to build a breadboard circuit that uses a transistor as a high speed switch Be able to explain the role of a snubber diode Be able to implement PWM speed control of a DC motor Additional references: Circuit 12 (p. 64) in the SIK manual 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 This device is designed for use as a medium power amplifier and switch requiring collector currents up to 600 ma 5

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

7 Use Digital I/O pin to switch LED on/off Digital I/O pin LED

8 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 use a single digital output pin, which in this code is LED_pin.

9 Use Digital I/O pin to switch LED on/off Use of a potentiometer Digital I/O pin LED Pot. Pin (Analog pin A0) Potentiometer

10 Use Digital I/O pin to switch LED on/off Use of a potentiometer Digital I/O pin LED Pot. Pin (Analog pin A0) Potentiometer

11 Use a Transistor to switch LED on/off Digital I/O pin Transistor LED Pot. Pin (Analog pin A0) Potentiometer

12 Use a Transistor to switch LED on/off Digital I/O pin Transistor LED Pot. Pin (Analog pin A0) Potentiometer

13 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.

14 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. C (1) is the collector. B (2) is the base E (3) is the emitter

15 What is a snubber diode, and why should I care? 15

16 Simplest DC Motor Circuit Connect the motor to a DC power supply 16

17 Current continues after the switch is opened Opening the switch does not immediately stop current from flowing in the motor windings 17

18 Reverse current Charge build-up can cause damage 18

19 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 19

20 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 20

21 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 Pot. Pin (Analog pin A0) DC motor flyback diode Potentiometer Motor Pin (Digital pin ~11) transistor 330Ω 21

22 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 Pot. Pin (Analog pin A0) DC motor flyback diode Potentiometer Motor Pin (Digital pin ~11) transistor 330Ω 22