Servo

Table of Contents

  1. Hello World!
  2. Library
  3. Details
  4. Examples

A library for controlling a Radio Control (R/C) model Servo.

R/C Model Servo

Hello World!

Import program
Servo_HelloWorld - main.cpp

00001 // Hello World to sweep a servo through its full range
00002 
00003 #include "mbed.h"
00004 #include "Servo.h"
00005 
00006 Servo myservo(p21);
00007 
00008 int main() {    
00009     for(float p=0; p<1.0; p += 0.1) {
00010         myservo = p;
00011         wait(0.2);
00012     }
00013 }

Library

Import library
Servo - Servo Class Reference

Public Member Functions

Servo (PinName pin)
Create a servo object connected to the specified PwmOut pin.
void write (float percent)
Set the servo position, normalised to it's full range.
float read ()
Read the servo motors current position.
void position (float degrees)
Set the servo position.
void calibrate (float range=0.0005, float degrees=45.0)
Allows calibration of the range and angles for a particular servo.
Servo & operator= (float percent)
Shorthand for the write and read functions.

This library controls a standard R/C model servo using a PwmOut signal, and provides control of the servo between min and max by setting it to 0.0 - 1.0.

As all servos respond differently, you can also use the calibrate function the range, so 0.0 - 1.0 is the maximum range of the servo.

Details

The underlying PwmOut period is set to 20ms, and by varying the pulsewidth (up to a maximum of 0.5ms to 2.5ms) the position of the servo is changed, usually by around 180 degrees. The library lets you calibrate the exact range.

The servo must connect to a PwmOut, and be supplied with a separate power supply, usually in the range 4.5-6.0v.

Warning

A servo requires higher current than the USB port can safely provide, and so it is essential that you power the servo using an external supply, such as a 4xAA (6v) battery pack or an appropriate DC power adaptor.

/media/uploads/simon/servoschematic.png /media/uploads/simon/servophoto.jpg

Examples

Import program
ServoProgram - main.cpp

00001 #include "mbed.h"
00002 #include "Servo.h"
00003 
00004 Servo myservo(p21);
00005 Serial pc(USBTX, USBRX);
00006 
00007 int main() {
00008     printf("Servo Calibration Controls:\n");
00009     printf("1,2,3 - Position Servo (full left, middle, full right)\n");
00010     printf("4,5 - Decrease or Increase range\n");
00011 
00012     float range = 0.0005;
00013     float position = 0.5;
00014     
00015     while(1) {                   
00016         switch(pc.getc()) {
00017             case '1': position = 0.0; break;
00018             case '2': position = 0.5; break;
00019             case '3': position = 1.0; break;
00020             case '4': range += 0.0001; break; 
00021             case '5': range -= 0.0001; break; 
00022         }
00023         printf("position = %.1f, range = +/-%0.4f\n", position, range);
00024         myservo.calibrate(range, 45.0); 
00025         myservo = position;
00026     }
00027 }
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