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A library for controlling a Radio Control (R/C) model Servo.
Public Member Functions |
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| Servo (PinName pin) | |
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Create a servo object connected to the specified PwmOut pin.
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| void | write (float percent) |
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Set the servo position, normalised to it's full range.
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| float | read () |
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Read the servo motors current position.
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| void | position (float degrees) |
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Set the servo position.
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| void | calibrate (float range=0.0005, float degrees=45.0) |
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Allows calibration of the range and angles for a particular servo.
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| Servo & | operator= (float percent) |
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Shorthand for the write and read functions.
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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.
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.
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.
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 }
Servos have a drive wheel that is controlled by a PWM coded signal. The servo shown below is a Futaba S3003 which is identical, internally, to the Tower TS53J servo. Radio control servomotors are mass-produced for the hobby market and are therefore relatively inexpensive and consistently available. They are ideally suited for robotics applications. Internally, the servo contains a DC drive motor seen on the left in the image below, built-in control circuitry, and a gear reduction system. They are small, produce a relatively large amount of torque for their size, and run at the appropriate speed for a small robotics drive motor.
The control circuitry of the servo uses a potentiometer (variable resistor) that is used to sense the angular position of the output shaft. The potentiometer is the tall component on the right in the image. The output shaft of a servo normally travels 180-210 degrees. A single control bit is used to specify the angular position of the shaft. The timing of this bit specifies the angular position for the shaft. The potentiometer senses the angle, and if the shaft is not at the correct angle, the internal control circuit turns the motor in the correct direction until the desired angle is sensed. The control signal bit specifies the desired angle. The desired angle is encoded using pulse width modulation (PWM). The width of the active high pulse varies from 1-2 ms. A 1ms pulse is 0 degrees, 1.5ms is 90 degrees and a 2 ms pulse is approximately 180 degrees. New timing pulses are sent to the servo every 20 ms.
Normally, a servo has a mechanical stop that prevents it from traveling move than half a revolution. If this stop is removed along with other modifications to the potentiometer, a servo can be converted to a continuously rotating drive motor for robots. You can find web sites showing how to convert a servo for continuous rotation.
If you have the .1" breakaway header pins with long pins on both sides or even wire wrap pins, you can break off a strip of three pins and plug them into the servo cable connector. The servo cable can then plug directly into the breadboard. This works a bit nicer than putting jumper wires directly into the servo cable connector.
http://www.pololu.com/catalog/product/1065 is seen above. Also available from Sparkfun, http://www.sparkfun.com/products/10158. Many of the right angle header strips will also work.
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All of the RC servos have the three wire connectors, but the different brands move the three pins around and use different wire color codes. Each company does this to try to lock users into their RC equipment. The info on which pin is which can be hard to find. There is even a company that makes conversion cables! Here is a handy pinout table I found with some pictures:
http://www.horrorseek.com/home/halloween/wolfstone/Motors/svoint_RCServos.html#ServoConnectors