WEEK 12 (11 - 15 Oct 2010)

RC Servos Motor

A "servo" is a generic term used for an automatic control system. It comes from the Latin word "servus" - slave. In practical terms, that means a mechanism that you can set and forget, and which adjusts itself during continued operation through feedback. The word "servo" really means an RC (remote control) servo motor. This is a small box designed for use in hobby airplanes and cars. Inside this box is a complete servo system including: motor, gearbox, feedback device (pot), servo control circuitry, and drive circuit. RC servos normally have 3 wires: +v, ground, control. The control signal is a pulse that occurs at about 50 Hz. The width of the pulse determines the position of the servo motors output. This would be easy to control with a digital controller such as a Basic Stamp. Most will run on 5-6 volts and draw 100-500ma depending on size. The control circuitry to perform good servo of a DC motor is much more complex than the circuitry that controls a stepper motor.

 RC servo controlling

Servo motors are small, compact and quite cheap. The servo motors itself have built in motor, gearbox, position feedback mechanism and controlling electronics. The servo motor can be controlled to move any position just by using simple pulse controlling. 

Figure 2.4: The rotation of servo motor

Servo motors have three wire interfaces for controlling and power supplying. The wires are colored using following color code:

Ø  BLACK     Ground

Ø  WHITE     Control pin

Ø  RED       +4.8V power supply (+5V works well in this)

Controlling of the servo motors is used using pulse controlling. The control pulse is positive going pulse with length of 1 to 2 ms which is repeated about 50-60 times a second. The details can be check in the figure below:

                         ____                               ____      _+4.8V

                        |    |                             |    |                  

                        |    |                             |    |

                    ____|    |_____________________________|    |____  _GND

                        |<-->|

                        1.2 ms

                               |<-------------------------------->|

                                      18-25 ms

Sending 1 ms pulses sets the servo to one end position and sending 2 ms pulses sets it to the other end position. Sending 1.5 ms pulse sets the servo motor to the center position. The controlling scheme is very easy to implement with some electronics.  

RC servos are hobbyist remote control devices servos typically employed in radio-controlled models, where to provide actuation for various mechanical systems such as the steering of a car, the flaps on a plane, or the rudder of a boat.

                                                                                               

Figure 2.6: Small R/C servo mechanism
1.Electric motor
2.Position feedback potentiometer
3.Reduction gear
4. Actuator arm

 RC servos are composed of a DC motor mechanically linked to a potentiometer. Pulse-width modulation (PWM) signals sent to the servo are translated into position commands by electronics inside the servo. When the servo is commanded to rotate, the DC motor is powered until the potentiometer reaches the value corresponding to the commanded position. The servo is controlled by three wires: ground (usually black/orange), power (red) and control (brown/other color). This wiring sequence is not true for all servos, for example the S03NXF Std.

Servo is wired as brown (negative), red (positive) and orange (signal). The servo will move based on the pulses sent over the control wire, which set the angle of the actuator arm. The servo expects a pulse every 20 ms in order to gain correct information about the angle. The width of the servo pulse dictates the range of the servo's angular motion.

 

2.4.1    Driving RC servos

As has already been mentioned, all RC servos have three connections: power (positive), power (ground or negative), and the controlling signal. The interesting part is the control signal. An RC servo motor doesn't just run when we give it power. It's an intelligent device, and we must tell it what we want it to do. We need something that drives the servo with that control signal.

2.4.2    Powering RC Servos

Most servos require a power supply between 4.8V and 6.0V. The higher the voltage, the faster the servo will move and the more torque it will have.

2.4.3    Theory of Driving RC servos

The servo is controlled by a series of pulses, wherein the length of the pulse indicates the position to take.

pulse width	angle	comment
0.6m Sec	-45 degrees	minimum pulse length
1.5m Sec	0 degrees	center position
2.4 mSec	-45 degrees	maximum pulse length

Table 2.1

Increasing the pulse width by 10 µSec results in about a degree of movement on the output shaft.

·         These numbers are nominal, and vary slightly between manufacturers and models. The rate at which these pulses are sent isn't terribly important - only the width of the pulse.

So, the short story is, if we can make a series of electrical pulses, we can rotate the servo shaft through a range of 90 degrees. And that 90 degree range of rotation can open and close the jaw of a skull, move eyeballs left and right, point a finger, or do all sorts of creepy animation.