16F Servo Motor Example

How Servo Motor Control Works

Also known as â€œclosed feedbackâ€ systems, Servo motors come with a control circuit, which senses if motor mechanism is in desired location and if not it continuously corrects an error until motor reaches the proper point. Servo's usually come in small plastic packages like the following, but keep in mind there is a whole system inside; motor itself, gears and motor driving and control circuit.

Servo Motor parts

Servo's usually have great torque because of their gearing. The motor speed decreases, but their torque increases, allowing them to drive high loads. The closed feedback loop mentioned earlier is actually an internal potentiometer that is connected to a mechanical shaft to sense the angle of turn. The signal from the potentiometer provides data to the control circuit to ensure the motor is at the desired angle.

Most servos are designed to operate within a limited range, e.g. 90Â° or 180Â°, but can be modified for continuous rotation. This accurate control of rotation and torque thanks to the gearing makes the servo a great device to include in any mechanical-robotic type project. Most servo's operate from 4.8 to 7.2V DC.

Servo control signals

Servo motor shaft is positioned with pulse width modulated signals (PWM). So all servos comes with three wires (Power, Ground and Control). Usually in hobby servos with rotation angle 90Â° signal width vary between 1 and 2ms. If pulse is more wide rotation continues until reaches mechanical limits. This signal is applied to the control line of the servo. Refer to your datasheet for more information, but for the example of servos in this tutorial, our servo has a maximum rotation of 180Â° and operates with a control signal from 1mS to 2mS.

With this in mind, a signal with a 1.5mS high pulse will tell the servo to go to half way, or a signal at 2mS will make the servo go to 100% deflection. This signal must be sent to the servo once every 1/50th of a second (50Hz), allowing your program to perform many other functions in between.

In Proton+ the Servo command does just as we have described above. By using it we can tell the servo to go to any position, and stay there. Some servo's have different pulse requirements for different angles, but most common RC Servos will operate from 1mS to 2mS.

Once a servo is in position, the potentiometer feedback to the servos internal control circuit will ensure that the servo does not drift from the desired position, (providing they are refreshed with the position every 50Hz or 20mS), if there is too much time between the control signals, the servo will drift, and to fast a signal will make the servo chatter. Refer to the datasheet of your servo for its optimal refresh rate.

Servo's have 3 wires, each will be for; +Ve supply, -Ve Supply and Signal. Here's a basic program to interface with Servo's (For more detail on specific commands, see the Proton+ Help File)

Device = 16F877
Xtal = 4

Dim Servo_Position as Word

LCD_DTPIN = PORTB.4
LCD_RSPIN = PORTB.2
LCD_ENPIN = PORTB.3
LCD_INTERFACE = 4
LCD_LINES = 2
LCD_TYPE = 0

All_Digital = True

Delayms 150
Cls

Servo_Position = 1500

Main:

	 Repeat

	 	 SERVO PORTA.0, Servo_Position	 	 
		 DelaymS 20
		 
		 Inc Servo_Position
		 
		 Print At 1, 1, "Servo = ", Dec4 Servo_Position
		 
	 Until Servo_Position = 2000
	 
	 Repeat

	 	 SERVO PORTA.0, Servo_Position	 	 
		 DelaymS 20
		 
		 Dec Servo_Position
		 
 		 Print At 1, 1, "Servo = ", Dec4 Servo_Position
		 
	 Until Servo_Position = 1000	 
		 
	 
	 Goto Main

Note the PIC's power supply/oscillator are not shown

Click here to watch this circuit in action!

One thing to keep in mind is that servo's generate a lot of noise in your circuit. This can lead to very inaccurate timing and strange things happening. To help stop this, be sure to have a large capacitor (around 100uF) near the output of your servo's +Ve supply, and a 0.1uF Tantalum capacitor as close as possible between the PIC's +Ve and -Ve pins. Optoisolators are great for isolating circuits from each other, and could be used to great effect with servo's.

A great supplier of Servo's is Hobby Engineering