Intro to RC Servos

RC servos, originally intended for remote control models, provide a versatile and inexpensive means of implementing "precision animation".

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Here is our collection of information on RC servos...
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Introduction to RC servos

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. Disk drives, for example, contain a servo system insuring that they spin at a desired constant speed by measuring their current rotation, and speeding up or slowing down as necessary to keep that speed.

What is "feedback"? Think about driving a car, and wanting to keep your speed smack on 55 MPH. You are cruising along, and occasionally glance at the speedometer. If your speed is under 55, you press down harder on the gas pedal. If your speed is over 55, you lift your foot. The speedometer gives you feedback - information about how fast you really are going. The brain uses this to decide whether to press down or raise the foot. This is also known as "closed loop" control.

An automobile's "cruise control" is a closed loop system that works similar to the way that you do while driving. You establish a set point, and if the speed is below that, the cruise control presses down on the gas pedal.

The most common consumer-visible servo is that used to operate radio controlled (RC) model planes, boats, and other gadgets. These are small boxes that contain:

The controlling intelligence, in this case the operator of the model, indicates to the servo the position that the output shaft should have. The position-sensing mechanism tells the servo what position the shaft currently has. The control circuitry notes the difference between the desired position and the current position, and uses the motor to "make it so". If the difference in position is large, the motor moves rapidly to the correct position; if the difference is small, the adjustment is more subtle. As for the operator, all he knows is that he moved a slider half-way up, and the rudder on his model plane moved to the center position, and will stay there until he moves the slider again.

RC servos have a huge amount of haunt potential for precision animation. Just imagine a skull head with eyeballs that can track left to right, following guests as they walk by....

Here are a couple of Royal "Titan" servos. 

 

Types and availability of RC servos

There are numerous types of servos. They differ in their precision, speed, and strength - all of which are reflected in price. This is as it should be - you shouldn't have to pay for a ball-bearing, metal-gear, high-precision servo, when your application can get by with a cheaper kind.

The radio controlled model market is evidently a lucrative one - there are numerous companies making RC servos: Airtronics, Cirrus, FMA Direct, Futaba, Hitec, JR, Ko Propo, Multiplex, Tower Hobbies.

Although the array of manufacturers may seem daunting, it is only a good thing. It gives you lots of choices, and lowers the prices. 

 

Servo Connectors

The good news is that all of these servos feed off of the same electrical connections: roughly 6VDC to power the thing, and a PWM pulse stream to indicate position.

The bad news is that just about everybody formerly used a different connector layout for these same signals. If you are using an Airtronics, you can't just plug a Futaba in its place. Luckily, this silliness has been noticed - there is an entire industry that revolves around replacing the various connectors with a plug-compatible arrangement. And, as time marches on, the various brands slowly converge on a standard.

If you need to mix and match different types of servos, this connection information may help:
brand universal positive wire negative wire signal wire
KO Propo NO Red, outside pin Black, middle pin Blue or White, inside pin
Airtronics
(old)		 NO Red Black, middle pin Black, White, or Blue
Airtronics/Sanwa T
(old)		 NO Red, outside pin Black, middle pin White or Yellow, inside pin
Airtronics/Sanwa Z YES Red, middle pin Black, outside pin Blue or Yellow, inside pin
Futaba J NO Red, middle pin Black, outside pin White, inside pin
Hitec S YES Red, middle pin Black, outside pin Yellow, inside pin
Japan Radio (JR) YES Red, middle pin Brown, outside pin Orange, inside pin
Tower Hobbies YES Red, middle pin Black, outside pin White, inside pin
Futaba G
(old)		 NO
Kyosho/Pulsar ? Red Black Yellow
Royal ?

Notes: 

 

Driving RC servos

As has already been mentioned, all RC servos have three connections: power (positive), power (ground or negative), and the controlling signal. We are going to assume that you are smart enough to hook up the power leads to a battery or suitable substitute.

The interesting part is the control signal. An RC servo motor doesn't just run when you give it power. It's an intelligent device, and you must tell it what you want it to do. You need something that drives the servo with that control signal. 

 

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.

Check the specifications before you buy. Example: the Hitec HS-50 Ultra-Micro "feather" runs only on 4.8 volts. 

 

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

Notes:

So, the short story is, if you can make a series of electrical pulses, you 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.

The driving pulse is usually specified as 3-5 Volt Peak to Peak, but I suspect that in many cases you can get by with whatever power the motor is getting. I would avoid using a drive pulse greater than the motor power. 

 

How can I try this out?

If you would just like to play with RC servos and see if they are suitable, you can drive them without need of any kind of computer. All you need is a train of pulses.

This circuit is from Ron Woodward's web page at http://members.aol.com/_ht_a/RonOuul/electronicnotebook.html.

[photo] Ron describes it as follows...

This 555 circuit will provide the signal required to control an RC servo. The output is a positive pulse between about 0.9 milliseconds and 2.1 milliseconds. The off period between pulses is about 40 milliseconds. This can be shortened by reducing the value of the 3.3meg resistor. The B+ voltage should be hooked to the same voltage as the servo. Servo connections are: red=B+, black=ground, and white=input signal. 

 
Here's another take on the 555 version from http://www.repairfaq.org/filipg/RC/F_Servos.html.
[photo] 
 
The 555 seems very popular in this application. The web is filthy with schematics for this kind of thing, all very similar. The web page "http://www.uoguelph.ca/~antoon/gadgets/gadgets.htm" has five of them! Just go to your favorite search engine and punch in "servo 555" to see some variations.

There is a common defect shared by all the single-timer circuits: as you change the pulse width, the frequency changes. Since servoes really only care about the pulse width, this probably isn't a big deal.

Fancier servo drivers use two timers: one counts off the basic pulse rate; the other generates the on pulse, setting the width. This is sometimes done with a pair of 555 chips; a 556; or a 555 driving some other monostable.

One elaborate servo driver is at http://www.electronic-engineering.ch/radiocontrol/circuits/servo-check/checker-fut.html.

[photo] It is described as follows...

This circuit shows a Servo-Checker for JR / Graupner / Futaba-Components (positive impulse). The ZPD2.7 (Zener-Diode 2.7V) at the output limits the positive servo signal to a maximum amplitude of 2.7 Volts ( Futaba-Receiver-Output ). The first Timer 555 creates a negative trigger signal of approximately 45 Hz (a trigger impulse every 22 ms, DO NOT exceed 50 Hz for proper servo function). The second Timer 555 modulates a positive square signal from 0.9 ms to 2.1 ms every 22 ms (Pulse-Width-Modulation). This signal provides the full information for the exact servo position. M is a trimmer for middle-position, P is the potentiometer. 

 
For more information on 555 timing circuits, check out our 555 timer calculator. 

 

Where can I get RC servo-driving chips?

Sending the pulses necessary to drive RC servos is actually easy for just about anything with a computer in it. But it does require some tight programming to make sure the pulses are of the correct width.

Lazy digital haunters might want to give over this job to some sort of co-processor or servo driver chip. These are systems where the CPU sends a character string over a serial port, to the controller, who interprets the command, and generates the necessary pulses.

Here are some makers of such chips:

If you don't want to add another chip, remember that most single-chip microprocessors (such as 68HC11) have an on-chip PWM generator that can drive servos. We are not, technically, doing PWM. We are doing Pulse Width Position Servo (PWPS). But the microprocessor doesn't know or care - he is merely being asked to generate a train of pulses. 

 

Commercial products

If you want to put RC servos to work, but don't feel up to the software or hardware, there are commercial packages out there...

Terror By Design

Terror By Design offers "Sync-It", a gadget that converts sound to servo movement. This looks ideal for use in animating skulls. Indeed, the web site includes a picture of a 42-inch skull animated with this unit.

For $125, you get the Sync-It, a servo, power supply, and connective tissue.

TBD is also coming out with a fancier servo controller soon...

Effective Engineering

Effective Engineering, "http://www.effecteng.com" has a full line of servo gadetry. This includes: 

 

Modification For Continuous Rotation

One popular modification to RC servos sets them up for "continuous rotation". Instead of moving through a range of about 90 degrees, a modified servo spins around and around. This modification is often used by those building their own miniature robots: the servo is turned into a combination motor, gearbox, speed control, and reversing switch.

I won't go into details of how this is done, but I will outline the general principle.

You can see how handy this is to robot builders. Just bolt a couple of modified servos to the bottom of the robot, and put tires on the output shafts. Then just generate a stream of pulses to control speed and direction. 

 

Digital Servos

All RC servos operate according to the instructions of a train of pulses, and could be thought of as digital. But in fact, they are easily and usually built from analog components.

A few companies offer servos that are actually digital inside. Such servos contain a computer chip that measures the width of the incoming pulses, convert the current potentiometer position to digital (or measure position digitally), compute the difference, and drive the internal motor digitally.

For the sake of discussion, the theory of operation is the same. The signals are merely processed in another way. 

 

Further information on RC servos

For more information on RC servos, try the following web sites:
author web site URL content
Matt Verrochi http://people.ne.mediaone.net/mverrochi/servoguide.htm table that compares servo types and brands
filipg@repairfaq.org http://www.repairfaq.org/filipg/RC/F_Servos.html assorted servo info
filipg@repairfaq.org http://www.repairfaq.org/filipg/RC/F_rc-offroad5.html assorted servo info
Hank Hagquist http://rcvehicles.about.com/hobbies/rcvehicles/library/eih/bleih_swiring.htm nice chart of connector pins for various servo makers
Kevin Ross http://www.rdrop.com/~marvin/explore/servhack.htm some servo basics; how to modify a servo for continuous rotation 

 

Related Pages

You may be interested in these related pages:

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