Servos? Just what is a servo (or a servomotor or servo mechanism, as they

are sometimes called)? Is that a year’scollection of this magazine? Most of uswho have built robots have used one ormore of these in our creations, but not allrobots use servos. Most of the larger vari-eties of robots don’t use servos thoughthey might employ shaft encoders to provide some sort of positional feedbackto a controlling microcontroller or computer. Most combat robots (like theones that seem out of control) don’t useany form of them, so why do so manyexperimenters utilize them?

Who would have ever thought thatthese small plastic boxes would have hadsuch an impact on experimental robotics? I remember playing with a fourchannel R/C system years ago, trying tofigure out how I could use it in a robot.Most of my robots were usually ratherlarge and the tiny servos could do littlemore than move small ‘special effects’appendages. Cute ‘decorations’ reallyserved no useful function, so I decided to

hack one to see what I could do with it.I believe that first thing I made was

a linear actuator. Pulling the 4.7K potout, cutting off the stops from the output gear, I attached a 25 turn leadscrew and a 25 turn 5K trim pot (in theplace of the other one) to the outputshaft and had an amazingly powerfulpush-pull actuator. Other experi-menters in our robotics group wereattaching them to arm and leg joints,and driving the servos with 555/556timer circuits or 6502 microprocessors,and a few started to use them as drivemotors for small robot’s wheels.

Typical Servos Usedin Robotics

The three servos shown in Figures1a, 1b, and 1c are just a tiny fraction ofthe many types, torque capacities, sizes,and weights available from the manymanufacturers today. Servos are quiteoften the only motive force of manyexperimenter’s robots. Most of the

beginner’s kits from Parallax and othersuse similar servos in small robots.

Tabletop robots can make use ofthe little motor/gearbox to drive a set of wheels and the associated electronics to receive the pulse trainsfrom a microcontroller and convertthem to drive signals. This is a cheapand effective way to get a robot designfrom a few sketches to a workingmachine in a few hours.

As robot experimenters, we thinkof those little black boxes that were originally developed for model airplanes as the only ‘servo’ that we’refamiliar with. Many of us have boxes ofthem; some hacked, some in pieces,and some actually in one piece.

Servo FeedbackWith the advent of specialized ICs

and electronics, modern servos haveemerged as marvels of mechanics andelectronics. Servos have been used inindustrial applications for years, long

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SERVOSb y T o m C a r r o l l

FIGURE 1a. JR servo. FIGURE 1b. HiTec robot servo. FIGURE 1c. Futaba coreless servo.

SERVO 12.2007 79

80 SERVO 12.2007

before model aircraft found them usefulto move various surfaces to change thedirection of flight. Newer applicationsare popular for CNC machine tool use.

Figure 2 shows three servos usedto move the three axes of a millingmachine by Servo Products. Way backin 1787, James Watt used a servo-likedevice — the flyball governor — to reg-ulate the speed of his steam engines.Figure 3 from the cover of a 1952Scientific American Magazine shows aclassic drawing of the flyball governor.It certainly was not what we think oftoday as an electrical/electronic servo,but it could be set in different positionsto control the speed of a steam engine.

The revolving set of balls was directly connected to the engine’s outputshaft and as the speed increased, centrifugal force caused the balls to moveoutward, pulling down the upper ringand connected lever. As this ring moveddownward, it would slowly shut downthe flow of steam by moving a valve, thusslowing the engine and revolving balls.

At one point, a stable speed wasdeveloped. By manually changing thedistance between the ring and wherethe valve cut down the steam flow, onecould set the engine’s speed whereverdesired. A relief valve was set to openat a specific pressure, thus preventingan exploding boiler.

No, this certainly is not a typicalservo that we’re familiar with, but it didutilize feedback to control a machine.

No 1.0 to 2.0 millisecond pulses weresent remotely to Watt’s engine to control speed, just a simple mechanicaladjustment by a human operator.

What is a Servo?Just like the definition of a robot is

so different to so many people, a servohas many definitions. Allow me to pres-ent four definitions of the term servothat I found at random through Google:

A servo is: “An electromechanicaldevice that uses feedback to provideprecise starts and stops for such functions as the motors on a tape driveor the moving of an access arm on adisk (PC Magazine).”

A servo is: “An automatic deviceused to correct errors in the operationof machinery, used in satellite-trackingsystems, power-steering systems onsome cars, and to control robots andkeep ships on course (encyclopedia).”

A servo is: “A small mechanisminside the RC vehicle, the servo is a devicewith a motor, gears, and circuits that con-trols things like steering and speed. Atypical RC car has a steering servo tomake the wheels turn and a speed con-trol or throttle servo to make it go fasteror slower. Other types of servos may bepresent to control other functions (radio-control car enthusiast’s definition).”

A servo is: “An electro-mechanicaldevice that is used to convert thereceived signal into mechanical move-ment. Servos are used to move controlsurfaces, throttles, retractable landinggear, or auxiliary functions (model airplane enthusiast’s definition).”

Servo is: “The name of a greatrobot experimenter’s magazine.” (Sorry,I just had to put that in.) If you Google‘servo,’ you’ll find most definitions andhits are about the model airplane types.

How Does a ServoWork?

Are you really any closer to knowing just what a servo is? So manyarticles in this magazine (includingsome of mine) have gone over how atypical model aircraft servo works. Themore popular and certainly cheapermodels utilize a pulse width modula-tion pulse train from the R/C receiver.

The pulse train consists of 50 to 60pulses per second with each pulsebeing one to two milliseconds long,though experimenters have used 0.8 to2.2 ms pulses to drive the servo furtherthan the typical 90 to 120 degrees oftravel. A shorter series of pulses willdrive the servo’s output shaft onedirection, and the longer pulses willdrive the other way — with positions inbetween for pulses closer to 1.5 ms.

In these older servos that havebeen used for years, there are threewires to the servo: a signal wire (for thepulse train) that can be a number ofcolors; a 4.8 to 6 volt power wire thatis usually red; and a ground wire that isusually black or brown. Note that thereis no output wire to inform an operatoror microcontroller just where theservo’s shaft is positioned.

Early Model AircraftServos

One of the first R/C systems that Iused was by Kraft. Figure 4 shows an ear-lier analog Kraft system with three servosmounted in the airplane, lying behind the transmitter and receiver in the foreground. Back in the ‘80s, several ofus from the Robotics Society of SouthernCalifornia were invited down to the Kraft

FIGURE 3. Flyball governor on thecover of Scientific American.

FIGURE 2. Three axis milling machineset up by Servo Products.

plant in Vista, CA and were given a tourof the facility. The guy leading us aroundthe Kraft facility gave us a lot of servos,receivers, and battery packs just for goodwill; maybe he saw that the end wasnear. Futaba from Japan was starting toreally hurt the US manufacturers andKraft’s days seemed numbered.

Experimenting with them at home,I found the Kraft servos to be quite wellmade. I also had an old Heathkit R/Csystem that I built that used two PS-4servos made by Orbit (remember kits?).Kraft later came out with the smallerKPS-12 servos that some people I knewbuilt into robot joints for walkers. I laterbegan to frequent the Hobby Shack(now Hobby People) in Fountain Valley,CA and found that Futaba and HS’Cirrus line of R/C equipment to be a lotcheaper for my R/C projects.

One of my first R/C robots for amovie used a Hobby Shack AeroSportfour channel system with two Vantecspeed controllers for the two wheelsand two very large Cirrus servos for thetwo arms. I used coil springs to compen-sate for the arm’s weight and the littlerobot could easily pick up over a pound.

Futaba took the lead severaldecades ago and is still one of the morepopular R/C systems with a full line ofservos for all applications, including servos designed specifically for robots.HiTec of Korea also has a line of servosspecifically designed for robots, as doesthe Robotis Bioloid line of Dynamixelservos (actuators), also from Korea.

Servo SelectionYou may be wondering just what

type of servo that you’ll need for yourproject. For economy’s sake, you canstart with the cheaper analog servoswith a three pole cored motor, plasticgears, and bushings for the shaft. Thesewill work great for almost all applica-tions where you need to study the basicsbefore advancing to your final design.

The next step for tougher applica-tions is to buy a metal geared servowith ball bearings on the main shaft.Coreless motors have quicker changesin speed over the three and five polecored motors. The most advanced arethe digital servos with an embeddedmicrocontroller to deliver a greater

number of PWM pulses to the motorfor quicker response, greater accuracy,and torque with less deadband. Theydo draw a bit more power to operate,but that is usually not too much of aconcern for robot builders.

Of course, servos vary widely intheir torque, weight, and size. The CirrusCS-3 Micro Joule SX servo weighs onlythree grams (its four channel receiverweighs a bit less), yet it only has sevenoz. in. of torque (see Figure 5). Monsterservos can weigh over a pound and putout 10 foot pounds of torque or more.It all depends on what you need.

This single paragraph certainly cannot narrow down the right servofor your application; you need to go to the Internet or to manufacturer’swebsites and do some research.

Servo Feedback vs.Feedback to aMicrocontroller

This magazine takes its name from

these devices that so many of us haveused in our robots for years, yet servosoffer no built-in intelligence. They onlytake commands from a microcontrolleror R/C receiver and move to a certainpoint and stop. But hack the little suckers and you have an intelligentdrive motor of sorts.

After reading my August columnon Robot Arms, Alex Dirks ofCrustCrawler wrote SERVO concerningwhat he felt were incorrect statementsthat I made concerning servos used inmany experimenter’s robots — thetypes used with radio controlled modelairplanes to move various wing andrudder surfaces. He referred to the following statements that I made concerning their use with robot arms:

“The advantage of using R/C servos is the positional feedback.“Potentiometric feedback, as in R/C ser-vos, allows the controlling computer toknow where each joint is positioned.”

Alex countered with the following:

FIGURE 4. Early Kraft radio.FIGURE 5. Cirrus CS-3 Micro

Joule SX servo.

FIGURE 6. Robotis Dynamixel AX-12. FIGURE 7. Robotis Dynamixel RX-28.

SERVO 12.2007 81

“There are no feedback mechanismsbuilt into any standard servo today withthe exception of the AX-12+ (the servofrom Robotis and several others in thatline) and a few specialized servos usedin biped type robots.” Alex knows servos as President of CrustCrawler, abusiness he started six years ago withthe HexCrawler and QuadCrawler.

The products he feels have themost promise today are probably theAX12 Smart arms designed around theRobotis smart servos that I’ll discusslater. However, it is CrustCrawler’s andthe other well-known suppliers ofready-to-roll robots and kits that havesteered the servo manufacturers intodesigns that are made specifically forrobot experimenters.

I felt that the best way to under-stand where he was coming from wasto talk with him personally. “When Italk about servo feedback,” he toldme, “I mean feedback to an externalcontroller. Feedback that is limited tothe servo itself without feedback to anexternal controlling/monitoring devicesuch as a microcontroller or host computer limits the usefulness of theservo motor substantially.” I convincedhim that I was speaking of the feedback of the internal potentiometer

to the internal circuitry, not to the outside world. Its internal feedback potserves only to tell the internal circuitryjust where the servo horn is positioned.

I feel that this is an advantage overthe use of a stepper motor as a steppercan become stuck and the microcon-troller will assume that it still hasmoved the required number of turns. Amicrocontroller connected to a typicalservo will send the appropriate seriesof specific width pulses and the servowill continue to try to move to the rightspot until it is there.

Intelligent ServosAlex feels that the standard servo

of today — whether analog or digital —will soon be phased out for walkingrobots, especially the higher end kitsand ready-builts. The ‘Robot ExclusiveActuator Dynamixel’ from Robotis isone of the most innovative servos tocome out in years. The Robotis line ofrotary actuators (as they call them)have some very good features forrobot experimenters. They certainlycommand a premium price buthumanoid builders will find one featurevery useful in their designs — the abilityto be daisy-chained rather that have

three leads from each of, say, 18 ser-vos leading back to a controller board.

Dynamixel actuators, such as theAX-12+ (Figure 6), speak to each otherthrough a TTL line, and units such as the RX-28 (Figure 7) communicatethrough the popular RS-485 protocol.CrustCrawler has developed the AX12Smart arm that uses the RobotisDynamixel actuators for the arm’s joints.Each servo in the daisy chain is assignedan address for control and feedback pur-poses. Yes, these devices have true feed-back to the controlling microcontroller,such as an Atmel or BASIC Stamp.

Most of the larger manufacturersand dealers of robots and robot kits inSERVO Magazine have numerous styles,costs, and capabilities of servos in theirlineups. Figure 8 shows a line of servosfrom Pololu. The Seattle Robotics Society(SeattleRobotics@yahoogroups.com) hasexcellent sources for hacking and modifying servos as do many othergroup’s sites. R/C and model aircraftsites offer much useful information.

In the two months that I havebeen working on this article off andon, I have come across so much information on the subject of servosand many of the articles are complete-ly contradictory with others. So, don’ttake everything that you read as fact.Talk with fellow robot experimentersand just take one apart and determinehow it works and what you’re going todo with it next. Modern servos are atrue bargain. SV

FIGURE 8. Line of servos from Pololu.

Tom Carroll can be reached via emailat TWCarroll@aol.com.

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