Then and Now: Servos

NOW Then and SERVOS by Tom Carroll ervos? Just what is a servo (or a servo hack one to see what I could do with it. beginner’s kits from Parallax and others Smotor or servo mechanism, as they I believe that first thing I made was use similar servos in small robots. are sometimes called)? Is that a year’s a linear actuator. Pulling the 4.7K pot Tabletop robots can make use of collection of this magazine? Most of us out, cutting off the stops from the the little motor/gearbox to drive a who have built robots have used one or output gear, I attached a 25 turn lead set of wheels and the associated more of these in our creations, but not all screw and a 25 turn 5K trim pot (in the electronics to receive the pulse trains robots use servos. Most of the larger vari- place of the other one) to the output from a microcontroller and convert eties of robots don’t use servos though shaft and had an amazingly powerful them to drive signals. This is a cheap they might employ shaft encoders to push-pull actuator. Other experi- and effective way to get a robot design provide some sort of positional feedback menters in our robotics group were from a few sketches to a working to a controlling microcontroller or attaching them to arm and leg joints, machine in a few hours. computer. Most combat robots (like the and driving the servos with 555/556 As robot experimenters, we think ones that seem out of control) don’t use timer circuits or 6502 microprocessors, of those little black boxes that any form of them, so why do so many and a few started to use them as drive were originally developed for model experimenters utilize them? motors for small robot’s wheels. airplanes as the only ‘servo’ that we’re Who would have ever thought that familiar with. Many of us have boxes of these small plastic boxes would have had Typical Servos Used them; some hacked, some in pieces, such an impact on experimental in Robotics and some actually in one piece. robotics? I remember playing with a four channel R/C system years ago, trying to The three servos shown in Figures Servo Feedback figure out how I could use it in a robot. 1a, 1b, and 1c are just a tiny fraction of Most of my robots were usually rather the many types, torque capacities, sizes, With the advent of specialized ICs large and the tiny servos could do little and weights available from the many and electronics, modern servos have more than move small ‘special effects’ manufacturers today. Servos are quite emerged as marvels of mechanics and appendages. Cute ‘decorations’ really often the only motive force of many electronics. Servos have been used in served no useful function, so I decided to experimenter’s robots. Most of the industrial applications for years, long FIGURE 1a. JR servo. FIGURE 1b. HiTec robot servo. FIGURE 1c. Futaba coreless servo. SERVO 12.2007 79 A servo is: “An electro-mechanical device that is used to convert the received signal into mechanical move- ment. Servos are used to move control surfaces, throttles, retractable landing gear, or auxiliary functions (model airplane enthusiast’s definition).” Servo is: “The name of a great robot experimenter’s magazine.” (Sorry, I just had to put that in.) If you Google ‘servo,’ you’ll find most definitions and hits are about the model airplane types. How Does a Servo Work? Are you really any closer to FIGURE 2. Three axis milling machine FIGURE 3. Flyball governor on the knowing just what a servo is? So many set up by Servo Products. cover of Scientific American. articles in this magazine (including some of mine) have gone over how a before model aircraft found them useful No 1.0 to 2.0 millisecond pulses were typical model aircraft servo works. The to move various surfaces to change the sent remotely to Watt’s engine to more popular and certainly cheaper direction of flight. Newer applications control speed, just a simple mechanical models utilize a pulse width modula- are popular for CNC machine tool use. adjustment by a human operator. tion pulse train from the R/C receiver. Figure 2 shows three servos used The pulse train consists of 50 to 60 to move the three axes of a milling What is a Servo? pulses per second with each pulse machine by Servo Products. Way back being one to two milliseconds long, in 1787, James Watt used a servo-like Just like the definition of a robot is though experimenters have used 0.8 to device — the flyball governor — to reg- so different to so many people, a servo 2.2 ms pulses to drive the servo further ulate the speed of his steam engines. has many definitions. Allow me to pres- than the typical 90 to 120 degrees of Figure 3 from the cover of a 1952 ent four definitions of the term servo travel. A shorter series of pulses will Scientific American Magazine shows a that I found at random through Google: drive the servo’s output shaft one classic drawing of the flyball governor. direction, and the longer pulses will It certainly was not what we think of A servo is: “An electromechanical drive the other way — with positions in today as an electrical/electronic servo, device that uses feedback to provide between for pulses closer to 1.5 ms. but it could be set in different positions precise starts and stops for such In these older servos that have to control the speed of a steam engine. functions as the motors on a tape drive been used for years, there are three The revolving set of balls was or the moving of an access arm on a wires to the servo: a signal wire (for the directly connected to the engine’s output disk (PC Magazine).” pulse train) that can be a number of shaft and as the speed increased, colors; a 4.8 to 6 volt power wire that centrifugal force caused the balls to move A servo is: “An automatic device is usually red; and a ground wire that is outward, pulling down the upper ring used to correct errors in the operation usually black or brown. Note that there and connected lever. As this ring moved of machinery, used in satellite-tracking is no output wire to inform an operator downward, it would slowly shut down systems, power-steering systems on or microcontroller just where the the flow of steam by moving a valve, thus some cars, and to control robots and servo’s shaft is positioned. slowing the engine and revolving balls. keep ships on course (encyclopedia).” At one point, a stable speed was Early Model Aircraft developed. By manually changing the A servo is: “A small mechanism Servos distance between the ring and where inside the RC vehicle, the servo is a device the valve cut down the steam flow, one with a motor, gears, and circuits that con- One of the first R/C systems that I could set the engine’s speed wherever trols things like steering and speed. A used was by Kraft. Figure 4 shows an ear- desired. A relief valve was set to open typical RC car has a steering servo to lier analog Kraft system with three servos at a specific pressure, thus preventing make the wheels turn and a speed con- mounted in the airplane, lying behind an exploding boiler. trol or throttle servo to make it go faster the transmitter and receiver in the No, this certainly is not a typical or slower. Other types of servos may be foreground. Back in the ‘80s, several of servo that we’re familiar with, but it did present to control other functions (radio- us from the Robotics Society of Southern utilize feedback to control a machine. control car enthusiast’s definition).” California were invited down to the Kraft 80 SERVO 12.2007 plant in Vista, CA and were given a tour of the facility. The guy leading us around the Kraft facility gave us a lot of servos, receivers, and battery packs just for good will; maybe he saw that the end was near. Futaba from Japan was starting to really hurt the US manufacturers and Kraft’s days seemed numbered. Experimenting with them at home, I found the Kraft servos to be quite well made. I also had an old Heathkit R/C FIGURE 5. Cirrus CS-3 Micro system that I built that used two PS-4 FIGURE 4. Early Kraft radio. Joule SX servo. servos made by Orbit (remember kits?). Kraft later came out with the smaller number of PWM pulses to the motor these devices that so many of us have KPS-12 servos that some people I knew for quicker response, greater accuracy, used in our robots for years, yet servos built into robot joints for walkers. I later and torque with less deadband. They offer no built-in intelligence. They only began to frequent the Hobby Shack do draw a bit more power to operate, take commands from a microcontroller (now Hobby People) in Fountain Valley, but that is usually not too much of a or R/C receiver and move to a certain CA and found that Futaba and HS’ concern for robot builders. point and stop. But hack the little Cirrus line of R/C equipment to be a lot Of course, servos vary widely in suckers and you have an intelligent cheaper for my R/C projects.

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