Spitfire222

Hi everyone, I'm writing a paper about the history of the servo, and I was wondering if anyone could provide me with some sources about early servos. Internet links are fine, but I'm mainly looking for non-internet sources. Anyone know of any books/magazines that have information about the history of the servo?

Thanks in advance!

TFF

Go to the vintage threads; they have some good history threads going on, some with people who were involved with the designs.

Spitfire222

Actually, I should have posted this question in that forum! Thanks for the info!

Dsegal

You might try here: http://www.rchalloffame.org/index.html

da Rock

You know about escapements? Their shortcomings were what pushed the development of servos for model airplanes. And once the servo showed up, it's development was somewhat colorless. Mostly just smaller and stronger.

Basically the function was accomplished by escapement, galloping ghost devices, "bricks", and then servos. But just writing about servos will do ok if you don't need a long paper. There were linear, rotary, and analog and digital. Of those 4 differentiations, 3 are still with us.

expo

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Shoot Rich LaGrange, AMA librarian an email.
[link=http://[email protected]]AMA Library[/link]

He will look up all that stuff for you.

dhal22

actually, linear servos are available if you goto a robotics supply website.

david

Spitfire222

Thank you everyone for the information! Da Rock, would it be possible for you to move this thread into to the Vintage R/C forum?

MinnFlyer

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Done

CoosBayLumber

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Actually, Expo...

Rich LaGrange of the AMA was unaware of the DMECo early servo of the 1950's, and that they were still being made. That is until I mentioned it to him couple years ago.

The Howard Bonner servo was in the back of his mind up until about ten years ago.

Wm.

Spitfire222

Thank you MinnFlyer.

HighPlains

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Are you looking for the development of the mechanics, electronics, or both?

otrcman

Could somebody provide directions to the Vintage R/C forum? I'd like to follow this thread but can't find the new location.

Actually, I think there are many significant steps in the evolution of servos. So many improvements that we wouldn't be anywhere near where we are today if the servos hadn't changed so much. Things which come to mind: Feedback elements, types of motors, electronic feedback logic, Center tapped battery packs vs switched power, noise suppression and it's effect on the receiver, etc. A whole world of improvements once you get into it.

The other interesting thing to me is that it seems like the radio pioneers were not necessarily the servo innovators. Makes sense when you think about it the servos are an electromechanical device requiring different expertise than RF transmitters and receivers. Maybe that's why aftermarket servos has been around pretty much as long as we've had radio control.

Dick Fischer

Spitfire222

Both mechanical and electronics. Essentially, who is considered the inventor of our hobby servos, and descriptions of the very first servos used to transform free-flight models in radio controlled ones. (How did they work, etc)

billmod12

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Spitfire, You have opened a can of worms! some memorable and some miserable. Don't worry the answers are here and you will have a full understanding of all before it is over, Like it or not!! You will also wish at times you were there. Cheers Bill

HighPlains

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The history of the servo is an intriguing idea. I assume you are considering only motorized versions, because besides rubber powered escapements, there were a few designs using pneumatics in the late 40’s and early 50’s.

Many of the early servos were home made through the sacrifice of various motorized toys. Commercially produced servos in the reed era were initially controlled by relays that allowed a small signal from a vibrating reed to close the relay, which in turn allowed much higher motor currents to run the servo. These servos were designed to either return to neutral when the signal to the reed quit, or maintain the position that the servo had when the signal stopped. This second type of servo allowed throttle position or elevator trim. So for the advanced systems of the late 50’s and early 60’s, it took two servos mechanically mixed together to operate the elevator. Most of these servos were produced by Bonner, deBolt, and Annco.

While most of the high end systems of the late 50’s and early 60’s were reed controlled, the experimenters such as Walt Good were working on early proportional control through the use of pulsers. As such, the servos were in constant motion, and still relay driven. So it took a lot of time and trouble to set up and maintain. Even so, the “proportional” control was just the time averaged response of the airplane to the wildly moving surfaces, and not very good. Servo centering was usually by springs or rubber bands and air loads on the surfaces.

True proportionals (I define this as servos with a feed back control loop) were initially developed with an analog scheme in the late 50’s. In these systems, the control position was encoded in tones sent to the receiver, and receiver discriminators would convert the tones back to voltages for each servo amplifiers. In a sense, these were FM control schemes sent over AM radio. Since the bandwidth of the signals were somewhat limited, problems with intermodulation and problems with signal drift due to changes in signal strength, battery condition and temperature changes usually meant that each flight required some degree of trimming. The servos also had very limited power to center, since these rudimentary amplifiers had less power as they got closer to the commanded position. While the pilot might compensate when moving away from neutral, the neutral positions were soft. So some of these early analog systems still depended on springs.

The competing “digital” designs in the early 60’s, were based on work done by Doug Spreng. He developed the concept of using the 1 to 2 mS pulse to encode the position information and the servo amplifier that drove the servo from it. At this time, most of these experimenters used modified reed servos like Bonner. The term “digital” only meant that the transmitter signal was either on or off, and the internal control pulses were configured like the simple logic circuits of the era. The servo amplifiers also created a pulse based on the position of the servo to compare to the control signal pulse. There they created the position error pulse (the difference between the internal pulse and the control pulse), and the pulse stretcher that allowed the small error pulse to drive the motor. Although there were a few missteps of other design approaches, the industry pretty much settled on this approach from the mid 60’s on.

At that time, the industry used simple resistor/capacitor time constants in the timing circuits. This worked pretty well if the components were well matched when the circuitry was mostly discreet components. But in the early 70’s, as integrated circuits were introduced the RC time constants gave way to linear charging of capacitors giving a linear time constants. This was noticeable if a linear ramp servo was used with a RC time constant encoder.

The servo amplifiers were pretty much all IC from the early 70’s on. The greatest improvement of all time to servo amps occurred when EK introduced the Transistor Bridge motor drive, which eliminated the center-tapped battery that earlier designs required. Since then, the rather recent digital servo has improved servo torque when the error is small. I designed one for an industrial application about 15 years ago, but never considered it for model applications.

The mechanics have evolved a lot since the 60’s. While Bonner made his own servo for his Digimite system, it was also used by a few other of the early systems. As was Kraft and Orbit. Looking back, I think the first modern servo would have been the Orbit PS-2 mechanics that came out in 1965 or ’66. This servo had a rotary output as well as a linear output. The advantage of using a linear output was that it gave the effect of expo control without any electronics modifications. Most of the companies used mechanics made by Orbit or later Kraft, and usually only offered up two or three sizes.

The big push to make light weight airborne system really heated up in 1969 when Orbit introduced a radio that only tipped the scales at 9 oz. Just a few years earlier, a proportional radio with 4 servos might exceed 25 oz. One of the significant weight reductions occurred when airborne battery packs dropped from 7 cells to 4 cells.

The most enduring servo design must be the Futaba S-30. Or you may have seen it as the S-130, or now the S3151. I don’t know, they have renamed this mechanics so many times in the last 20 years, I’ve lost track.

Now you can buy a servo that weighs less a couple grams up to torque monsters that are around a half pound. Even one that uses a 3-phase motor, though a 3 pole iron core is most commonly used. Upper end servos often have 5 pole or coreless motors. The primary advantages to coreless motors are low inertia and zero cogging. The disadvantage seems to be higher costs, and a tendency to fail when exposed to high operational temperatures, high vibration, and high G loads. But even these problems have been improved over the years.

How’s Zatt? to paraphrase an early servo designer.

Spitfire222

Highplains, you are a Godsend! I cannot thank you enough!

rcacro

There were some manufactrers of servos before the ones listed by Highplains. Two that I know about were sold in the early to mid 50's and were supplied with Rockwood and Schmidt systems? Both of these were reed systems using relays to contol the servos which had small motors, a gearbox, limit and neutralizing contracts operated by a cam on the output shaft of the servo. Not sure who actually maufactured the servos.
John W.
Scottsdale, Az

jaymen

I amswered this guys e-mail to me, he's writting a paper for school, actually we are writting his school paper for him as I'm sure he's going do some cut and pasting using our replies!

Servos go way back, the ancient Greeks had robotics and mechanically opperated devices that used primitive eescapements, and cog driven mechanics, the forerunners of modern servos. Nicolai Tesla had the autrotron? it was developed around 1900 and was a radio controlled vehicle, it has very advanced and ingeniuose servos. The military had servos long before they were introduced to the hobby, we basicall have cheap commercial versions of them today for hobby usages.
Walt Good probably deserves alot of credit for the first servos that were used in his TTPW proportional system.
Hershel Toomin gets credit for the first r/c analog servos that were commercially availlable and were sold as Solitronics Space Control.
Doug Spreng made the first PPM digital proportional servo for hobby usage.