NEW Spektrum AR635 Receiver with AS3X - Page 3 - RCCanada - Canada Radio Controlled Hobby Forum
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Old 04-25-2013, 08:51 PM   #21
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I am: Robert W
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Re: NEW Spektrum AR635 Receiver with AS3X

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Originally Posted by polix View Post
digital servos only? really? wow.... what am i made of money? i've used various flight stabilizers and they all work good, this is the first one that's insisted on digital servos. For that i'll just get some good analog servos and put the extra money in a good flight stabilizer.
[FONT="Comic Sans MS"]As I bought an extra Spectrum AR635 A3X receiver to use in my the Acromaster or the Carbon-Z I was puzzled as to why the AR635 manual said you needed to use “Digital Servos” because you will find analog servos that have better speed and torque ratings than some digital servos; so why not get one of them over a more expensive slower digital? So I did some research. Phew! I didn’t think the humble little RC servo would that complicatedly picky (did not fully appreciate this before)

In a nutshell a digital servo has all the same parts as an analog servo, even the three-wire plug that plugs into the receiver is the same. The difference is in how the pulsed signals are sent to the servomotor.

In analog servos the speed & torque is controlled with electric pulses of varying length every 20 milliseconds.

As you can imagine in an analog servo a short power pulse every 20 milliseconds doesn’t get the motor turning that quickly or allow it enough time to produce much torque. This is the problem with all analog servos; they don’t react fast or produce much torque when given small movement commands or when external forces are trying to push them off their holding position. This area of slow sluggish response and torque is called deadband.

In a digital servo a small microprocessor inside the servo analyzes the receiver signals and processes these into very high frequency voltage pulses to the servomotor. Instead of 50 pulses per second, the motor will now receive upwards of 300 pulses per second. The pulses will be shorter in length of course, but with so many voltage pulses occurring, the motor will speed up much quicker and provide constant torque. The result is a servo that has a much smaller deadband, faster response, quicker and smoother acceleration, and better holding power.

Today much of the control in RC planes, especially with RC helicopters is done with small quick stick movements moving the servo back and forth in very small increments. There are also many changing loads on the rotor system (both main and tail) that are always trying to force the servo off its hold position as well. Don’t forget about the gyro either. The new heading hold gyros or electronic flybarless systems are sending hundreds of small correction changes to the RC servos every second and most of the movement actually happens within the deadband area of an analog servo.

This is not really that big of deal for slow human response times, but becomes a problem for lightning fast gyros and electronic flybarless systems or advanced 3D pilots.

So if I want to use the AR635 (without problems) in either the Acromaster or the Carbon-Z I will have to buy new digital servos for these planes & they are not cheap plus this increase in speed, torque, and holding power does come with a small disadvantage. Power Consumption!

One last thing we need to remember those good-looking analog servo specifications are given at full stick movement when the servo has ramped up to full speed and torque. The slower spec digital servo in this case will still provide much more speed and torque where it is needed most during small, fast command inputs.

EVEN AFTER MY RESEARCH I AM STILL NOT CONVINCED THAT IT IS MANDATORY TO USE DIGITAL SERVOS WHEN YOU CONSIDER SLOP THROUGHOUT WHOLE CONTROL SYSTEM – Slop in the control surface horns/clevises – Slop in the pushrods and Clevises – Delay in the plane responding to directional changes the actual slop contribution by the analog servos must be a very small percentage.:confused
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