analogue -> Digtal Servos what the difference

[arshia]

I had a look at a servo board of a analogue servo and a digi servo.

The circuit seems to be the same. :eek:

So what makes a digi a digi compared to analogue?

[vintage1]

Pretty sure all the digital servo does is to take the incoming PWM and use it to generate servo signals using a microcontroller, so that you get continuous power fed to the servos, rather than a blip every time the channel input comes along.

This makes them faster and more powerful.

[arshia]

like i said its the same as analogue servos. The boards are nearly identical. Analogue servos are driven using a micro too which has confused me some what.

There must be something in the programming that makes them different :confused:

[vintage1]

like i said its the same as analogue servos. The boards are nearly identical. Analogue servos are driven using a micro too which has confused me some what.

There must be something in the programming that makes them different :confused:

Its not really a micro. Its an ASIC ...what it does, is take the pot signal, and put that into a monostable.

When the input pulse comes in, that monistable is triggered, and depending on wheter the input pulse is longer or shorter than the monostable, the difference - a VERY short pulse - is 'stretched', and used to drive the motor in one direction or the other.

In a digital servo I magine the input pulse is taken and turned into a digital number, and an ADC picks up the pot signal, the two are compared, and the motor turned on FULL until the two match, or the input signal timnes out..(loss of signal) ...so the motor is more or less full on or full off all the time, whereas in the analogue servo it gets little puluses every 50ms or so.

If you like the 'pulse stretcher' is repalced by a digital switch...its a lightly better way of doing a very similar job. The net result is that the servo as I said, gets full power for more time. Alowing a more powerful servo to be built in a smaller footprint.

[Acetronics]

Hi,Vintage

You've just forgotten a digital servo can give its full torque without positionning error ... THE main difference.
also more sophisticated position control than the conventionnal PID regulators = faster, but not oscillating ...

I think there's already a thread about a digital servo - amp ... on this same forum, XTAL wrote it ( the thread ...) !!!
It is based upon a 12F675 or 683, but alas ... a PID algorythm has been implemented, so it's not the perfect thing at this time.

see : http://www.rcgroups.com/forums/showthread.php?t=475518

Alain

[pmackenzie]

An analog servo is not based on a PID controller. It is just uses proportional feedback with a bit of deadband to stop it from hunting.
Torque is proportional to the position error.
As Vintage pointed out it only provides feedback when it gets a new input pulse, about every 20 msec.

The digital servo can use a much more sophisticated feedback algorithm which allows higher loop gain and a much smaller effective deadband.
Full torque is available for even tiny position errors.

In time all servos will probably be digital. The cost difference between the ASIC and the microcontroller is tiny.

Pat MacKenzie

[gedaso]

So what makes a digi a digi compared to analogue?
The amount of profit the manufacturers get....

[Punkie]

One thing digis do different to analogue servos is they eat power, according to the guys I know who have been using them on their competition pattern aircraft, they use a lot more power. But if you want the extra accuracy they are the business. They wouldn't help someone like me who are just about capable of getting the plane down in the right field but for someone who requires absolute accuracy say a competition flyer they can make the difference. Some you can program the servo itself, things like the width of the dead spot, the throw. These are useful if you are building a plane that requires more than one servo in each control surface, not something you get with many electric aircraft but for large scale you can make sure your servos match when you pair them up.
http://www.futaba-rc.com/servos/digitalservos.pdf
http://www.hitecrcd.com/homepage/product_fs.htm

[andrew b]

The amount of profit the manufacturers get....

Well said!!

If it somehow improves your system AND you're prepared to pay for a marginal increase in performance, then so be it, I guess models never flew well until we got digital servos. :rolleyes:
I prefer servos that are faster than me (which is all of them!!)