Analog vs Digital Servos

Steve Joblin

I know, I know, I know... there have been like 6 billion threads about this, but....

I am reading an article about servos in the Spring 2007 issue of Robot Magazine where they talk about different attributes of servos.· They state that digital servos only require one pulse and that once it receives a command, it will hold that position until a new command is executed.
If I buy a digital servo and modify it for continous rotation, does that mean that I don't have to keep sending pulses every 20ms... just one "Pulsout" type command one time will keep the servo rotating at that speed until a send a new command?
Seems like everyone would be using digital servos if that was the case... any thoughts?

John Abshier

That is true for some digital servos, but not all. For some servos it is a feature that can be set if you have the digital servo programer. This was mentioned a couple of years back in Servo magazine.

John R.

I can vouch for the "single pulse" "set and forget" working on Hitec 5995 servos. I've read and heard confilicting information on other servos, and at some point will be purchasing a couple to test out. Manufacturer's spec pages are kind of vauge. I am almost thinking that this is a "feature" that happens by accident.

With an analog servo, the pulse is received, and the motor "updated" every 20 ms.

With a digital servo, the pulse is received, and the motor is generally updated 6 times or so between signal pulses (300 x per second). Well, the servo has to have a value for these updates, and it will use the last value. I suspect that it just "remembers" the value until it gets another pulse. If it doesn't get one, it just keeps itself in the same position. Wether this is by design, or by accident, and wether it works with "all" digital servos, I don't know. Based on the fact that we "know" that digital servos update (and potentially adjust) the motor position more frequently than the signal pulses are transmitted, it would seem a fair assumption that it shouldn't matter if the next pulse gets there in 20 ms or 2,000,000 ms.

I do know that this seems to be a very "emotional" discussion for some. Personally, I don't know what to think, beyond the fact that at least SOME digital servos do NOT need a pulse every 20ms. I also have seen/heard that this may not be true for ALL digital servos.

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John R.
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