Servo Resolution:

Lliam20789

New Member
I know this has been brought up before, but there has yet to be a clear answer...

Is there a way to increase pulse resolution; on say, an 08-M 4Hz?
Research shows that you can only get down to 10us blocks? there must be a fairly simple way to increase this, surly the developers didn't restrict us to this crude resolution.

I have recently seen a PIC chip (same as PICAXE 40) running a servo from -2048 to 2048 resolution!

Does anyone know how to at least increase current resolution to degrees using a picaxe?

Thanks,
Lliam
 

hippy

Technical Support
Staff member
Yes, the developers do restrict users to the 'crude resolution' of 10uS ( 150 steps ), but that's usually plenty good enough for the sort of projects used in education and by many other users. There are practical and technical reasons for that, not least the limited memory in smaller PICmicro devices which also have to run PICAXE programs.

The PICAXE has never claimed itself to be a perfect solution for everything, but is a balanced processor offering good ( if not perfect ) performance across a whole range of applications. A dedicated PICmicro will always be able to perform better than a PICAXE.

The best option for highly accurate servo control would be an external servo driver. Using PULSOUT on an over-clocked PICAXE would perhaps be the next best thing.
 

Grant Fleming

Senior Member
.....surly the developers didn't restrict us to this crude resolution.


Maybe they were being a little surly!

Cheers,

Grant
 

BeanieBots

Moderator
Also, remember that the average hobby servo is a mechanical device with a high ratio gearbox that has backlash. The resolution of the PICAXE does not compramise the mechanical resolution of the servo by very much.
 

Wrenow

Senior Member
I know this has been brought up before, but there has yet to be a clear answer...

Is there a way to increase pulse resolution; on say, an 08-M 4Hz?
Research shows that you can only get down to 10us blocks? there must be a fairly simple way to increase this, surly the developers didn't restrict us to this crude resolution.

I have recently seen a PIC chip (same as PICAXE 40) running a servo from -2048 to 2048 resolution!

Does anyone know how to at least increase current resolution to degrees using a picaxe?

Thanks,
Lliam
Llian,

Remember, most hobby servos only have 90-180 degrees of motion maximum. So, you pretty much have 1 degree positional accuracy as it is. Also keep in mind, as mentioned prior, that the backlash, pot resolution, internally programmed deadband, etc. all introduce their own positional errors, if you are going for incredibly overkill positioning accuracy. However, they do great operating multi-thousand dollar jet aircraft models, so think about how much micrometer precision you really need versus what will work.

In other words, if you are designing a surgical robot or a combat laser aiming system for shooting down satellites, you might want to look somewhere other than RC servos.

Cheers,

Wreno
 

Lliam20789

New Member
I understand the issue of backlash but still, it would be nice to say... double the steps. The minimum step size is indeed noticeable on a robotic arm.

Thanks,
Lliam

PS. what does increasing the clock speed do to the servo command?
 

Wrenow

Senior Member
You can increase the resolution, I believe, by increasing the clock speed and using pulsout, but you will need to handle the 20MS pulse frame issue (not difficult, and not terribly critical for many servos, which will accept a shorter pulse frame). Of course, you lose running in the background.

I believe you can also get much finer resolution with a dedicated SSC (Serial Servo Controller Board) to offload the servo routines. The 21 servo controller board, for instance. Tonnes of these out there, including a very nice 8 servo board that is tiny from http://www.pololu.com/products/pololu/0207/.

Check out the docs for resolution, but I believe it allows 5,000 steps of resolution.

Cheers,

Wreno

Wreno
 

BrendanP

Senior Member
Hi Wreno, I've looked at the SSC you mentioned and skimmed through its data sheet. Have you used this thing with picaxe? Im going to order a couple and I just want to be sure Ill be able to get it to work prior to doing so.
 

Wrenow

Senior Member
I have the micro SSC, but have not tried running it off the picaxe yet - I got it to be in stock for MkII or MKIII fire control assistant project. I am still working on the MKI, based on the Picaxe 08M three servo controller board. The only issue that concerns me with the non-uart, lower end Picaxes is the occasional dropped characters I found interfacing to LCDs, but with the x1's I would be less concerned (and you get higher baud rates). At under $20 per, not too bad a budget buster to see if it works in your appplication.

For what it is worth, I love Pololu. I have ordered a lot of custom laser-cut parts from there, and they worked great at a reasonable price. Great for robotics projects, when you need several precisely cut bits.

Cheers,

Wreno
 

BrendanP

Senior Member
I had a look at their site, 3 guys out of MIT started the opperation. Impressive. I also saw their laser services, I have two suppliers here in Melbourne I use, one a laser cutter and another a plastic machining place that has a lot of CNC gear and can machine just about any shape out of plastic.
 

Wrenow

Senior Member
Good site. I actually wanted to machine a relatively small piece of plastic for a proof of concept of a CO2 powered gun breech and valve/loading system for the Warship Combat. Machining the prototype was going to cost around US$200. Figured out, I could do it in slices, and stack them. Pololu cut them for me for $28, engraved O-ring seat and all. By the way, the prototype worked just fine once bolted together.

Cheers,

Wreno
 

Wrenow

Senior Member
Brendan,

Wish I could claim it. I got the idea from the story of the guy at Raytheon who figured out how to mass-produce the highly difficult to machine British designed Magnetron cheaply and easily in WWII. And, he didn't even have a degree of any sort. He was just Raytheon's go-to guy for figuring out how to make things. He was reportedly inspired while stacking up his loose change on the table at dinner.

Go Raytheon. ;-)

Wreno
 

BrendanP

Senior Member
One of the few regrets I have in life is that I didn't do a engineering rather than humanities degree and work on missile research/design. Got to love those Raytheon guided missiles. My dad worked on the Rapier missile, he said it was a good project. (Dud weapon though if its perfomance in the Falklands is anything to go by.)
 
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