servo output accuracy

[NextPerception]

Is the accuracy of the two servos controlling the camera limited by the servo potentiometer's output or would I be able to modify the program to give me degrees with one number past the decimal? (like 43.7 instead of just 44)

I realized while i was righting this that it may be the pwm's themselves causing the limit (0-255) but there may be around it so i am still going to post this ...

This is my first year programming one of these things so...

[Eldarion]

Originally Posted by NextPerception

Is the accuracy of the two servos controlling the camera limited by the servo potentiometer's output or would I be able to modify the program to give me degrees with one number past the decimal? (like 43.7 instead of just 44)

I realized while i was righting this that it may be the pwm's themselves causing the limit (0-255) but there may be around it so i am still going to post this ...

This is my first year programming one of these things so...

The limitation is in the entire servo system, I.E. both the servos and the standard PWM signal used to control them.

You will only have 255 "ticks" of resolution regardless, so there is no reason to go into decimals.

Hope this helps...

[NextPerception]

Wow. thanks for the quick reply!
well i guess if it can't be done it can't be done... (although it could be done with a 10 turn pot geared to the servo output shaft so 10 turns=180 deg then have a circuit that puts that onto two inputs to give me 65536 values to work with instead of just 256 but no way in hell i am going to go through all that...)

I just did the math and it looks like at half court and over about 12 feet to one side it would be only about a 3 inch error in either direction without decimals. (and that isn't including the allowable error to prevent oscillation in the default code)

[Eldarion]

Originally Posted by NextPerception

Wow. thanks for the quick reply!
well i guess if it can't be done it can't be done... (although it could be done with a 10 turn pot geared to the servo output shaft so 10 turns=180 deg then have a circuit that puts that onto two inputs to give me 65536 values to work with instead of just 256 but no way in hell i am going to go through all that...)

I just did the math and it looks like at half court and over about 12 feet to one side it would be only about a 3 inch error in either direction without decimals. (and that isn't including the allowable error to prevent oscillation in the default code)

That's pretty much where I ended up. I guess FIRST made the target 30" wide for a reason after all!

[sciguy125]

Originally Posted by Eldarion

The limitation is in the entire servo system, I.E. both the servos and the standard PWM signal used to control them.

You will only have 255 "ticks" of resolution regardless, so there is no reason to go into decimals.

Hope this helps...

If I understand the system correctly, the limitation is in the signal being put out by the controller. It may have changed recently, but I'm pretty sure that older servos (probably from before the mid-90's) were completely analog. Older hobby radios (and lower-end new ones) were also analog. This gives you whatever resolution you are mechanically capable of having.

My newer futaba radio has 10-bit resolution. The earlier digital futabas had 9-bit. Barring an ingenious marketing scam, this would imply that the servos themselves are capable of at least 10-bit resolution.

As for the IFI system, it'd be an interesting trick to increase the PWM signal resolution.

[NextPerception]

well i checked the rules and R67 says something like we can't control anything with our own pwm and the output has to go straight from the servo to the robot controller

that means we cant use a 16 bit digital analog converter to convert the servos into "high def" servos...

come on first! the fcc wants us all to switch to high def!
(that wasn't serious for those who take everything seriously)

i don't think the 3 inches is going to a problem as long as our teams launcher is accurate and very repeatable. (and from what we've tested so far this shouldn't be a problem)

[Keith Watson]

Servo accuracy includes 2 other things not mentioned.

One is "centering", or how well the output wheel exactly goes to the commanded position. It can vary between brands and between models in a given brand. Digital servos have much better centering accuracy.

The other is the range of motion of the output wheel versus the PWM range IFI uses of 256. Servos can have ranges in degrees of 60, 75, 90, 120, 180, etc. So one PWM step can give a different angle depending on the servo.

[KenWittlief]

there are other restrictions on the accuracy of a servo: the load (resistance to movement), temp, voltage of the supply battery.....

Servos are designed to use on radio controller vehicles, where the operator applies a command to the servo, sees what the vehicle does, then fine-tunes his inputs

I have never seen an accuracy or precision spec for RC servos.

[Keith Watson]

I have never seen published centering specs either, just word of mouth. But it is easily measurable. People with high speed planes typically need servos with high accuracy.

[Mike Bortfeldt]

If you want to try using a higher resolution PWM signal to see what happens, I posted some code for PWMs 13-16 here that can easily be modified to give you more control over the signal width.

Mike