EDURobotics Servos

[Jacob S-O]

I have two questions about the servos, first can you read the angle the servo has moved? second can you tell the servo to move a specific number of degrees? if so does anyone have the code?

[Lloyd Burns]

I've not looked at the EDUbotics specifically (Ian has ours), but most servos I've seen work on the basis of you sending it a "number", and it turns to the angular position corresponding to the number. It uses a feedback loop inside the case : the motor is geared down inside the case, and the output shaft is coupled to a potentiometer, which sends rotation data back to the internal controller. There is no way to get feedback from inside the case without illegal "tampering".

You should determine the position you need the servo to assume, and try things out to find the "number" to send to get it there.

If you need to find 'x' in the formula number = x degrees, send zero, and note the position. then send 254 and measure the number of degrees (about 180 with the full fledged RC). Each Number then gives 180/254 degrees, using these numbers.

That is some school building you have there. :-)

[rbayer]

The EduRobot doesn't actually include any servos. Those little boxes that look exactly like a servo are actually very small multi-speed, reversible motors that work exactly like a regular motor: values greater than 127 cause it to turn one direction, and values less than 127 cause it to turn the other direction.

If you want position data, you'll need to hook up an extra potentiometer. If I remember right, any 100k ohm pot should work just fine.

[MattK]

Ask my team coach Mr. Kelso, he was on the team that helped design the EDU kits His CD name is David Kelso

[Jacob S-O]

thats helpful, I just have to steal a real servo from the old robot.... and yes it is a nice building.

[David Kelso]

A correction....I was on a team that helped design and plan the FIRST activities(A Primer for Success) that are posted on the FIRST site. Actual design and lots of other development time was/is done by Innovation First. (and for rookie teams, FIRST and Innovation First are two different organizations.)

[Adam Y.]

second can you tell the servo to move a specific number of degrees? if so does anyone have the code?
Thats just a matter of figuring out the exact zero pulse range of the servo. I believer parallax has something on finding out how to mover a servo to a specific postition. http://www.parallaxinc.com/html_files/downloads/downloads_sic.htm I believe their robotics guide should do the trick since their robots use severos. But if what they say is true and their not true servo motors then this won't work because you need servo motors.

[Lloyd Burns]

You can buy servo-motors from any RC hobby store: I have priced them at several pplaces around town, and the cheapest was in Markham Village (16th and 48). They're also available from web-savy hobby stores.

Unmodified, they work as I wrote before - but Parallax, in the link above, wants to make them into two-directional, full-speed-until-the-control-signal-stops "motors", as used in model blimps and some helicopters (and EDUBots ?). If you have one of these, then coupling to a pot is required since the internal pot has been taken out of the loop.

In effect, you would be making a bigger servo. You'll probably want pulleys or gears to allow the motor to go someplace before the new pot reaches its limits. (With powerful motors, eg Drill Motors, Atwoods, the pot can be broken.

[Lloyd Burns]

I just saw the motor last night, and yes, they aren't servos, having no internal position feedback loop (a pot geared to the o/p shaft).

What appears to have happened is the circuit inside has a microcontroller A la Victor 883, driving the servo-style motor, giving you a range of speeds. It also has another input method, giving full speed either way, or stop.

As I say, I just saw it last night.