Newbie with an idea... Where to start...

Status
Not open for further replies.
*smile*

Well luckilly at least for this project there shoudn't be any crashing. But my real situation may be camera equipment could be pretty heavy so I would want to take advantage of anything that would help my torque.

OK, so servo is absolute position, this is what I need most definately.

Should I be worried about checking to make sure the servo makes it to the position or is that too much micro managing? Maybe I'll not worry about it for now and come back to that if positioning is a problem.

I'd like to have as close to 360 motion, but it's not required. More than 180 though would be needed.

Someone posted a link to an R/C servo site, but I couldn't tell what the ranges of motion of those servos were. Anyone have a specific servo they've used and know it's stats like range of motion, precision (how many stops can I address along the path) and price?
 

Whats to worry?, it's a VERY simple scheme, with really no chance of any problems.


Rotation of RC servos is small, possibly only the 40 degrees mentioned in the spec sheet I gave the links to? - but you can drive them further by exceeding their drive specs. Although you would be doing well to exceed 90 degrees.
 
Well I'm glad I'm getting all of this information from you guys. As I learn more I'm shaping my part list after every post almost.

See, this PTZ mount for a camera will need to have at least a 180 degree range of motion, closer to 220 or so would be better.

Maybe my best bet is a normal small DC motor, gear it down for maximum torque, and use a high quality potentiometer to determine position.

This way I'd get a full 360. But I need a really consistent potentiometer. Do they make 'digital' ones? The only exposure I've had to them is for volume knobs and those seem fairly low 'resolution'. I'm going to have to be able to determine the arc position of my device to 1 or 2 degrees consistantly.
 

hi,
You can get rotary encoders with photo optic angular sensing, rather than a resistive pot.

The optical devices have a continous rotation thru 360 deg, the 1deg resolution requirement would not be a problem.

They also have the advantage in that they are low noise and dont wear out.

>> normal small DC motor, gear it down for maximum torque,

A word of caution regarding gear trains on positional motor drives, is 'backlash',
this occurs due to the degree of meshing in the gear teeth.

If the gears are 'over meshed' the gear train will be stiff, 'under meshed' the gear train will be too loose
and the positioning system will have problems called 'hunting'.

Thats where the motor drive cannot drive to the precise postion, so it will continously overshoot the desired position.

You can buy special gears, which basically two identical gear wheels, face to face,
with springs mounted within body of the gear wheels.

The idea being to keep the gears in the optimum mesh.
 
Last edited:

Potentiometers only rotate 270 degrees, so you couldn't get 360 degrees in that way.
 
Nigel Goodwin said:
Potentiometers only rotate 270 degrees, so you couldn't get 360 degrees in that way.
Click to expand...

hi,
The closest I have seen are the VISHAY 357 series pots. [about £8GBP]
Rotational life 50 million revs.

Continous 360 deg mechanical rotation, but electrically 340 degs.

I 'suppose' it should be possible to step thru the 340deg+ point and test for open circuit on the wiper.
 
The cheapest servos are the $10 standard servos. But they have limited travel, so you may have to make a gearbox.

One other idea is to use two servos, one of them being able to rotate the camera and the other to rotate the camera assembly with the other servo. So you can double the throw, but it will get more complicated to design.
 
Nigel Goodwin said:
Potentiometers only rotate 270 degrees, so you couldn't get 360 degrees in that way.
Click to expand...


Not exactly correct.. There are servo pots that can do a full 360 rotation. RS Components sell loads... They have 340 degree sensing, with an allowable 360 degree continuous rotation capability. Good for over 10,000,000 rotations. But at over £25 they aint cheap!

go to RSWWW.COM and do a search for servo pot...
 
OK, that servo mount pot is sounding like a good idea, but we're talking about mounting it to a dc motor (non-servo) because of the range of motion issue, right?

Anyone have a quick sketch or picture of a motor and pot actually hooked together? Am I just attaching their shafts together with some kind of splice piece of metal that is also a cog?

(crude drawing attached)

**broken link removed**
 
No, you need to gear the motor down - a LOT!. The pot needs to be connected to the rotating table, and not the motor.
 
Yes, something like that - I would also suggest adding limit switches to prevent it going too far.
 
Well the limits may vary between cameras (different cameras will be used on these mounts) that's on my list of things to deal with though.

So can I find a motor and gearbox combo somewhere or is this the kind of thing that I'm going to have to build myself somehow?
 
archaic0 said:
2. read servo motor positions and make decisions based on it
Click to expand...
As stated earlier, this will be the difficult part, but it is doable. You should check out the **broken link removed** and the **broken link removed**.

This involves replacing the electronics in a standard servo with a custom Open Servo electronics board. This gives you many added features and capabilities with servos, including reading back the position, control via standard R/C PWM or I2C, and other features too numerous to mention. You can now purchase assembled and tested Open Servo control boards as well as get your servo(s) converted for a small fee per servo.

8-Dale
 
Nigel Goodwin said:
That's the only thing that's any problem, because you can't read a servo position, unless you add some type of positional sensor, or modify the servo to take an output from the internal poteniometer.
Click to expand...

For anyone interested, Beau Schwabe over at Parallax (does that get me dinged here) has in fact come up with a way to get "Proportional feedback from a Standard Hobby Servo"!

He has developed a Propeller (another ding?) application and video for demonstration. Principles should be applicable regardless of the uProcessor used.

From the post: "Here is an interesting application that allows you to accurately sense the position of a standard Hobby Servo without modifying the servo."

**broken link removed**
 
Actually, I just (FINALLY) ran across something someone already built to acomplish what I need. It's a little more expensive than I was shooting for, but it's far cleaner and retail ready than my project would have been.

Thanks everyone for your input! I'll revisit this idea myself if this other product doesn't pan out.
 
Status
Not open for further replies.

Similar threads

K
  • Question
Help with STM32G0
Replies
2
Views
4K
rjenkinsgb
L
  • Question
Pic16c57 to Pic16f57
Replies
4
Views
2K
aliarifat794
A
F
  • Question
Issue with Interfacing LCD with PIC16F877A
2
Replies
22
Views
6K
Farhaad
F
J
  • Question
Trying to convert an IR Torch
Replies
7
Views
3K
schmitt trigger
S
E
  • Question
High res cap meter with PIC 16F628
Replies
7
Views
2K
Elekktrishun
E
Menu
Log in
Register
Cookies are required to use this site. You must accept them to continue using the site. Learn more…