Accurate Pan/Tilt Unit

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1

Hi,

I have a project in mind which requires a fairly accurate pan/tilt device. I’m planning to attach a laser pointer to a pan/tilt controller, hanging in a roof. I want to be able to quickly and consistently direct the laser beam onto specific locations on walls 4-5 meters away, with at least ±5 cm accuracy.

I’d like to be able to control it from a regular PC running Linux via USB or similar.

Can anyone recommend good solutions to this problem?

Cheers, Lars

2

Hi Lars,

Sounds like a great project! Because a laser isn’t very heavy, the Pan/Tilt Kit from DFRobot would be perfect.

It’ll allow you to choose any of our standard servos to use with it and give you the accuracy that you need.

As far as controlling it from your computer. You could use an Arduino board (or any other board with a serial port) and just control it via the serial monitor. For example, you could create a bunch of strings which the Arduino accepts as commands. When that string is sent via the serial monitor it could snap to a known location. Or you could use a cartisean coordinate system which can be entered via the serial terminal (x,y).

3

For the absolute in smooth pan/tilt control, it will be worthwhile looking into brushless motor gimbals.
Backlash in a servo’s gearbox probably won’t make it repeatable over those distances.
Over 5m it only takes 0.6 degree error to put you out 5cm.

4

Thanks Sam,

I had a look at your standard servos, but I’m not sure how to tell if they’ll be accurate enough. I can’t find any numbers on their angular resolution.

To get an accuracy of a few cm, beaming a laser on a wall 5m away, I’d need an angular accuracy/resolution of about 0.25 degrees or so.

5

Thanks Michael, do you have any recommendations/links when it comes brushless motor gimbals? My budget for this part of the project would probably be limited to a couple of 100AUD max.

6

Haha, that’s my bad, I misread the distance and for some reason thought you meant 40-50cm. You are both quite correct. How quickly does it need to move positions?

7

I don’t, sadly. I’ve never experimented with them but they appear to be the go-to for accurate and smooth pan-tilt.
There’s a plethora of projects built around them, typically for cinematography. there are loads of products specifically designed drone-footage so they can be easily driven by regular servo signals. Nothing that core carries though. Check out some brushless gimbals on RC drone sites as a starting point.

8

I’ve used brushless motors a bit for a few projects, but because they require more sophisticated drivers, they end up being a lot more expensive.

9

It doesn’t need to move super fast… something like 90°/sec would be ok.

10

When you buy a drone type of brushless gimbal unit, won’t that usually mean you’ll be paying extra for stabilization etc though? Since this device will be hanging solidly attached to a roof, it won’t really need stabilization, just consistent and fairly accurate pointing.

11

Given your budget, something that comes to mind, which I’ve used for a similar project, is you could mount potentiometers to the pivot points, and use a high-resolution ADC to monitor the values. Something like a 12-bit ADC should cover your 0.25 degree resolution. The only downside is that it restricts your rotation angle. But, provided you didn’t need more than ~270 degrees of movement (standard for most pots), you should be fine.

That would allow you to have an absolute reading, simplifying your code, and you could use standard brushed DC motors.

12

Thanks Sam, 270 degree range is plenty. I’ll definitively look into this kind of solution. This being my first physical hardware project (I got a fair bit of software experience) I realise I have a lot to learn though.

I was recommended these motors as part of a feedback loop system that may be able to give me the required accuracy:

They could be attached to this pan/tilt bracket:

If it works, it would certainly be good value. What do you think?

13

Hi Lars,

I haven’t used those servos before myself so I couldn’t really say. We’ve got both of those products available, but I couldn’t really advise on the resolution of that motor.

14

Ok, thanks for your suggestions guys. I will keep them in mind will continuing to do some research…