How to automatically screw an adjustable resistor by motion?

[yangyang-rexxar]

Hello guys

I am facing to a PCBA problem. There is an adjustable resistor with a small screw on it to adjust. The size of the resistor is 9mmX4mmX8mm, and the diameter of screw on it is about 2mm. This resistor is designed for adjust the sensitivity of the device. In production, I need to adjust it automatically to make the device at a default value.

In my opinion, a step motor is a good choice for this kind of application. The risk here is if the resistor is not solderred accurately, i.e. a little bit lean or shift, it's difficult to  handle the screw. I'm hot to look for a better solution on this.

Thanks very much.

[Buechsenschuetz]

As you already have pointed out, the problem is the mechanical interface between a step motor drive and the adjustable resistor. Besides the problems with force applied to the resistor, you also have to make sure that the mechanical interface hits the slot of the adjustment screw precisely. The slot may be in different positions. Anyhow, if you need to make adjustments automatically maybe an "electronic adjustable resistor" would be an option. The drawback is that these devices are no longer adjustable manually, i.e. with a screwdriver. There are several manufacturers, Maxim has quite a wide range of these devices: http://www.maxim-ic.com/products/digi_pot/

[Jochen]

Hi paulbin,

as long as the screw is in place, I agree, that a stepper motor (or 2, 3 or 4, depending if you need to position the screwdriver over the screw or if this is done by the guidance of the PCB) should be a good solution for your applilcation.

If there is a chance of significant tilt, things could become very complex:

First you would have to detect the tilt, which could be done by a machine vision system (potentially two cameras required). By the way, a camera might be required anyway to find the initial angle of the slot of the screw. Then you would have to adjust the angle of the screwdriver which might require up to 6 degrees of freedom. So you might end up with a full blown articulated robot here.

On the other hand I could also think of a low cost solution: Maybe you could replace the screwdriver by a kind of a hard rubber tool that could be pressed onto the screw. If this tool has enough friction without pressing the screw too hard, this might be an option.

Jochen

[yangyang-rexxar]

Hi Buech

Thanks for your advice. Here I can not change the resistor to a "electronic adjustable resistor" due to the cost, I'm not sure how much is it, will talk to electronics engineer. But on hand adjustable resistor is 0.12 buck..

I think a rubber tool is an appropriate solution here, but I'm still looking for  a better one.

[AnalogKid2DigitalMan]

paulbin:

Can you post an image of the pot? There is a tool I have used in the past that is like a small screwdriver blade that is enclosed within a plastic nonconductive tube. The tube's ID is sized to the OD of the screw on the pot. You slip the tube over the screw and rotate and eventually the blade of the driver lines up with the slot in pot's screw.

-AK2DM