Help Appreciated With RC Servo Control Circuit

[Russell Hayes]

Hi Guys,
This is my first post here although I have been lurking around here and other electronics forums for a while now.

I am in the middle of a small project to build a simple wired control device to remotely control the focus ring of a camera lens and need a little help. The device consists of a hand held box with a ‘control knob/wheel’ that, when rotated, proportionally drives a standard RC servo with a gear attached. The gear then engages to another gear fixed to the lenses focus ring, thus, focus can be controlled remotely. There are similar commercial products available like the images below:

**broken link removed****broken link removed**

The heart of the device is a small commercially available servo controller, which comes shipped with a 10K potentiometer. The servo is connected to this controller; also 4x AA batteries are connected to power the system.


3 wires from the controller are then connected to the 10k potentiometer and, as the pot is rotated, the servo turns proportionally and in a linear fashion to the control knob pot. So far so good! Below shows how this is wired up:

**broken link removed**


The servo can rotate 180 degrees. When the main control knob is turned fully left, the servo is at 0 degrees. When the main control knob is turned fully right, the servo is at 180 degrees. Perfect!


Now, my goal (and where I’ve hit a hurdle) is to add two ‘end point limiters’ into the mix. By this I mean 2 extra control knobs/potentiometers, which reduce the travel of each end of the servo. So when these ‘limiters’ are both set to ‘full’ the servo rotates its full 180 degree travel when the ‘main knob/wheel’ is rotated from end to end.


If the ‘left limiter’ is turned, the servo only travels from 0 degrees to 100 degrees (depending on setting), from example. Or, if the ‘right limiter’ is turned, the servo only travels from 80 degrees to 180 degrees, from example. If both are turned, the servo only travels between 80 to 100 degrees, for example.


I have tried a simple wiring of 3 potentiometers, like below:

**broken link removed**


This works but the problem is that the servo no longer turns proportionally to the control knob. When the knob is turned, it starts slow and then accelerates as the pot is turned. It is not linear to the control knob.

Does anyone know why this would be or how to solve it so that the servo rotates proportionally and in a linear fashion to the control knob pot?

 

[erikl]

Did you already try to exchange the yellow and the red (currently: left - guess you're a sailor, aren't you? ;-) ) wires at the Servo Controller? And perhaps the left & right potentiometers should have lower values, say 5kΩ. Presumably brigding both 10kΩ pots with a 10kΩ resistor would suffice. Guessing, just give it a try!

 

[Russell Hayes]

Hi,

Ok, I wired it up like how I think you mean:

**broken link removed**

What happens now is, that the left limiter is working as an 'additive' regardless of the position of the 'main knob'. The right limiter has no effect what so ever.

So if the 'main knob' is fully left, the left limiter move the position, but also when the 'main knob' is on the right.

Now, looking at the PCB of the 'circuit controller', there are indeed 3 separate soldering tags for red yellow and green, but, the yellow and green seem to be connected by a pcb track?

 

[erikl]

... the yellow and green seem to be connected by a pcb track?

You could measure this with an Ohm meter (power supply off, and make sure the right potentiometer doesn't produce the short circuit).
If actually so, the right potentiometer cannot produce any effect, of course.

 

[Russell Hayes]

Ok, lets back track to clarify want I want to achieve as I not have described it too well

I want the lower limit control to set the servos 'left' position and the upper control to set the servos 'right' position. The control knob should mix linearly between these two positions.

With both upper and lower limit set to full, the servo linearly travels from 0 to 180 degrees when the control knob is turned fully from end to end.

If we set the lower limit control to set the servo to 20 degrees and the upper limit to 120 degres, the servo linearly travels from 20 to 120 degrees when the control knob and turned fully from end to end.

So the main control knob should linearly mix between the 2 limiter settings.

 

[erikl]

Right, Russell,
I had already understood your intended function this way, and the above connection should actually provide it (if there is no short circuit between the center and right input terminals of the servo controller, as you mentioned as a possibility before).

But: It is possible, that the servo controller input relies on receiving a total resistance of 10kΩ, which is not the case if you add various resistances at both ends to set the corner limits. This will surely not vitiate the very function, but could well affect the linearity - that's why I had suggested to bridge the outer potentiometers to lower the total resistance. Did you try it?

With this arrangement of course you cannot expect that the end point settings are independent of each other - simple math shows that.

You could possibly increase the linearity if you connect the outer - currently unconnected - terminals of both the left and right (and in this case: un-bridged) potentiometers via a 10 .. 15kΩ resistor. This will decrease the total resistance change which your servo controller will see.

 

[Russell Hayes]

Hi,

When you say bridge the outer pots, what exactly do you mean? Could you show me a quick sketch of how to wire that up?

Here is a link to a scan of the instructions provided with the servo controller (look at the pot solder tags, you can see the yellow and green tabs are linked:

**broken link removed**

 

[erikl]

When you say bridge the outer pots, what exactly do you mean? Could you show me a quick sketch of how to wire that up?

With bridging I meant to connect the 2 outer connections of the pot via a resistor such as to decrease its total resistance by this parallel circuit. Unfortunately the pot's linearity will adversely be affected by this. But this isn't relevant any more, due to the following:

Here is a link to a scan of the instructions provided with the servo controller (look at the pot solder tags, you can see the yellow and green tabs are linked:
**broken link removed**

From this figure it's quite clear that the above arrangement cannot work properly. It says: "Use of potentiometers other than 10k will limit the angular range of the servo." And the schematic below shows clearly that the intended position is proportional to the percentage of 10kΩ between the left (red) and the center (yellow) input terminals - the right (green) terminal actually seems to be connected to the center one - did you measure this?

The servo adjustment works quite differently then I (and perhaps you, too) thought.
That's why we have to find quite a different schematic arrangement. It's not quite easy, but I've got a working proposal for you, pls. s. below (I'm sorry I don't own a good drawing program for schematics containing potentiometers - IC designers don't use such bulky devices ;-) ).

For your example above (left limit 20 degrees) you'd need a total resistance (always between the left and the center=right terminal) of 10/9 = 1.11..kΩ, to reach the right limit of your example (120 degrees) you'd need 10*2/3 = 6.66..kΩ, i.e. 5.55..kΩ more.
It should be possible to attain this with the arrangement below. But beware:

• the trimming of the 3 potentiometers affect each other, i.e. must be set iteratively a few times in series for the intended limits
• the adjustment between the 2 limits wont be very linear
• this is just a suggestion how to start, you could still play on the resistance values, firstly on the Rs value
• No guarantee! ;-)

Good luck! erikl

 

[Russell Hayes]

Hi Erikl,

Firstly I would like to thank you for your time on this so far, I really appreciate your sketch, it makes things much clearer for me.

I will try this circuit today and report back. If it is useful to you, I can take resister readings via my multimeter.

One thing I have not mentioned until now, is that a total resistance of 10KΩ only results in a servo movement of about 120 degrees. 180 degrees requires approx. 12KΩ but I will report back that reading today.

Again, many thanks for your help, you are very helpful!

 

[erikl]

... a total resistance of 10KΩ only results in a servo movement of about 120 degrees. 180 degrees requires approx. 12KΩ ...

The necessary range can easily be covered by using my above sketch. Rgds, erikl

 

[Sander108]

Hi,

I'm very curious if your min-max limiters are working successfully. If so, would you be so kind to place your final diagram?

Thanks,

Sander