Touch sensitive encoder

Hello all,
I would want to detect the touch of my incremental encoder (metal case) with its caps (plastic). Do you know how can I do ? I know that is possible probably with capacitance detection, but I don't know how can I do detect touching

hope that you could help me
regards

here is the encoder : **broken link removed**

Hi,
You can use Atmel capacitive sensors to detect the touch to the encoder or even approaching to the encoder. For single channel you can use Atmel's QT110 sensors - it's a 8 pin device - **broken link removed**

Hello,
Thanks for your reply, do you know if it can detects the touch with the plastic cap of the rotary encoder ? and what do the "sensing electrode" suppose to be in contact with ? metal body of the encoder ?

regards

houly said:

Hello,
Thanks for your reply, do you know if it can detects the touch with the plastic cap of the rotary encoder ?
regards

Click to expand...

Sure, you can detect the touch through the plastic cap. Of course if it's from thick plastic you have to increase sensitivity of the sensor which is not very advaisable but still can work. I have projects with 50mm plastic which work well but normally the plastic should be betwen 2 and 3mm (best results).

and what do the "sensing electrode" suppose to be in contact with ? metal body of the encoder ?

Click to expand...

It's quite difficult to answer without having the mechanical design in front of me - how is connected the encoder, are there metal objects around. Capacitive sensors is a new area and usually people don't have experience and make stupid mistakes, like result their designs are not stable and don't work well. The sensing electrode design is strongly affected by the mechanical design of the device, what you want to achieve like outlook, etc.

One question - why you want to use encoder? You can put some capacitive slider / wheel and complteley remove the need from this encoder, I would say that these encoders are "not fashion" and "not shick" anymore.

BTW, you can buy any Atmel controller and if you insert the QTouch library you can turn it into capacitive sensor (key, slider or wheel) - Atmel Products - QTouch Library - QTouch Library

Thanks for your interest and your help Luben,
I used products like QTouch (from Cypress : PSOC microcontroller capsense technology) for another projects but I don't like it (I prefer a real contact with a button or encoder).
I would like to create my own circuit instead of using a chip, I started to think of it with a simple oscillator which will be "out of tune" when the equivalent capacitor of the human body touch the encoder then a circuit stop to oscillate.
I know that it will not easy but I think it's an interesting challenge...

regards

PS : I'm going to try to explain my application environment : the system is a placed in a rack case 19u (metallic case) and the metal body of the encoder is directly connected to the rack case. This rack case is connected to the earth (because it contain a power supply unit which must be connected to the earth for protection). So what is the best way to "add" the human equivalent capacitance to my sensor circuit ? I will try to make a diagram of the system to present you the problem and then try to know could I do to connect the "sensitive oscillator unit" to make the out of tune when the finger touch the encoder...

If you put the encoder on top of metal box and everything is connected to earth you'll have real challenge to make the capacitive sensor working well. I would suggest you completely remove the encoder and you put pure capacitive wheel in place of the encoder (think about iPod control wheel). If you still want to use the mechanical encoder I would suggest to use capacitive coupling between the sensing electrode and the sensing line of the chip - there is no possibility to connect reliably mechanically the sense electrode with the sense line with wires.

Oscillator is not the best way to measure capacitance. Atmel uses probably the best way for measuring the capacitance - charge transfer, as I said if you buy ANY Atmel uP and you put the QTouch library inside your project (payfree) you can turn this uP into capacitive sensor. One other way (which works well) is to charge for fixed time the sensing electrode through high value resistor and measure the voltage.

Hello again,
No I have to use this encoder and you're probably right for the method, the transfer of charge or charging the sensing electrode is probably better... I really want to use a custom way, I know that it will be be more difficult but it may be realizable. The question is what will be the sensing electrode in my configuration ? is it the metal case of the knob ? The charging method is interesting... but whant you said that, you talk about current or voltage ramp ?

The sensing eletrode should not be in contact with the GND. The simplest idea is to place on top of the knob a metal plate insulated from the GND and connected to the sensing pin of the uP. It's strongly not recommended to touch directly the capacitive sensing electrodes, so on top of the sense electrode should be placed some dielectric layer like polycarbonate 0.3-1.0mm.

You should charge with fixed current for fixed time and then measure the voltage. Charge transfer is working in different way - you charge a measured capacitor to Vc and then you transfer the charge over sampling capacitor