Thank you - it matches up with my understanding, but getting the practical details of the implementation really help. I'll let you know what I finally decide and how it turns out. -Adam On Mon, Jan 3, 2011 at 2:47 AM, Scott Dattalo wrote: > On 1/2/2011 4:41 PM, M. Adam Davis wrote: > >> Hopefully someons has done this before, but if not perhaps you know >> some references or resources that might get me started. >> >> I'd like to be able to attach a device to, say, an iPad, and cause the >> ipad to register touches to the screen as though a human touched it. >> In essence I need to simulate a human body electrically. =A0The contacts >> will be in constant contact with the iPad, but I need to turn them on >> and off with another device. > > Hi Adam, > > I've done something like this. I don't know about the iPad sensor but > other capacitive sensors are often comprised of two orthogonal grids of > ITO electrodes; each grid is generally on its own layer. The layer closer > to the touch surface is non-space filling while the one below is. There i= s > a capacitive coupling on the order of a few hundred femptofarads at each > grid intersection between the two layers. When a grounded conductive > object is placed close to this junction, the capacitive coupling > decreases. When a non-grounded conductive object is placed close to this > junction, the capacitive coupling increases. > > A finger may be approximated by a grounded cylindrical slug of metal. The > mean normal touch diameter is about 9mm, although diameters between 5mm t= o > 20mm are common. > > Since you require the simulated touch to remain in constant contact with > the surface, you will have to be careful with the design. I would suggest > a metal cylindrical disk about 9mm in diameter (maybe a few mm thick) > soldered to the center lead of a piece of coax cable (like RG-58 or > RG-59). Tie the outer conductor of the coax to "ground". Use a > low-capacitance switch to connect the coax conductors together. > > "ground" is nebulous. You only need a sufficient enough free space > coupling to achieve ground. Free space coupling exceeding 10pF. The human > body is on the order of 150pF and for all intents and purposes may be > considered "ground". > > There is an optimum cable length. If the cable is too short, then the dis= k > will look like an electrically floating piece of metal (and it's signal i= s > the opposite of a grounded piece of metal). If the cable is too long, the= n > the disk will always appear to be touching even if the center electrode i= s > not grounded. > > You can estimate the length of the cable by grounding the outer conductor > (and leaving the center one floating) and touching the sensor with the > disk. If the touch registers, then the cable is too long. Once you find > the maximum length for a non-registered touch, shorten the cable by about > 50%. This calibration must be performed while the switch you plan to use > is connected (but obviously not activated) since it contributes to the > capacitance. > > With this arrangement, you can ground the outer conductor of the coax and > the 9mm disk will not be seen. However if you ground the inner conductor > it will. So to complete the job, use a low capacitance switch to make the > connection between the two. I would suggest a JFET, but a MOSFET or a > relay contact may work just as well. > > Note, the optimum cylinder length depends on several parameters like the > dielectric of the glass or PET, the grid spacing, the separation between > layers, the free space coupling of the sensor, etc. > > I don't know if you have access to the raw signals coming from the sensor > or not, so you may not be able to know when the optimum length. > > BTW, the coax serves two purposes: 1) provides some capacitance to counte= r > the negative signal the "floating" metal disk induces and 2) shields the > finger signal. > > >> What is the electrical equivalent to the human body that would be >> suitable for this type of sensor actuation? =A0What is the appropriate >> contact material (ie, should I use a PCB with pads, a wire, a >> conductive cloth or foam, etc)? =A0What should be on the other end of >> the contact that will let me switch the human capacitance on and off >> reliably? >> >> I've done a few trivially simple tests, but it looks like I need more >> reading material to understand exactly what's needed to triger such a >> system. > > The finger coupling to the sensor is sub-picofarad. The human body free > space coupling is ~150pF. You can simulate the 150pF with a wire returnin= g > to the iPad's ground or with a wire connected to some large metal object. > I think you can get by with metal contacts, although a more accurate mode= l > would have resistance. The sensor's spatial resolution is fairly coarse, > so fine details like finger prints are not discernible. > > Scott > -- > http://www.piclist.com PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist > --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .