2
\$\begingroup\$

I am currently doing my master internship to get a degree in mechatronics. For an experiment I would like to conduct, I am interested in using electroadhesion. For those who don't know about it, the principle is easy. Get two electrodes (side to side or interlaced), put them on a flat surface and apply a high DC voltage across the electrodes. The forces are developed on a molecular level, so I won't go into details. The electrodes are usually well separated (by a gap) and embedded in a dielectric material (silicone or overlaid in tape for the cheapest versions). When I say high voltage, it is around 2-6kV DC, more might cause air ionization between the electrodes.

I know this process was used in old HP plotters to hold the sheet of paper while the tracing pen moves on the paper, and I remembered touching the surface of one and feeling some kind of buzzing in my fingers. In my experiment, there might be contact between humans and the electroadhesive surface, and I am concerned about safety. The HV DC-DC converters used usually provide several kV at less than 1mA of max output current (Emco produces the C50 that gives 5kV at 0.2mA). I know a DC current around 1mA can give a slight sensation.

The question is: Provided the electrodes are covered by dielectric, will there be this buzzing feeling when touching the surface? If yes, is there any danger? Provided there are holes or imperfections (due to time damage) in the dielectric, is there a danger, if the person touches both electrodes at the same time (with a finger crossing both electrodes for example)? If yes, what kind? Simply a shock similar to when you rub a balloon and get shocked, or something nastier?

Thanks a lot for your answers! Sebastien

\$\endgroup\$
1
\$\begingroup\$

This is a 'best guess' answer based on lots of general experience but not specifically with shocking HP plotters. (I've used them long ago but they did not shock me).

The energy delivered and the current capability are what (probably) matters. In a static adhesion system with good insulation you can use a series resistor that limits supply fed current to whatever level suits.

The energy delivered short term depends on the capacitance of the electrode structure on the load side of the protective series resistor. This obviously depends on electrode areas, inter-electrode separation, dielectric constant of the insulating layer and Murphy. At an off the cuff non calculated guesstimate capacitance figures in the 10-1000 pF range.
At 1 nF and 5000V the energy (0.5 x C x V^2) would be about 10 milliJoule = about 10 gram centimetre of potential energy. I'd expect that be enough to be felt quite well as it's delivered rapidly so the power is high for a very brief period. As the period is very small, even a large short period current will not be "cardiac dangerous" and 10 mJ should not otherwise be a threat.

Data point: Static charge accumulated on a human body on a suitably enthusiastic carpet can convey an unpleasant shock but otherwise unimportant to both parties if contact is made with another person.

\$\endgroup\$
  • \$\begingroup\$ Thanks for your contribution. Unfortunately I haven't found any ballpark figure regarding the capacitance of the electrode, since the capacitance is not what is the most important in the electrodes' design, but rather the strength of the electric field on the electrode. However, you used 500V, but as mentioned earlier, voltages are usually around 5kV. And the strength of adhesion is proportional to the voltage squared, so it has some pretty heavy importance. Therefore, I consider that the charge obtained with 5kV could start presenting some danger. \$\endgroup\$ – Seb.D Mar 23 '17 at 8:29
  • \$\begingroup\$ 500V was a typo - I just changed it to 5000V. 0.5 x C x V^2 = 0.5 x E-9 x 5000^2 = 0.01 Joule at 1 nF and 5 kV. Capacitance can be calculated or measured (as I'm sure you know. ) \$\endgroup\$ – Russell McMahon Mar 23 '17 at 12:22
  • \$\begingroup\$ Even at 1 Joule (100 nF, 5 kV) that's 0.1 kg.m - you start to notice that much energy, but it's unlikely to do vast harm. (Choose :-) - 1 kg from 100mm or 100 grams from 1 metre. ). From 1m 100g ~= 4.5 m/s \$\endgroup\$ – Russell McMahon Mar 23 '17 at 12:27
  • \$\begingroup\$ Sorry, I didn't redo the calculation myself. Thank you for all the details, I'll do more research and decide afterwards. Have a nice day! \$\endgroup\$ – Seb.D Mar 23 '17 at 12:35
  • \$\begingroup\$ According to my information the limits for DC that you don't need to protect against touching are 60V (SELV), 12mA and 350mJ, but I can't find a good source to cite. For AC it's 25V and 3mA. \$\endgroup\$ – starblue Mar 23 '17 at 20:07

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.