{"id":125,"date":"2006-10-22T22:55:51","date_gmt":"2006-10-23T03:55:51","guid":{"rendered":"http:\/\/www.thegatesofdawn.ca\/wordpress\/electronics\/emg-amplifier\/"},"modified":"2021-04-12T23:05:28","modified_gmt":"2021-04-13T04:05:28","slug":"emg-amplifier","status":"publish","type":"page","link":"https:\/\/www.thegatesofdawn.ca\/wordpress\/electronics\/emg-amplifier\/","title":{"rendered":"EMG Amplifier"},"content":{"rendered":"
\"EM-16<\/div>\n

The second project undertaken for Jeff’s research was a 16-channel Electromyography Amplifier.\u00a0 At the time, Jeff was working on his PhD thesis<\/a> at Queen’s University<\/a>, in the lab<\/a> of Dr. R. Mel Robertson<\/a>.<\/p>\n

Electromyography is recording the electrical signals that activate muscles.\u00a0 Jeff was interested in recording the the activity of the wing muscles of a locust in tethered flight.\u00a0 Many wing muscles are involved, and he wanted to record them all simultaneously.\u00a0 The lab had a number of Grass<\/a> Model P15 amplifiers that were used for this type of work, but each one has only a single channel, and they were in short supply.<\/p>\n

EMG\/EEG\/ECG are among the major applications of instrumentation amplifiers.\u00a0 The enormous input impedance and common-mode rejection ratio (CMRR) of an instrumentation amplifier allows them to extract usable signals from the poorest, noisiest, weakest sources.\u00a0 We had previously used an Analog Devices<\/a> AD620<\/a> instrumentation amplifier chip for the Position Transducer<\/a> project.\u00a0 We also used it for the Myoelectric Locust<\/a> hack, an application which was basically just EMG.\u00a0 On that project, we learned a number of useful things about how to do EMG, which led directly into this project.<\/p>\n

The most important lesson we learned that fed into this project was that a really large input impedance can cause its own problems.\u00a0 On M.E.L., we found that the circuit worked fine for a while, but after a time the output of the amplifier drifted all the way to the power rail and stayed there.\u00a0 The reason turned out to be that the huge input impedance of the AD620 tended to collect charge, until eventually the DC offset exceeded the range of the amplifier.\u00a0 The problem was solved by adding very large resistors (22MOhm) from each input to ground, to drain off the collected charge.<\/p>\n

The amplifier is intended to be used inside a Faraday cage, to reduce the amount of 60Hz hum picked up.\u00a0 I made a separate +\/- 12V DC power supply, which would remain outside the Faraday cage.\u00a0 I tried to filter the output as well as I could, to prevent 60Hz hum from getting into the cage.\u00a0 A shielded cable delivered the power to the amplifier chassis inside the cage.\u00a0 I ran all the output signals on a single cable to a rack-mountable BNC terminal panel outside the cage.<\/p>\n

Each channel of the amplifier provides three switch-selectable gains (x10, x100 and x1000), and low-pass and high-pass filtering. To keep costs down, the corner frequencies of the low-pass and high-pass filters are not easily changed. They are determined by capacitors installed inside the unit on a screw-terminal strip.\u00a0 In addition, the amplifier has a monitor channel.\u00a0 Any or all of the channels can be mixed onto the monitor bus, which is further amplified and played through a speaker.\u00a0 The monitor allows an audible indication of the quality of the electrode implantation.\u00a0 Finally, the amplifier provides an accurate calibration voltage pulse of either 50 or 100 uV (derived from a precision voltage reference chip), which can be sent to any input to calibrate voltage levels accurately.<\/p>\n

For electrodes, Jeff used very thin “magnet wire”.\u00a0 Thin as a hair, and with a lacquer insulation.\u00a0 To implant the electrodes, Jeff put the locusts in a refrigerator for a while to put them sleep.\u00a0 Then he would poke a hole in the exoskeleton just outside the muscle.\u00a0 The magnet wire was then inserted through the hole to the appropriate depth to reach the muscle of interest.\u00a0 He did not strip off the lacquer insulation, so that the only point that made electrical contact was the tip of the wire.\u00a0 The electrode would then be secured in place with a drop of melted wax.<\/p>\n

\"EMGHere is a picture showing EMG signals Jeff captured from 12 wing muscles simultaneously.\u00a0 Six muscles on the left, and the same six muscles on the right.\u00a0 The region labelled “stimulus” is where a simulation of a bat echo-location sound was played, to trigger the locust’s “evasive manoeuvers”.\u00a0 At that point, you can (barely) see that the muscle enervation pulses start double or triple firing.\u00a0 In the magnified region on the right, you can see the left and right sides become slightly asymmetric, which has the effect of causing a turn (or it would if the locust wasn’t glued to a stick and flying in a wind tunnel.)\u00a0 These results were published in a paper, J.W. Dawson, F.H. Leung, R.M. Roberson (2004) Acoustic startle\/escape reactions in tethered flying locusts: motor patterns and wing kinematics underlying intentional steering<\/em><\/a>.\u00a0 Journal of Comparative Physiology A (2004) 190: 581-600.<\/p>\n

I wrote this user’s guide for the amplifier: EM-16 User’s Guide<\/a><\/p>\n

PCB Construction<\/h4>\n

The circuit board was fabricated using a postive photographic etching process.\u00a0 The photomask was printed on laser-printer-compatible transparency film at 1:1 scale.\u00a0 A photomask produced this way is not sufficiently opaque to produce good PCB results.\u00a0 If you hold it up to a bright light, you can see light through the toner.\u00a0 I found a decorative special-effect foil product that can be used with laser-printers to make metallic imagery.\u00a0 Normally, it’s used only to add gold or silver lettering at specific places on the page.\u00a0 I used it to put a metallic foil coating over the entire photomask.<\/p>\n

The way the film normally works is you print out your page normally.\u00a0 Then you cut out small pieces of the metallic effect foil, and secure them over the specific parts of the page where you want the effect, using small adhesive dots that come with the film.\u00a0 Then you run the page through the printer again, printing out a blank image.\u00a0 The idea is to just run the page through the fuser of the laser-printer.\u00a0 The heat of the fuser will bond the metallic film onto the preexisting toner on the paper.\u00a0 Then you can peel off the plastic carrier, leaving the metallic effect on the paper.<\/p>\n

When using the metallic film to make a photomask, I use the entire sheet at once.\u00a0 I found it tended to jam up in a laser-printer, so I actually use a laminator to fuse the film onto the toner.\u00a0 It took some experimentation to get the laminator feed rate set properly.\u00a0 I put the transparency photomask and metallic film together through the laminator. When I peel the metallic film carrier, the photomask has been covered with metallic flim, and is almost perfectly opaque.<\/p>\n

Unfortunately, I find that with the transparency, a lot of the metallic film sticks to the open areas where there is no toner.\u00a0 So, I have to do a fair bit of cleaning up with an X-acto knife to scrape off metal where it shouldn’t be.\u00a0 In the end, I get a good quality positive photomask that can be used to fabricate many PCBs.<\/p>\n

Of course these days, low-volume quick turnaround PCB shops are increasingly common, so my days of fabricating PCBs at home may be over.\u00a0 Good riddance, I say.<\/p>\n

Front Panel Construction<\/h4>\n

Making nice professional-looking front panels for my electronics has been an obsession for some time.\u00a0 I still do not have a technique that gives me satisfactory results at a reasonable cost.\u00a0 I’ve tried many things along the way.<\/p>\n

For this project, I used a homemade decal<\/a>.\u00a0 I wanted white lettering.\u00a0 I approximated white, using the same silver effect foil that I used for PCB fabrication.<\/p>\n

Links<\/h4>\n