During the past month I've been refining the hardware part of my project. Here are some updates:
First of all, I had to buy a better amplifier. The previously used op-amp (ST L272) couldn't handle the required voltage, so I got a National Semiconductor LM3875 power amplifier that is able to handle voltages up to ±82 volts. In the current setup I'm using two 30V DC supplies that are connected in series in order to supply the power amplifier with +30V and -30V, respectively. The two chips are depicted in the photo, the big one on the left is the power-amp:
Because my actuators are very weak, I had to look for something light to actuate. I bought a cheap (maybe too cheap because it's not very accurate) Higgstec 9" 5-wire resistive touchscreen-overlay and built a wood housing for it to see whether this setup could work. I put a piezo actuator in each corner and lay the touchscreen-overlay on top. I found out that it works better when the overlay is supported by very small rubber-patches because the actuators can move more freely. The whole frame is put on top of a lying 19" monitor and the display is fit into the small frame through software scaling.
After I got this all working I re-made the frame with acrylic glass because it was quite thick before and thus too far away from the monitor. Also I took care of all the thin wires which could be easily torn off the actuators before. This was a very, very fiddly kind of work as I wired them around a bigger cable using two tweezers and a very calm hand …