Having witnessed some small photo detector changes reported in the previous blog it is possible I am measuring some small Mach Effect forces when the piezoelectric stack is energized. The obvious requirement though is to eliminate false positive and misleading data. The objective for Build 2.0 is to develop a completely reversible stack so that apparent thrust can be measured on subsequent runs and common mode errors subtracted out.

Figure 4. shows the overall Build 2.0 design from below with reversible stack (negative stack). The brass counter weight is now divided into two pieces and electrically isolated from each other so that polarity sensitive power can be brought down via the support wires. In the figure the uppermost mass is mechanically attached to the piezoelectric stack and electrically connected to the negative, right side of the piezoelectric stack. The middle mass is electrically connected to the positive, left side of the stack. Each brass mass has a hole drilled to accept brass screws soldered to the power/support wires. This design allows the entire stack to be unscrewed and reversed 180 degrees between data collection runs. This test is called the Orientation Reversal test. As you might guess this reversal or any change to the stack disrupts alignment meaning the laser must be readjusted.

Figure 4. Thrust Measurement Device using a reversible negative stack

Figure 5. shows the Build 2.0 laser sled which addresses the adjustment issue and the alignment problem in general. Laser and photo detector are aligned and mounted on an aluminum plate that moves back and forth on nylon slides mounted on a second aluminum plate which slides independently left and right. The adjustment screws have a thread pitch 0.5 mm allowing very accurate alignment with the stack in two dimensions, forward/backward and left/right. The third dimension, up/down is adjusted by moving the pendulum support with nuts/screws. Figure 6. is a close-up of the infrared detector used to measure the piezoelectric chip temperature. Since all power runs are at the anti-resonance point (~600 kHz) heating is very rapid and must be monitored carefully. These piezoelectric chips loose their polarity and their usefulness beyond 260 degrees Fahrenheit.

Figure 5. Laser Sled

Figure 6. also shows the negative stack directly above the IR detector. It is electrically connected so the large mass end has negative polarity. This means that the electrical connection above the supporting wires must agree. When the stack is rotated 180 degrees the connection above must also be switched 180 degrees to maintain correct stack polarity. The support wires are not moved therefor the current flow before and after the switch is also reversed. To eliminate all false positives we want to keep everything constant before and after the switch. To eliminate the current flow issue we build another stack (positive stack) identical in design but with the piezo stack electrically connected to the large mass end with positive polarity. For this test we collect data with the positive stack, replace the positive stack with the negative stack and collect new data. Differencing the sets removes any common mode errors due to current flow direction. This test is called the Stack Swap test. Results of these tests suggest small Mach Effect forces and will be detailed in the next blog. As before  let me know other error modes or suggestions that come to mind. Until next time, thanks for viewing. – Larry

Figure 6. Infrared Detector aligned under the reversible negative stack