Month: April 2020

This week, I helped handle a handful of shipping/ordering mishaps. The PCBs that we had originally ordered were somehow mixed up with another team’s order, and Kenny ended up receiving a networking/audio controller board instead of the unipolar motor driver I had designed. Quinn re-ordered the motor controller PCBs, and they should arrive by the 17th.

In addition to dealing with the PCB mix-up, I had to revise our Digikey order for the NEMA 17 motor driver parts after reviewing simulation data of the motor driver power supply and the motor driver itself. After extending the transient simulation time for the motor driver power supply, I found that the SMPS’s inductor current oscillated between 0.6 and 6A, with an RMS average of 3.73A:

Since this was far beyond the current rating of the original inductor I’d specified for the SMPS, I asked Joe to cancel the original Digikey order, and changed the Digikey cart to use inductors that were rated for a maximum of 9A RMS. Since this maximum current would result in the maximum survivable temperature rise for the inductor (roughly 60 degrees C assuming the ambient temperature is 25C), I estimated that the newly selected inductor would experience temperatures of roughly 50C in operation. Kenny will need to be careful not to touch the SMPS inductors with his bare hands while the NEMA 17 motors are running, but they will otherwise be 35C below their maximum operating temperature.

Furthermore, a review of the transient currents through the motor driver core’s bootstrap diodes in LtSpice revealed that they experience regular spikes on the order of 1.5A and an inrush current in excess of 2.5A. Since their maximum current spike rating was 2A, I judged this to be potentially hazardous and switched them out for MBR360s, which are rated for 80A spikes. We were already ordering a handful of MBF360s for the motor driver SMPSs, so this wasn’t much of a load on the budget.

Lastly, I checked the power dissipation of the LM2576 regulator that I was planning on using for the motor driver SMPSs in simulation, and found that it would dissipate an average of 11W in steady state operation. Since the thermal resistance of the regulator’s package was specified to be 32.4 C/W, this would have resulted in a temperature rise of about 360 degrees Celsius if we ended up operating it without a heatsink. Obviously, this would have destroyed the device rather quickly. To prevent this, a heatsink with a natural thermal resistance of 3.8C/W was added to the original Digikey cart, in addition to a mounting kit with a mica insulator for the regulator tab. This thermal resistance is slightly less than half of the maximum required thermal resistance to keep the regulator at or below the maximum operating temperature of 125C (9.04 C/W), and should keep the regulator temperature down to 67C.

Lastly, I made sure to put in an order for 3 ULN2803 Darlington transistor arrays as a contingency in case the PCB re-order for the motor controller doesn’t get shipped to Kenny’s house in time for us to make the deadline for the demo.

Thus far, I am on track with my contributions to the project. Next week, I hope to consult with Kenny on the final construction of our camera mount’s electronics, and with Joy on the interface between the Raspberry Pi and the motor controllers.

This week, I worked with Kenny to specify through-hole components suitable for driving the large NEMA-17 motors that were determined to be necessary for implementing the pan and tilt functions of our camera mount (see Team Status Report). Spinning out the PCB that was designed for this task earlier in the semester was found to be infeasible due to the risk of shipping delays impacting our ability to receive the boards from whichever manufacturer we decided to use (the boards would have needed to be manufactured overseas to keep costs low). We examined the possibility of simply ordering the motor drivers that were recommended on the motor supplier’s website (notably the Big Easy Driver on Sparkfun), but found them to be rated for far higher supply voltages than the motors were capable of handling.

First, we specified through-hole components for a 12V to 3V SMPS for powering the motors. We chose to use a LM2576T-ADJ/NOPB as the SMPS controller, and simply calculated the resistor feedback values for setting the output voltage to 3V. The inductor and capacitor values were kept to the recommended values in the datasheet. LtSpice was used to verify that the design would indeed produce the desired output voltage:

Then, I spec’d out through hole parts for implementing the actual H-bridges for controlling the motors (2 H-bridges per motor). IRFZ24N transistors and IR2302 gate drivers were selected, along with 24.9 Ohm gate resistors so that the driver circuit can tolerate up to 50 nH of parasitic inductance at the MOSFET gates (to reflect the implementation of the circuit on a breadboard). The gate drivers have built-in PWM and ENABLE inputs, so no additional logic was required past a single inverter for buffering the input of the 2nd gate driver. Because the gate driver ICs are synchronous, no special measures were required for generating dead time. High speed 1N4148 diodes were chosen to clamp the inductor voltages to the supply rails.

Verification was carried out in LtSpice:

Note that there are no spikes in the I(V1) waveform, indicating that the gate driver ICs are successfully mitigating shoot-through in the MOSFETs. Furthermore, the current through the inductor representing the motor coil falls rapidly to 0 when the ENABLE signal goes low, meaning that this functionality works in simulation.

There are voltage spikes at the motor coil during operation, but the flyback diodes successfully keep them within 1 diode drop of the supply rails. There’s also some ringing when the ENABLE signal goes low, but it eventually dies down to a steady-state.

Thus far, I believe I am on track for the 1st demo-in-lab, and the Gantt chart schedule. The first demo will consist of a slide show with simulation results for the motor controllers, and the layout for the motor controller PCB that was designed for the smaller compensator motor. Next week, I will first need to compile the slide show for the demo. Then, I will need to compile a Digikey cart for the final through hole components, and work with Kenny to ensure that we aren’t missing any mechanical components for the test-setup for our mount.