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Scorpio Picaxe 08M2 SPPC Test Results Summary • Tested and written by Ian Gardner. Edited and published by the TMSC on

3/12/2020

A Scorpio Picaxe 08M2 Solar Panel Power Controller (SPPC) was tested and compared with the Automax electronics unit. This unit is theoretically inferior to the Automax but also much less expensive ($25 vs $105).

The shortfalls of the Picaxe unit are:

1) It does not track power in real time and instead sets it to the predicted maximum power voltage at turn on, holding this setting until being switched off. This prediction may be a little off on certain solar panels, particularly those with nonstandard Fill Factors.

2) Users must ensure that the solar panel is unshaded and held at the angle of racing at turn on. Failure to do so may see the unit target an incorrect setting and give reduced race performance.

3) Unlike the Automax, the Picaxe-based unit does not continuously adjust for the effects of panel heating or any light level changes during a race.

4) The Picaxe unit uses a diode for commutation instead of active switching used by the Automax. Active switching has lower losses when the unit is working hard with a motor stalled down or running at well below its optimum operating RPM.

Some of the areas identified above have been investigated in the pages below to evaluate their effect on car performance during racing. Conclusion: Despite the theoretical advantages the Automax has over the 08M2 Picaxe unit, testing has proven that there is in fact little difference between the units when it comes down to model solar car performance. Active switching instead of diode commutation, as well as tracking panel power in real time (ie adjusting for panel temperature or light level changes during a race) has no significant effect on car performance. Test details and results are summarised in the pages below.

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Mathematical Simulation Predictions of Race Performance: The dynamometer test data included in Appendix A was used for predicting solar car performance using a Mathematical Race Simulator. This data was recorded for both the Picaxe and Automax units in the form of motor torque (mNm) against RPM at a range of sun levels from approximately 10% up to 100%. The curves depict how a Faulhaber 2232 6V motor converts the electrical energy from the electronics unit into the mechanical motion of the motor shaft. This is what actually drives the car. These data curves were then linearised for input into the Mathematical Race Simulator (please see the Simulator instructions file at www.modelsolar.org.au/tech-support-old/simulator for further details on this) and the performance of a “Standard Evaluation Car” evaluated as follows: (actual real-world car times are typically within 3% and so a 3% time variation is provided in brackets). Sun % Predicted 1 lap race Time in Seconds Picaxe 08M2 Automax 10 53.3 (±1.6) 50.3 (±1.5) 21.9 31.8 (±0.95) 32.55 (±0.97) 32.6 26.4 (±0.79) 26.05 (±0.78) 42.9 23.4 (±0.70) 23.8 (±0.71) 52.2 21.95 (±0.66) 22.3 (±0.67) 60.4 21.2 (±0.64) 21.35 (±0.64) 71.2 19.9 (±0.60) 19.8 (±0.59) 84.5 18.45 (±0.55) 18.7 (±0.56) 90.7 18.1 (±0.54) 18.3 (±0.55) 100 17.7 (±0.53) 17.8 (±0.53) In an attempt to minimise the effect of linearising the dynamometer curves, another set of simulations were then run with slightly modified motor data: Sun % Predicted 1 lap race Time in Seconds Picaxe 08M2 Automax 10 35.1 (±1.05) 35.45 (±1.06) 21.9 30.1 (±0.90) 33.0 (±0.99) 32.6 26.35 (±0.79) 26.75 (±0.80) 42.9 23.1 (±0.69) 23.6 (±0.70) 52.2 21.3 (±0.64) 22.4 (±0.67) 60.4 21.05 (±0.63) 21.25 (±0.63) 71.2 19.95 (±0.60) 19.65 (±0.59) 84.5 18.55 (±0.55) 18.45 (±0.55) 90.7 18.15 (±0.54) 18.35 (±0.55) 100 17.75 (±0.53) 17.65 (±0.53) From the simulation predictions in the two tables above it is obvious that there is no significant difference in car performance based on the electronics unit used.

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Panel Heating Test: The following test results were obtained on a light box set to 100% Sun. The panel temperature was the same (26 Deg C) at the start of each test. A 2 lap race is well and truly over by 35 seconds at 100% Sun. Electronics unit Power to motor Watts Time seconds Picaxe 08M2 5.54 13 5.47 25 5.37 35 Automax 5.48 10 5.45 25 5.38 36 Light Level Change Test: The light box was set up at 80% Sun with the ability to quickly switch down to 21% Sun. This test gave the following results. The panel temperature was the same at the start of each test. Electronics unit Power to motor Watts 80% Power to motor Watts 21% Picaxe 08M2 4.56 1.03 Automax 4.48 1.04 The results obtained from the panel heating and light level variation tests carried out above prove that the ability to track panel power in real time provides no noticeable advantage over the course of a normal race.

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Commutation Diode vs Active Switching: This difference is difficult to measure directly so an indirect method was employed. Using the power vs RPM graphs obtained from the dynamometer tests and comparing the energy difference between the two units from start, where the motor is stalled up to the time the powers coincide, a quantitative value can be put on the difference in performance. The dynamometer graphs for 60.4% Sun level were used to produce the composite graph below. These graphs were chosen as they showed the largest torque variation obtained in this series of tests, thus giving the “worst case” condition.

The graph shows power difference but comparing energy is a more useful tool for evaluating differing performance. Energy is power acting over time, one watt for one second is one Joule. The graph depicts power difference. To obtain energy we need to know over what time period the power difference acted. On the graph, power outputs from the motor coincide at 4500 RPM. The mathematical simulation predicted that 4500 motor RPM occurs at 2.6 seconds. Using this time an energy difference of 0.4 Joules was calculated (green cross hatching shows power difference). Interestingly, the Picaxe 08M2 unit produces slightly more power than the Automax once motor RPM exceeds 4500 RPM. This slight extra power is available for the remaining 18.75 seconds of the race and amounts to potentially an extra 3.75 Joules, well and truly negating the 0.4 Joule deficiency of the first 2.6 seconds. In fact, race simulations show the Picaxe powered car to be slightly faster (lower race time) in this particular set of results.

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To understand how significant this extra 0.4 Joules of energy is over the course of the complete race, consider how much power it equates to if spread over the complete race and then how much current from the solar panel is required to make up for it. By dividing the 0.4 Joules by the simulated race time of 21.35 seconds we get 0.0187 Watts which equates to 0.00276 amps at the typical 6.8 volts maximum power point of the solar panel. With the Picaxe unit requiring only 0.003 amps to run the electronics and the Automax requiring 0.011 amps it comes as no surprise that the Picaxe unit can easily make up this initial energy deficiency. Evaluation of active switching used in the Automax, compared to the simple diode used in the Picaxe 08M2, indicates that no significant advantage is derived from the active switching commutation.

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Appendix A - Dynamometer Graphs: Red trace is Automax results, Blue trace is Picaxe 08M2 results.

100% Sun

90.1% Sun

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84.5% Sun

71.2% Sun

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60.4% Sun

52.2% Sun

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42.9% Sun

32.6% Sun

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21.9% Sun

10% Sun

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4.3% Sun In general, the graphs coincide very early on in a model solar car race, from several hundred mm to around 4 metres. Consequently the apparent advantage of the Automax in the low RPM region is not particularly significant. Most of the race is run in the area where the units are virtually identical in performance.