Kenmore 158: Current Sensor Calibration

The first task: produce an equation that converts raw ADC values into actual motor current. This is not quite the same as the DC calibration, because the motor current is neither clean nor stable.

Step the output current setpoint in 50 mA increments from 450 mA to 1100 mA and remain at each setpoint for 10 seconds while dumping measurements every 500 ms. The ADC count comes from the sampling / sorting / selection process that attempts to pick out either the not really flat top of the current-limited waveform or the peak of the non-limited sine wave.

Convert the raw data dump into a spreadsheet to get a block like this for each current setpoint:

Motor RPM	Shaft RPM	Setpoint mA	DAC count	ADC count	Noisy mA	Comp mA
Setpoint: 600	DACvalue: 2372
3797	334	600	2372	266	724	540
4465	399	600	2372	263	715	532
4734	416	600	2372	265	721	538
4834	438	600	2372	263	715	532
4829	433	600	2372	264	718	535
4857	438	600	2372	264	718	535
4900	438	600	2372	265	721	538
4859	436	600	2372	266	724	540
4887	445	600	2372	265	721	538
4926	446	600	2372	263	715	532
4884	438	600	2372	265	721	538
4890	442	600	2372	264	718	535
4913	440	600	2372	264	718	535
4866	436	600	2372	263	715	532
4895	434	600	2372	264	718	535
4890	442	600	2372	266	724	540
4884	438	600	2372	266	724	540
4913	442	600	2372	265	721	538
4913	441	600	2372	266	724	540
4878	436	600	2372	264	718	535
				265

The lone number on the bottom row is the computed average of the ADC counts for the block, which I did in the spreadsheet rather than in the firmware.

During each ten second interval, set the scope voltage cursor to the eyeballed “correct” value of the motor current waveform, as measured on the Tek current probe. There’s no way to automate this, because only the human eyeball can pick out the, ah, true current measurement amid all the clutter:

For each current setpoint value, create a line with the manually measured true voltage from the scope trace, the calculated true current (using the Tek probe’s front panel scale), along with the DAC setpoint and the average ADC values extracted from each block of that giant data dump:

Setpoint mA	Scope mV	Actual mA	DAC count	ADC count
450	21.80	436	2205	197
500	25.94	519	2261	225
550	29.06	581	2316	245
600	31.56	631	2372	265
650	34.38	688	2427	285
700	36.88	738	2483	304
750	39.69	794	2538	324
800	42.19	844	2594	340
850	45.00	900	2649	350
900	47.50	950	2705	361
850	46.86	937	2649	356
800	43.75	875	2594	348
750	41.25	825	2538	335
700	39.06	781	2483	318
650	36.56	731	2427	302
600	34.38	688	2372	285
550	32.50	650	2316	270
500	30.31	606	2261	253
450	27.81	556	2205	237
400	25.63	513	2150	220

Plot each actual motor current against the corresponding average ADC value:

The linear fit breaks down toward 1 A, because measuring the actual peak of a noisy sine wave doesn’t work well, but the values aren’t all that far off.

Given an ADC value, that equation converts it directly into the actual motor current as estimated by the human eyeball, taking into account all the measurement weirdness. The Hall sensor produces a voltage that’s linearly related to the current, so the reasonable linearity of the data says that the sampling / sorting / selection process actually produces pretty nearly the correct result across the entire operating current range.

Note that the equation doesn’t depend on the DAC output calibration; the ADC and Tek probe simply measure whatever current happens to pass through the motor for that DAC value. The current through the ET227 transistor doesn’t seem to change over the ten seconds required to take the manual measurement, so it’s all good.