KMA200 Programmable angle sensor

Michael Peshkin, 2006-11-08

The KMA200 is an angle sensor system. The Magneto Resistive (MR) element and the mixed signal IC are integrated in one package. This angle system is already pre-programmed, offsets are compensated and, therefore, ready to use.

Magnetic field angular sensing

  • One package angle sensor system
  • High accuracy
  • Resolution better than 0.05°
  • Operating temperature range from -40 °C to +160 °C
  • Digital or analog output signals (user programmable)
  • Online diagnosis
  • Overvoltage protection at all external pins
  • Bidirectional digital interface (SPI)
  • EEPROM (user programmable)
  • Programmable maximum angle up to 180°
  • Programmable zero point
  • Pre-calibrated, ready to use device

Read about its principle of operation in the datasheet.

This device is delivered (default) as an analog output device. It doesn't say that on the datasheet anywhere.

Importantly, its output is linear in magnetic field direction, and independent of field strength.

The field strength must be above a saturation threshold, ideally far above; the bigger the better.

The output repeats every 180 degrees (not a full rotation)

We needed a very small sensor, so we special-ordered NdFeB magnets from China, the home of all NdFeB magnets. A set of 200 magnets cost $40.

They are 8mm in diameter, 0.75mm thick, and magnetized across the diameter (not in the usual face-to-face direction)

They arrived about 5 weeks later. Magnetized rim-to-rim, they stick in a rosette:

I soldered ribbon cable onto the 5 leads and stuck a magnet on a motor with tape, to spin it.

The question to be answered was: are these thin magnets strong enough to saturate the sensors, as they are intended to operate.

At a proximity of about 1mm: yes. Very nice linear trace on the scope. There are two analog outputs that change opposite each other. (Presumably these are provided so that you can read these in differential mode if you want to)

Even out much farther the output looked linear. It began to deteriorate near 1cm

The wiggles are the non-linearity. The double trace is merely a scope trigger issue. Beyond 1cm proximity, the trace started to break up (useless).

 

To measure hysteresis I put it near its magnet and perturbed it with an coil driven at 60Hz.

I monitored the current through the coil (and thus, presumably the perturbing field) with a series resistor, and this voltage became the Y deflection of a scope.

The output of the KMA200, hi-pass filtered at >1sec, became the X deflection of the scope.

I saw this trace, a lissajous figure. The X scale is 10 mV/div.

Ideally the KMA200's output would retrace its same values as the perturbation field does: we should see a diagonal line.

Hysteresis would give us a parallelogram, where the vertical flat edge height projected onto the X axis is the amount of hysteresis, as a voltage.

Phase delay would give us an ellipse.

I see a lot more ellipse (~10mV) than I see parallelogram (~1.5mV). 1.5mV is 0.06 degrees.

Hmmm - maybe this thing just has an output step size of 1mV. Regardless, the hysteresis is small.

Here's a graph the mfg made (calculation, not measurement) of field strength vs. distance from center.

The horizontal axis is the measurement position, 0.25mm above the surface of the magnet and a distance x from the center along the magnetization axis.

By, green, is the field strength parallel to the magnet face. It is maximum at the center of the magnet where we intend to operate the MR sensor. 1mm off center, it's half as large.

Bx, red, is the field strength perpendicular to the magnet face. It is zero at the center of the magnet (by symmetry.)

Ignore purple, Bm, (magnitude)