Weekly Report #3: Electronics

Hi, my name is Johannes Kugele. I am a member of the electronics team so this issue of the weekly report will focus on the progress of one part of our electronic gear.

After our hold-down release mechanism has been activated, we want to measure the angular position and speed of the solar panel while opening. Because the solar panel is very lightweight, we have to take into account that our measurement devices can easily disturb the movement of the solar panel. For example we should not have a physical contact between the solar panel and our measurement device, because this would slow down the movement of the solar panel due to friction.

Currently I am testing two possibilities:

  1. An array of light barriers each consisting of an infrared LED and a photoresistor
  2. A magnetic rotary encoder (kindly provided by ams. Thank you!)

Both of these possibilities have advantages and disadvantages, so I am building prototypes of both of them.

The first two pictures show the infrared light and the three photoresistors on a bread board. The distance between IR LED and photoresistor has to be at least 23 cm, so the solar panel suits in between.
In the picture of the IR LED you can see a purple glow. This is caused by my camera – in the real world infrared light is invisible to human eye.

Bild1 Bild2Moving an object through the light barriers produces a nice signal on the oscilloscope. From the delay of these two signals you can calculate the speed of the object passing the light barrier. And of course you know where the object is located if a light barrier is interrupted.

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Now that it’s proven that the light barrier works on a bread board, it’s time to build a prototype PCB. The next picture shows the prototype PCB at its current stage. The PCB is almost completed – only drilling and soldering hasn’t been done yet.

Bild4Our second alternative is a magnetic rotary position sensor, named AS5145 from ams. The sensor measures the magnetic field caused by a small cylindrical magnet and calculates the angle between the magnetic field and itself with this information. The angle can then be read out via SPI or PWM.
This is a very lightweight and contactless method. The sensor is easy to use thanks to two led indication outputs showing if the magnet is well positioned above the sensor.
For a first test I soldered the sensor on a breakout board, connected the PWM output to my oscilloscope and glued the magnet on a pencil.

Bild5 Bild6On this picture you can see the PWM output. 600 µs duration of the positive edge and a period of 4160 µs equal to an angle of 51.9° between the sensor and the magnet. The sensor has a good absolute accuracy and low noise, but the magnet has to be centered quite precisely above the sensor.  If you don’t do this the sensor still provides position information but the accuracy is getting worse the further you move the magnet away.

Bild7Because of this it is not possible to do a good test of the sensor while holding the magnet by hand, so I will construct a little mount for the magnet next week and continue testing.

So I hope you liked this little report, although it covered only a little part of the project.

See you next week

Johannes