Redesign of the BeoloverRPM Device: Precisely Measure and Log the Platter RPM

I recently decided to redesign my BeoloverRPM device, which I started developing in 2015 for the detection of intermittent RPM variations mainly of DC platter motors. I thought after ~8 years it could use a little update in terms of ergonomics and performance. The original device was based on an Atmega 328P processor (aka 'Arduino') and this limited the performance a bit. I recently got into using the much more advanced ESP32 platform. This enabled a few new tricks due to the much higher speed.

The new BeoloverRPM can be placed on the aluminum panels for a quick and convenient RPM check

but it can also be configured for frame mounting when working on a Beogram:

This was achieved by a design where the frame adapters can be removed from the main device:

The BeoloverRPM comes with two frame adapters that have different spacing between the 'legs' that straddle the enclosure wall. B&O used slightly different enclosure designs with varying wall thickness.

Another significant update is that the BeoloverRPM now has two data acquisition modes:

The modes can be selected via buttons on the device. The 'slow' mode performs similar to the previous designs. It shows the RPM and min/max values on the display, and sends a RPM snapshot to the serial port every 10s (faster if the RPM changes quickly).

The 'fast' mode is new: It graphs all datapoints as they come in whenever a rib passes under the sensor. The display shows a rolling snapshot of the last 64 datapoints. The data is also sent out via the serial port. Here is an example measured on a Beogram 4004:

The main graph shows the data output from about 60 platter rotations. The graph shows a repeating pattern modulated by a sinusoidal envelope. Closer inspection yielded that the pattern repeats every 24 points, i.e. everytime the platter makes a full turn (there are 24 ribs on the platter). This can be explained by minute changes of the rib spacing (it varies by about 0.1%).

The sinusoidal modulation is a result of the feedback based RPM control of the DC platter motors of the later Beogram 4002 and 4004. Feedback control always results in an oscillation of the actual value around the set point. And that is what we see here. The analog feedback mechanism used by the later Beograms is based on time constants defined by capacitors and resistors, and this results in sinusoidal RPM variations.

Fascinating stuff!...;-).

I made a short video about the new BeoloverRPM. Enjoy!