Load Cell Reference

The load cells are temperature compensated and they are designed to measure only the force in one direction. That means that off-centered loads will have a very small effect on the thrust measured (<1% in most cases). Nevertheless, off-centered loads can damage the load cell if the load causes a plastic deformation of the aluminium.

The stain gauge are connected in an H-bridge configuration. Please see the image below for electrical details.

Load cell measurement circuit

The load cells are measured using an ADC. Each channel has an instrumentation and gain amplifier to condition the signal, as well as a small low pass filter.

Electrical RPM measurement

The electrical RPM is measured from a square wave generated with by passive circuit. The circuit works in most cases, but due to the widely varying signal shape and frequency of the voltage measured on one of the motor phase, the circuit may not generate the square wave correctly. This happens mostly for motor with a Kv under 700.

Accelerometer

The accelerometer is intended to be a safety feature. The magnitude of the acceleration is calculated multiple times per second and a low pass filter is applied on this magnitude. You can use it for comparison purposes only.

Electrical protection and ground loop.

The circuit is not protected for reverse polarity or over-current. However, the current measurement circuit is designed with a safety margin.

In some cases, a ground loop may occur when using a power supply. This happens when the ground level from the USB is not the same as the ground level from the power supply. The circuit may reset when a ground loop is present. There are a few solutions:

  • Use a USB isolator providing at least 200 mA.
  • Use laptop power with a battery.
  • Use a battery instead of a power supply

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