Projects from the grapevine: Bárány chair repair

Note: this blog post has been produced with permission from the client.

Update (2020-12-31): After receiving and installing the replacement parts, we have another successful repair! Here’s a video of the result:

Recently, a repair job came across my plate for some exotic test equipment: a “Bárány chair” used for testing pilots. This is one of those odd projects that comes through a friend-of-a-friend-of-a-friend. No electricians or repair technicians in the city were willing to touch this niche equipment – and I can’t blame them! I couldn’t find any information on this machine whatsoever.

Like most of you, I don’t specialize in exotic chairs – so what is a Bárány chair? From Wikipedia:

The Barany chair or Bárány chair, named for Hungarian physiologist Robert Bárány, is a device used for aerospace physiology training, particularly for student pilots.

Via Wikipedia

Here’s what the unit looks like:

The chair is motorized, with a wireless control to operate it:

The control allows the operator to adjust the rate and direction of rotation. The chair had stopped spinning, and the manufacturer was unresponsive and/or unwilling to repair the unit. It was clear from dead-end google searches that this unit is from a very limited run of a very niche product – there wasn’t an operator’s manual or service diagram to be found. Note the use of 3D printing for the remote control enclosure. If I had to guess, this probably came out of an R&D lab.

Upon arriving, the remote complained that it had no power to the base unit (“Dashes” observed). The only troubleshooting info available was listed on the side of the controller:

A quick investigation revealed that the power supply was plugged into a switched outlet, which had been turned off. Moving to a regular outlet caused the power supply to come online, and the controller sprang to life! No error code – just “00” on the two-digit readout. The chair was still not responding to the rotation settings – but when I manually spun the chair, it displayed the sensed rotation speed on its LED display. I concluded that the logic power supply and wireless communications were working. At this point, I suspected that the problem was in the motor, motor power supply, or the motor controller.

I investigated the power supply for the motor and found it to be outputting a steady 5V – which seemed unusual for a giant motorized chair. I would expect 12V or 24V for a large DC motor. I entertained the idea of adjusting the power supply to a higher voltage – could the power supply have degraded in its ten years of operation? The motor controller datasheet suggested that 6V was the minimum supply voltage, after all. Luckily I noticed that the motor controller had been modified for this application: it passes the motor power directly to the control logic. You can see a jumper wire that bypasses the 5V regulator in the image below. I expect that the 6V minimum is to accommodate the voltage drop incurred by the 5V regulator. Note, modifying off-the-shelf parts for out-of-spec operation is not a design technique that I recommend.

With this knowledge, I was reasonably confident that the entire system was designed to run at 5V, unintuitive as it may be. Good thing I didn’t adjust the power supply! I attached my scope to the control signal from the control logic to the motor controller to see what signs of life existed.

The control signal pictured above appeared to be a servo-style control signal – a carefully timed pulse controls the motor’s speed and direction. When I adjusted the speed control on the wireless remote, this control signal was unchanging. This suggested to me that the wireless receiver, control logic, and motor controller were likely all working – but receiving a “don’t spin” input signal. I turned my attention to the remote control:

And there it is – the rotary encoder for the speed control knob was split in two! One half was bolted to the plastic housing, but disconnected from the rest of the encoder. It still had an actuating “click” feel, without being electrically connected.

With the remote still disassembled, I pushed the rotary encoder back together to make a temporary connection – and the chair started spinning. If I had to guess, someone was a little too rough with this remote.

The replacement parts are ordered – with some careful soldering, I expect a successful repair and many dizzy pilots.