Exploring an Otis door controller

I've always considered Otis to be the most iconic elevator brand, with a very interesting technical history. This door controller is very "Otis". It seems to be well designed at every level with well laid out circuitry, proper input/output protection and no corner cutting.

The controller is surprisingly complex. It's a computer controlled variable frequency drive specifically designed for one application. It can operate on a variety of networks including a proprietary one and CAN Bus, plus accept simple switch signals for use with older equipment.

It takes a surprising level of torque to open and close an elevator door, as the drive mechanism isn't just driving the car door, but also the mechanically interlocked landing door, which has a self-closing bias. Then there are various friction factors like debris in the door track and general wear and tear. But the unit must also be able to detect obstructions and avoid using excessive force to avoid causing injury to users.
To give good torque and precise speed control the controller has an integrated VFD with an IGBT module that converts the incoming single phase supply to a frequency controlled 3-phase supply for the motor.

The classic Otis service tool is a simple handheld serial display and keyboard that will display whatever data the module it's connected to sends it. The menus can be quite deep, and also used for diagnostic and development purposes. A notable warning in the pocket service tool guide says "Warning: This function is used to to monitor and change the RAM memory hexadecimal codes. DO NOT USE THIS FUNCTION FOR ANY REASON!!!"

The unit is designed to self-learn the door operator it's connected to, and work out motor-phase and encoder sequence by slowly trying the door in both directions until it hits a limit switch. By watching encoder movement and switches it can determine position based on door size data entered by the engineer setting it up. During the learning sequence it also displays questions regarding door movement and direction if needed.

The IGBT block looks like it might be a Mitsubishi module like a PS21553-N. If it is then it does require external control from the processor for the phase control. That makes the software in the board's processor quite complex as it has to control the motor phases while also handling network communications.


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