Captian Marvel

The Keck domes can be controlled three ways… From a set of manual controls on a panel at the base of the dome, via computer control when observing, and from a radio controller that you can use from anyplace in the dome. This radio controller has long been called Capt. Marvel around Keck, the original versions looked like a prop from a 1930’s sci-fi serial movie.

The dome radio controller, called Capt. Marvel
The dome radio controller, called Capt. Marvel
The radio controller is actually a crane controller, a standard unit you can buy that can safely control large machinery, including enormous factory or dockyard cranes. Being a standard unit it has a number of safety and security features built-in… Fail-safe operation, coded communications, and more insure reliable operation.

The unit is standard, the panel is custom. When buying the transmitter you need to specify the front panel layout including the switches and labels for your application. Thus our transmitter has a panel arranged to our specification with switches for dome rotation and opening the shutters. In the middle is a bright red-emergency stop switch to insure you can immediately stop everything if something goes wrong.

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I took a break from working with a “big” microcontroller to work with a little one. The PIC12C671 is definitely little when compared with the PIC18F66K80 I am using in the GenPIC utility PCB. With less than 2k of program space and a mere 128 bytes of RAM it is definitely limited. Consider that the chip only has eight pins, two of which are power and ground, leaving six I/O pins to get the job done. No problem, I only need two I/O pins for this task and that few bytes of RAM is more than enough!

A simple heater using a bang-bang controller.
A simple heater using a bang-bang controller.
This project is pretty basic… A bang-bang heater controller. This simple form of controller simply turns on and off as the temperature (or some other controlled parameter) goes up and down, there is no attempt to vary the output, all or nothing. Bang on, bang off, or simply a bang-bang controller as it is known in the trade.

A bang-bang controller is inherently reliable and stable because it uses two different control setpoints, a high and a low. Because these control points are separated by a large margin, called hysteresis, the controller will not oscillate or rapidly turn on and off. In this case the heater will not turn on until the temperature falls below 10°C and will not turn back off until the temperature rises above 15°C. That five degree margin is called hysteresis, and ensures a good period of time between on and off.

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