low voltage microcontroller
That is a difficult question to answer, it depends on the situation.
Normally, the company needing the equipment, or a contractor supplying equipment to them would draw up a precise specification. It would state exactly what motors or other electrical loads have to be controlled, for how long and under what conditions the equipment is activated. That specification would be passed to an electrical or electronic engineer (or team) who using their experience and knowledge of available parts would design, build and test the equipment.
For example, the specification might say:
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Supply is 3-phase 440V at 50Hz.
Three single-phase motors to be switched. Switching to occur at zero volts.
Motor 1 controls, vertical motion of a crane, maximum load 3KW. Extent to be limited by mechanical switches. Position setting is by joystick in crane operators cabin.
Motor 2 controls rotation of crane about vertical axis, maximum load 5KW. Angle of rotation determined by steering wheel in crane operators cabin. Rotation to be limited to 200 degrees clockwise from reference point and 50 degrees anti-clockwise from reference point.
Motor 3 extends crane boom, maximum load 4KW. Extent limited to 1.5 metres from operators cabin to 12.3 metres at full extension. Control by lever in operator cabin.
Opertional temperature range -20C to +50C under 90% humidity.
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That is much shorter than a real request!
It would be up to the designer to work out how the control linkages work and how the motors are actually switched. Usually, there would be much talk between designer and the end user to optimize the system for their needs.
The name for the person or team that do this is "Application Engineers". Typically, they would design it using the best technology then look at ways of reducing costs so they fall within the allowed budget. Other engineers may be involved to design certain parts of the whole system, such as software engineers, environmental engineers and mechanical engineers.
Brian.