I’ve spent the last week or so modeling all 68 parts in an MPS that @cushjbc kindly leant me to take apart. In this thread I will show what is inside an MPS and how it all fits together, along with engineering diagrams. I will post to this thread as and when I get time.
The name for the parts are mine. I have no idea what names Bosch used.
First a general view that we are all familiar with.
Now we remove the case.
The MPS uses a stack of two aneroid cells, the same idea as can be found in common aneroid barometers. In a barometer the cells are sealed with air at a specific pressure inside, however in the MPS the inside of the cells are exposed to atmospheric pressure via the adjustment screw.
A wax ring is used to create friction, stop accidental movement of the adjustment screw and create an air tight seal between the adjustment screw and the case.
When the adjustment screw is turned the entire aneroid cell stack rotates. The effect is to add or remove pressure on the metal core inside the coil windings.
As the atmospheric pressure increases the stack expands pressing more on the core inside the coil, pushing it further rearwards.
The two cells are not identical. The second cell is slightly smaller in diameter and thinner. This must be part of the precision design to work out exactly the combination and sizes of cells to get the desired amount of motion as the atmospheric pressure changes.
The second cell is sealed and has a factory-set pressure inside.