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Figure V.B.1: Close-up photo of Metro M4 and Add-On Interface Circuitry
The most significant difference between the ADA2210 and the Adafruit Metro M4 to address is the high voltage level. The ADA2210 has a high voltage level of 10 V. The Metro M4 has a high voltage level of 3.3 V. The original Rokhvarg circuit has analog inputs and outputs with a maximum voltage swing of up to 10 V. It is desired that the original circuit wouldn't be modified.
Thus, I built a new interface circuit composed of 3 scaling amplifiers on an Adafruit protoboard. A photo of this interface is in Figure V.B.1. A circuit diagram is in Figure V.B.2. This new interface circuit connects the original interface circuit built by Alex Rokhvarg to the Metro M4. The NTE928M, a single-supply operational amplifier (op-amp), is used in the scaling amplifiers. 1 op-amp reads the linear potentiometer interface and converts its 0-to-10 V swing to 0-to-3.3 V for the Metro M4 to read. Its power supply input is connected to the 5 V power pin from the Metro M4. The other two op-amps drive the solenoids' VCCS. They convert the Metro M4's 0-to-3.3 V swing to a 0-to-10 V swing suitable for the VCCS. Their power inputs are connected to an external 12 V power supply mounted to the protoboard.
Figure V.B.2: Circuit diagram of Retrofitted Interface with New Controller. Letters correspond to parts shown in Figure III.1.
There was a force imbalance due to the original compression spring on the linear potentiometer's shaft pressing on the payload. This spring was on the right side and exerted a leftward reaction force. Because of this, the solenoids would have to provide a rightward force during steady state to keep the payload from moving. I removed the single spring and replaced it with two identical Danco 88101 faucet springs. One is on the left side of the payload while the other is on the right. The median steady-state input force is now more balanced between left and right.
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