This is a wheatstone bridge. Something you will rarely encounter when messing with E30s, except we need one in this case.
The linear pot is the equivalent of a strain gauge. It exhibits small changes in resistance when measuring the physical property of interest. Wheatstone bridges are the signal conditioning circuit of choice for these applications.
Why? It's not because of sensitivity. If you look closely the bridge only produces about 150 mV/in of voltage change. Same as the simple voltage divider. Difference is in DC offset. Resistor divider has a large DC component, on top of which there is a small DC change. If you put that into an amplifier, the DC offset will just drive it to the rails. Example: If R1=2500 and the pot equals 2500, Vout is 2.5V. Amplify that by more than 2X and the output saturates.
However, the bridge can be balanced, such that when R1=R2=R3=Rg, DC output is zero. This means we can put the output into an amplifier and get more gain on the signal, i.e. get better measurements. We'll use a gain of 6.
The AIM system accepts 0-5V analog input. By setting R1=R2=R3=2K the bridge will balance out at a six inch draw on the pot. With an "at rest" pull of nine inches, we'll get three inches each of compression and rebound within the 5V range.
NOW I can build the damn thing. :devil:
The linear pot is the equivalent of a strain gauge. It exhibits small changes in resistance when measuring the physical property of interest. Wheatstone bridges are the signal conditioning circuit of choice for these applications.
Why? It's not because of sensitivity. If you look closely the bridge only produces about 150 mV/in of voltage change. Same as the simple voltage divider. Difference is in DC offset. Resistor divider has a large DC component, on top of which there is a small DC change. If you put that into an amplifier, the DC offset will just drive it to the rails. Example: If R1=2500 and the pot equals 2500, Vout is 2.5V. Amplify that by more than 2X and the output saturates.
However, the bridge can be balanced, such that when R1=R2=R3=Rg, DC output is zero. This means we can put the output into an amplifier and get more gain on the signal, i.e. get better measurements. We'll use a gain of 6.
The AIM system accepts 0-5V analog input. By setting R1=R2=R3=2K the bridge will balance out at a six inch draw on the pot. With an "at rest" pull of nine inches, we'll get three inches each of compression and rebound within the 5V range.
NOW I can build the damn thing. :devil:
Comment