I am participating in a student project where we are building small, portable vehicles, starting with a pocket bike and some materials. We are also allowed to buy parts on our own.
Our current plan is to make a motorized long-board (with 49cc gas engine, really dangerous plan, I know!).
What would be really cool, is to have load cells in the front and rear, to control throttle and brakes. The throttle and brakes are controlled by wires, and an attempt at measuring the force needed for braking gave a result around 20N.
So I thought "Maybe we can get some kind of proportional pull solenoids, and connect them to a control circuit and to the wires". And now I have sketched a schematic for one such circuit. The full thing is HERE!
On the left I reduce (divide) the supply voltage with a potentiometer for calibration according to three uncertainties: 1. The gain of the op-amp. 2. The exact weight of the rider. 3. The bias of the cheap Chinese flat load cell.
Note that I have tried to model the sensor as a Wheatstone bridge, and the state of it with a small pot-meter in one of the lower arms of the bridge.
Then in the middle, I lead the tiny two-voltage signal into an op-amp without any feedback.
After the op-amp I have placed a diode, which is supposed to represent a forward reference diode that basically just subtracts a fixed voltage (ideally), representing about half of the weight of the rider, so that the board does not easily accelerate and brake simultaneously, and of course to avoid that it accelerates or brakes when you are standing neutrally.
Then there is a resistance, dimensioned to limit the base-emitter current of the transistor that switches on the current to the actuator that pulls the wire. I have, for now, represented the actuator by an inductor.
Practically, the calibration will be done after the rider mounts the board. He/she will bend down and turn the pot-meter (which, I assume, can be shared between front and rear pedals) slowly until the actuators start pulling the wires, and then just a bit back again, so the transistors don't leak too much current through the actuators, emptying the battery too quickly.
Note that strict linearity between load and acceleration is not a requirement, although some predictability and reasonable behavior would be nice. It is a learning project, most of all. (For the real thing we would be using electro-motors and a digital circuit, I suppose.)
So my question is simply: Does this make sense at all?
I do have specific doubts:
Some of the sensors I have read about specify "input impedance" and "output impedance" both around 1kOhm. Am I right to assume that means the resistance of the arms in the bridge are around that value, or is that a complete mix-up?
Is it OK to lower the voltage over the sensor instead of doing difficult (hard to find) feedback tricks with the op-amp, or will this incur unacceptable noise or something?
Does the gain of cheap op-amps vary too much that a pot-meter will suffice for calibration?
Can a forward reference diode be used this way?
Will running all parts of this circuit from the same battery be a problem?
Can a real proportional solenoid be modeled simply as an inductor, and if so, what would be a typical inductance value of one such device running on 5V, pulling about 10mm max with a force of 20N?
Will such an actuator be fast enough for emergency braking, yet slow enough that driving on uneven ground won't throw you off the board? If not, do I need to run it on a higher voltage than 5V, or (in the second not case) soften out the signal another way?
Now if this all makes sense: How, in what way, am I going to fail trying to implement it? (According to your experience, that is.)
I really welcome a recommendation on what kind of actuator I should look for. The trade websites I am searching do not help me well narrowing down the search.
Thanks in advance for helping me sort out some concerns in the planning process. I have tried to find answers on my own, but it seems the essentially relevant pieces of knowledge are a bit hard to locate and get a grip on.
Update: After reading this tutorial it turns out that it may be possible to use a simple pull solenoid, but it will require some tricks. I think making a biased, periodic control signal with proportional bias and amplitude could be the way to go. And it seems I need something called a "flywheel diode" to protect the BJT.