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# how to control this??quite urgent

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#### blibala

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i want to build a car which its motor speed can vary according to the terrain of the field. i mean on the flat surface, the car will move at constant speed.when it is going up the hill, the speed will increase to mathc with the constant speed.when driving down the hill, it decreases the speed to match the constant speed.what feedback circuit should i build?and how?or anyone has different suggestions?

blibala said:
i want to build a car which its motor speed can vary according to the terrain of the field. i mean on the flat surface, the car will move at constant speed.when it is going up the hill, the speed will increase to mathc with the constant speed.when driving down the hill, it decreases the speed to match the constant speed.what feedback circuit should i build?and how?or anyone has different suggestions?

Well firstly you obviously don't want the speed of the motor to vary, if it does, the speed of the car will vary!. What you need is the power to the motor to vary, according to the speed of the car, in order to keep the speed constant.

It's a pretty normal servo feedback circuit, the biggest problem is probably getting a feedback signal from the speed of the car - some kind of tachometer signal is needed, opto-pulses, an electric motor used as a generator, hall effect sensors - there are loads of options, the problem is probably going to be fitting them to a wheel.

control

what you need is a simple form of a control system.

First figure out a way to convert your car's speed into voltage. Another poster mentioned a tach, and that may be a good way of doing it.

Now - let's say you want your car to move at 10 cm/sec, and lets say this corresponds to 5 volts when converted to voltage using a tach circuit.

This means you need a 5 volt reference.

Here's the basic version of the circuit you'll need. Now you have two signals - your 5 volt reference, and output from the tach. You take an op-amp, and connect your 5 volt reference to the positive input, and the output from the tach to the negative input. You connect the ouptput of the op-amp to the drive of your motor.
What happens is this:
a) your car is running at the desired speed (in our case 10cm/s), which means that the tach produces 5volts, and both inputs of the op-amp are equal, and you get 0 at the output (meaning you don't need to apply power to your motor, because you are already going at the desired speed)

b) your car is going too fast (let's say 15cm/s), which means that the tach will produce, let's say 7.5 Volts. Now you have 5 volts reference connected to the positive input, and 7.5 volt reference connected to the negative input - which means that the output of an opamp will be negative - thus slowing down your motors, because you are going to fast.

c) your car is going too slow (let's say 5cm/s), in which case your tach produces 2.5V. Your reference is 5Volts, and you input 2.5 Volts to the negative input of an opamp - which means the output will be positive, thus increasing power to your motors, and speeding up your car.

This is the basic version of a control system (in fact you are using proportional version of the controller). If you want to go deeper into the whole thing, you can do PI controller (proportional plus integral), or PD (proportional plus derivative). And if you have plenty of time you can even do PID version (proportional, integral, and derivative). Integral lets you maintain constant speed more closely, and derivative allows you to adjust the speed faster. Look up control systems on-line to find examples. For your particular case I'd recommend starting with proportional, and doing integral later on. Also - don't use amplifier in open-loop configuration - apply negative feedback.
Hope this helps.

i want to build a car which its motor speed can vary according to the terrain of the field

Appears you mean a vehicle cruise control system, you can get those things ready-made that you can install yourself using a manual that comes with it. I think you can get them in either electric or vacuum types, the latter being cheaper.

There is no need to convert the speed to an analog voltage. You can use a tachometer that measures wheel speed, shaft speed, transmission speed, etc. It is easy to convert any of these to a pulse signal. A micro monitors the pulse frequency or period to compute the speed. Then a feedback loop (perhaps a PID) is used to adjust the motor power by way of PWM or linear amplifier.

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