First, let me explain that I know very little of electronics. Thus, please, forgive me if I am asking something very obvious or abstract. I have tried to Google this but I am obviously not using the right words as I find very little.
The problem: I have an auto-pilot that is supposed to control a hydraulic pump for a ram which will actuate a rudder. Well, I would like to use an electric 12 vDC brushed motor to control such rudder instead of the ram. The motor has a reduction and is connected directly to the output power for the hydraulic pump, which is 12 vDC, up to 25 amp and reverses the polarity when it needs to actuate in different directions. It works more or less and I would like to get rid of the 'less'.
Is it possible to have an interface between the output of the autopilot and the motor? …so that:
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It fades the voltage of the output. When the autopilot decides to apply power to the motor, it does it straight away. I would like to have some dumping on it so that the motor starts and stops smoothly. I have put some capacitors attached to the motor but am not sure is doing much; probably just absorbing peaks and filtering noise?
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It brakes the motor when there is no voltage applied (by shorting it?) So, when the autopilot does think that the rudder is in the right position and does not provide power to the motor, the motor cannot spin freely.
In short, the power is 12VDC and reverses on demand of the autopilot. The motor is 12 VDC and there is no need to alter voltage or amp in the power.
I have seen many motor drivers but I believe there must be something simpler? This one can do what I want but I would then need to transform the -12/+12 output from the autopilot to 0/2.5/5 in order to use the analogue input of the driver. I am sure it must be simpler than this.
I obviously appreciate very much a full solution but I am very much willing to do my homework. Please, any advice or heading so that I can research further in the right direction?
Edit:
It seems I didn't explain well.
The autopilot is a controller and is fed by many sensors -heading, rate of turn, wind, rudder angle, GPS. It basically can follow a track or maintain a heading compensating for any forces. The problem is that it controls the rudder with a +/- 12 VDC, which is actually activating a DC motor is both directions. What I would like is to improve that signal and brake the motor when there is no current out of the autopilot, by shorting the motor I guess, and to apply those +/- 12VDC progressively, let's say with a period of 1/4 of a second, so that the motor starts and stops smoothly.
Added:
For a soft start. Could I use a simple circuit with a resistor, a capacitor and a MOSFET? Initially, the MOSFET is off and the current goes through the resistor, once the capacitor is charged, the MOSFET turns on and the resistor is bypassed.
And for braking it. Could I use a high power resistor in parallel with the motor controlled by a MOSFET?
Best Answer
You first have to decide what signal exactly the autopilot is producing. Is it something that says go more right or go more left, or is it saying go to this rudder position? Note that one is the derivative of the other.
In either case, it sounds like you need a real control system, not just something that applies power to the motor sometimes. You should probably implement a rudder position controller, then either use the signal from the autopilot directly or integrate it, depending on what it actually is.
The position controller will apply whatever current to the motor to maintain the required position. If it doesn't take much torque, then little current will be applied. If the rudder is at one end of travel and there is a lot of back force on it, then the system will automatically apply more torque. The point is, what you really care about ultimately is rudder position. So control that, then use whatever signal you get to adjust the desired rudder position.
On a separate topic, someone who "knows very little about electronics" shouldn't be messing with a flight control system. You have no business being in there. If you want to kill youself, that's your business. But it's my business if you want to do it in a airplane because I might be underneath when it comes crashing down.