Hello,

Very new to the forums and have searched a ton and read lots but haven't quite found what I'm after.

I am looking for the ability to take a variable DC input of -12vdc to +12vdc and have that control the output of a separate 18v power source of -18vdc to +18vdc to control a motor (30amps)

I have little to no info on the component that I am getting my input from other than it will output 0vdc (off), 6vdc (speed 1 fwd), 9vdc (speed 2 fwd) and 12vdc (full speed fwd) and reverse -6vdc (speed 1 rev), -9vdc (speed 2 rev) and -12vdc (full speed rev).

What is our there that can do this for me ?

 

To control a DC motor it is probably best to go with a Pulse Width Modulated (PWM) H bridge driver all driven from a micro controller.
But more info would be a help. What is your application?
Will the speed be variable or just stepped?
Do you want built in delay on motor reversal? That is a good idea as it does help limit smoke
What is the motor you are wanting to use? do you have data on it?
If it can run at 30Amps, your controller must be much larger to allow for stall currents on over loads.

 

Welcome to AAC.

Lacking some important information my first impression is that you are stating your problem incorrectly. Rather than saying your problem is translating -12V ~ +12V to -18V ~ +18V, I would say your problem is:

I have a black box that outputs several voltages intended to control a motor. There are 6 speeds, 3 each in forward and reverse, like this:

Speed	Direction	Voltage
1	Forward	6V
2	Forward	9V
3	Forward	12V
1	Reverse	-6V
2	Reverse	-9V
3	Reverse	-12V

The motor I need to control requires -18V ~ +18V at 30A. I need to use the values above to control this motors speed. I have no access to the black box other than its output.

I am considering a simply linear translation between the two voltage ranges but I don't know how to do that. Is it a good idea or is there some better way?

Still missing important details, it is my first impression that to solve your actual problem, you should be considering some scheme to command a motor controller rather than translate the voltage. As mentioned by @dendad PWM is the most common scheme for this, for good reasons.

Keep in mind that "simple" solutions can often become the most complicated as you attempt to work around their shortcomings. What looks like complexity in a good solution is just a reflection of complexity in the problem when fully stated.

It would help a lot to know the actual application for this. What is the problem that all of this is a solution to?

It is also important to know your time and money budgets, at least basically.

 

First, "Y" is correct. However, PWM (pulse width modulation) is not a trivial control scheme to build, and even with a ready-made module it is still not simple.
So my questions are: first, what is the intention with the desired arrangement? and second, why 18 volts? it seems like the motor is probably rated as a 12 volt motor.
And what is the actual application? That is a fair amount of power, 18 volts X 30 amps= 640 watts, approaching one horsepower.
Whatever is created will not be cheap.