I would like to build a controller for this actuator: http://www.firgelliauto.com/product_info.php?cPath=77&products_id=4 or a similar device.

I need a relay that provides normal and reversed polarity 12V DC power, and that can be turned off, and I would like to control it with a computer via RS-232.

I have been looking around the National Control Devices website, but I can't find anything with three positions. Can I use a 2-channel DPDT relay and wire the motor up to both outputs, with one providing normal and one reversed polarity? Is there an easier solution?

 

For starters, you need a relay like this one:
http://www.mouser.com/ProductDetail/NEC/EP2-3N1S/?qs=sGAEpiMZZMtSzCF3XBhmW7Rmwp1m9ujJ%2bH0Q0/2ZXro=
1 Form C * 2; or two independent SPDT relays in one package. Alternatively, you could use two SPDT automotive relays, but that would be more bulky.

The basic idea is that the actuator would be connected between the two common terminals. The two NC contacts would connect to ground, and the two NO contacts connect to +12v. That way there is no combination of coil energizing that could cause a short; and when no power is applied to either relay coil, the actuator is being braked by the two actuator terminals being shorted.

I don't have my schematic images available at the moment to show the basic wiring for the relays. The RS-232 is a separate issue; probably easiest to handle with a MAX232 as a front-end to a microcontroller that has a UART or other serial communication capabilities.