If you do not have any feedback signals from your motion system you will never know about the exact position of your drive.
You will not necessarily need an encoder. If you do a plain movement, often it is enough to generate a feedback signal when the drive reaches a certain position. Usually a mechanical switch or a an opto switch is used for this application. The output signal (provided it is TTL level) can be connected to the HOME input of the axis in question on the NI board or the UMI box respectively. Mechanical switches suffer from contact bouncing which might be mis-interpreted by the "find home" routine of the NI boards. If possible, use an opto switch but take care that the switch does not get "confused" by ambient light.
