How should we do market research without compromising our IP?

Since you would like to field-test the device with a multiplicity of customers, I do not believe that having the customers sign NDAs would truly be a viable solution. Therefore, I would suggest, at the minimum that you file a provisional patent application for the device/system before you disclose the device/system to the public through means of your field-testing. As you receive feedback from your customers, you may desire to make improvements to the device/system - you would then file additional provisional applications for the improvements. When filing the additional provisional applications, simply add the improvements to the original provisional application and then file the composite disclosure as the new provisional application. You can keep doing this multiple times until you are ready to file a non-provisional patent application which must be filed within one year from the filing date of the first provisional application and which can effectively comprise your last provisional application since all improvements will have been included within the last provisional application. Note that a provisional application by itself will never lead to a patent because provisional patent applications are never examined - they must be perfected by means of the non-provisional application which will be examined in due course and hopefully allowed to proceed to grant of your patent. The provisional applications merely establish your priority date as to when you effectively developed your device/system. If you also intend to pursue foreign patents, you will also need to file a PCT application within the one year time frame from the filing of the first provisional application.