"Adding the motors basically made it a push-button operation," said Daniel Montgomery, vice president of engineering at RedX. "The user does not have to think about what their environment is or what they need. They just turn the unit on and push the 'activate' button, and no matter where they are, they're going to get good detection."
It wasn't easy, of course. The 18-inch x 12.5-inch x 4.25-inch unit had a signature look and form factor that the company wanted to preserve, both from an efficiency standpoint and from a branding perspective. The problem was that the design was already as compact as possible. Now, the designers needed to fit a motor, gearbox, drive, and wiring inside a housing that was already full, particularly around the sampling baton.
At first blush, it would seem that the application would not present significant torque requirements, but a sampling baton loaded with a full roll of paper can weigh as much as 350g. The motors needed to be small enough to fit in the constrained space while overcoming inertia and reaching target rpm rapidly and reliably, no matter how cold the environment. The motors also needed to be efficient enough to support the analysis of a minimum of 50 samples on a single charge from a 7.2V rechargeable battery, even at freezing temperatures. "That was basically the big trade-off, size versus power," said Montgomery.
Automating the baton
The team went with a Micromo brushed motor that delivered 16mNm of torque, adding a planetary gearbox that brought the total reduction ratio to 14:1. Including a 512 line magnetic encoder helped ensure accuracy. Although performance was important, perhaps the biggest worry at the beginning was reliability.
"When we first proposed the motorized approach to our management, there was some pushback to creating a critical dependence on a fairly complex electromechanical component. If the motor failed or performed unexpectedly, the XPAK would be compromised, leaving our users with no capability when they needed it," said Montgomery.