Break Time 08-3-98

August 3, 1998 Design News

BREAKTIME The lighter side of engineering

RF control lets sleepers choose how they snooze

Tom Brockman, Vice President of Engineering Winland Electronics Inc. Mankato, MN

Select Comfort claims to lead the air- bed market it established in 1987 following a developmental period to pioneer a new consumer product category. Consumers have snuggled up to the idea of a mattress that adjusts to each sleep partner's firmness preference.

Winland Electronics is a custom electronic controls designer and manufacturer. We participated in this emerging market by advancing the radiofrequency (RF) technology used to regulate mattress pressure.

Our objective? Add RF functionality to increase the transceiver's maximum control distance. Our challenges included the interaction of parts and harmonics, the size and length of the battery power, and the cost to manufacture.

In the original version, people would put their hand around the remote-control unit in regular use, causing frequency problems for the radio. We pretuned the radio so that it was on frequency when a person put their hand on it. The new RF control also had to be backward compatible with previous Select Comfort models. I always tell my engineers that if it were easy, everyone would be doing it.

We ultimately created an entirely new RF design, transmitter, and receiver in less than two months by working around the clock. The design increased the control range by 200% without adding transmit power, eliminated manual frequency tuning, and optimized manufacturability.

RF distance performance can be enhanced by overcoming naturally occurring "noise" or interference. We added "low insertion loss" surface-acoustic- wave (SAW) filters. The selectivity of the SAW filter on the receiver's front end lets the radio see through extreme interference conditions. As a result, the control range increased 200%.

Transmit and receive antenna design topologies let us enhance control performance and pc-board manufacturability for mass production. In the case of the transmit antenna, we cut assembly time, materials, and tuning costs by producing the antenna during automated pc-board assembly instead of using a discrete antenna element. The solution was a "trace" antenna, tuned for the circuit, which allows the pc-board to become the transmit antenna.

The receive antenna is a typical coil design. We refined the antenna further by working with Winland's manufacturing engineers to simplify assembly on the pc board. The result is a hook system that the supplier bends into the antenna.

We used a Philips Semiconductors (Eindhoven, The Netherlands) radio receiver integrated circuit (IC) because its dense functionality increases performance while maintaining the small transceiver dimensions. The IC combines the intermediate frequency mixer, the data filter, and final amplifier, all on a single, compact chip appropriately sized for use in a small, hand-held transceiver.

We pre-screened early for FCC and Industry Canada compliance through our lab. Our final approval came from an outside laboratory. Through this pre-screening process, we tracked down and reduced excess transmitter harmonics during the board layout stage.

Developing this RF control was a fun project. We slept on a lot of the mattresses during the research phase, and many Winland employees have subsequently bought them.

Design Objective

To add RF functionality that would increase an air mattress transceiver's maximum control distance.

Upgrade performance while, keeping the hand-held transceiver small. Reduce the cost of the unit by minimizing parts and leveraging the cost efficiency of high-speed, pick-and-place pc-board assembly equipment.