Electrical System (B Model and Newer Beds)
Chapter 3: Theory of Operation
CareAssist® and CareAssist® ES Bed Service Manual (MAN330 REV 4) Page 3 - 17
3
Detection of Bed Connected to AC Power but Brakes Not Set
The detection system operates by a normal on switch, mounted on the foot end
crossbar and is activated by the brake/steer bar. The power supply unit controls
an audible alarm if the following two elements occur together: AC power is
present and the switch not activated. The audible alarm stops when the brake is
applied or when the bed is unplugged from the power source.
Trendelenburg/Reverse Trendelenburg
The Trendelenburg/Reverse Trendelenburg is a fully electric function accessed
by the caregiver. The function is enabled by the Enable control on the caregiver
siderail controls. The principle involves controlling the two hilow columns in
opposing directions according to the position required, regardless of the initial
height and position of the sleep surface. As long as one of the two controls
remains pressed, the two columns continue to move until their internal EOT is
reached. The maximum inclination can be reached as long as AC power or
battery backup is available.
Scale and Bed Exit Systems—B Model Beds
Power is supplied to the Scale/Bed Exit System from the siderail interface P.C.
board inside the power supply. The power supply supplies a regulated 8.6 V
DC referenced to signal GND at 500mA max, 200mA typical. The GND is
common among all nodes in the bed. This is important to the network
transceiver explained later.
The scale P.C. board has a single chip analog solution specially designed for
scale transducers. The load beams are resistive bridge sensors excited by a
regulated DC voltage. This regulated voltage is supplied to the beams and to
the analog to digital converter (ADC) reference input to minimize the effects of
variations.
Each beam is connected to the differential input on the ADC. The multiplexer
and gain (up to 128) are internally configured through software. The ADC has
the ability to detect open or shorted sensors. It also has a second stage sigma-
delta converter. The serial interface is SPI compatible and transfers between
the micro controller on the scale P.C. board and the microcontroller on the
display P.C. boards.
The Scale/Bed Exit System user interface consists of 5 seven-segment LED
digits, LED indicators, and switches to start scale and bed exit functions. The
scale P.C. board sends controller area network (CAN) messages to the scale
display P.C. board which, illuminates the LEDs. The scale display P.C. board
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