Hill-Rom VersaCare P3200, Service Manual

Page: 125 / 379

Electrical System
Chapter 3: Theory of Operation
The raw digital information is translated and further filtered in software before
being placed on the network. If needed, the accelerometer or motor position
can be used to detect the level of the bed with respect to the floor and then
correct for the cosine error.
The display consists of 5 seven-segment digits. The scale node sends a network
variable for the caregiver to display. Switches, located on the display board are
available to the user to initiate functions like Zero and display weight. When
activated the network variable gets updated and read by the scale node.
A model beds: Three FSRs are located on the sleep surface in strategic
positions. They are buffered by an LM324 configured as a DIFF amp. The
head sensor is the only one used for the sensitivity mode. The other two are
located in the seat section. All Three FSRs are used to detect an out of position.
Load beams are used to detect that a patient has left the bed. The FSRs and
load beams information is sent to the air node. The FSR is in A/D counts while
the load beams information is formatted into pounds using the factory zero as
the offset. The factory zero is calculated during calibration only.
B model and newer beds: Three FSRs can be located on the sleep surface in
strategic positions. They are buffered by an LM324 configured as a DIFF amp.
The head sensor is the only one used for the sensitivity mode. The other two
are located in the seat section. The load beams are used to detect patient
activity. The FSRs and load beams information is sent to the air node. The FSR
is in A/D counts while the load beams information is formatted into pounds
using the factory zero as the offset. The factory zero is calculated during
calibration only.
The FSR output will decrease in resistance with increased force. Each FSR is
connected between a 3.3K and a 2.2K resistor to limit the current. The 3.3K are
connected to reg5 V and the 2.2K are connected to signal GND,
forming a simple voltage divider. A differential amplifier then takes the
voltage across the FSR. This is fed into a single ended 4 channel 8-bit A/D
device internal to the embedded controller.
Switches located on the outside of the head rail are available to the caregiver to
initiate functions such as mode selection, off, and alarm volume level. The
LEDs indicate status of the PPM system.
All of the Scale and PPM functions revolve around the microprocessor on the
control board. This is an 8052 variant with internal 32K flash program and 256
+ 1.2K EEPROM data memory. External to the microprocessor are the CAN
transceiver, accelerometer, watch dog reset circuit, 2K EEPROM and audio
enunciator with three volume settings.
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VersaCare™ Bed Service Manual (MAN333 REV 2) Page 3 - 11