Hill-Rom Advanta P1600, Service Manual

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Theory of Operation
Chapter 3: Theory of Operation
Page 3 - 22 Advanta™ Bed Service Manual (man168ra)
Patient Position Monitor (PPM)
Power is supplied to the Scale/PPM module via connector P5, by the P1600
central power supply. The power supply provides a pre-regulated 12-14V DC
referenced to signal VSS at 400mA maximum, 200mA typical, and another
unregulated 9-18V DC referenced to signal chassis digital ground at 200mA.
The voltage referenced to chassis digital ground is only used for the bed exit
ARMED signal that goes to the CIB. This is for field compatibility. The
supply with respect to signal VSS is distributed throughout the rest of the Scale
system/PPM from this assembly.
There is an on-board piezo speaker. The microprocessor allows it to turn on
with a logic level high to the base of a NPN transistor which sinks enough
current to turn the buzzer on. There are three output lines from the
microprocessor for three volume levels. This is achieved by placing multiple
resistor values in series with the piezo. A capacitor is connected across the
piezo to keep the voltage up when a resistor is used in series with the piezo.
The piezo is connected to 12 volt DC to achieve maximum decibel (db) level.
Steering diodes on the base of all three NPN keep the piezo attenuated when
the microprocessor is in a reset condition.
The Watch Dog Timer and Reset Circuit monitors VCC and when it comes up
to 4.75 volts, the reset line is released after a 250ms delay. The reset line goes
high if the strobe line is not strobed within 1.2 seconds. The inverse reset pin is
used to keep the piezo off when the controller is in reset.
Three Force Sensing Resistors (FSR) are located on the sleep surface. The
head sensor is the only one used for the position mode. Two FSR are located
in the thigh area and are used in the exiting mode as well as the head sensor.
Load beams are used to detect that a patient has left the bed. The FSR output
decreases in resistance with increased force. The PPM sensor (FSR) input
signal enters the board through P6, P7, and P8. Each sensor goes into one of
the four channels on the 8-bit A/D. The data is then read serially by the
embedded controller.
The inside (left) siderail LED is on whenever the PPM/bed exit system is on.
This lets the patient and caregiver know the system is on from the right side of
the bed. It is controlled by the microprocessor and switched via an NPN
transistor on P5.
The PPM caregiver interface, located on the outside of the left siderail is used
to initiate functions, mode selection and alarm volume level. The LEDs’
indicate status of the PPM/bed exit system.