Medical Applications User Guide (pdf) - Freescale Semiconductor

Diagnostic and Therapy Devices
17.2
Using Motors for Patient
Positioning
Pressure ulcers or decubitus ulcers (bedsores)
are one of the most common complications of
patients who cannot change position in a bed.
Bedsores can be caused by sweat, humidity
and temperature but are mainly the result
of unrelieved pressure applied by the bones
to the skin and tissue. This is why the most
common places for bedsores are the sacrum,
elbows, knees and ankles.
To avoid bedsores, hospitals and health
care providers use irregular bed surfaces to
distribute pressure along the whole body while
electric motors allow the patient easily switch
positions with just the push of a few buttons.
Electric motors are clean and relatively
efficient. This makes them a much better fit
for use in hospital beds rather than pneumatic
or hydraulic alternatives. An electronic motor
system can be used to adjust the height of
the bed and provide movement to the bed’s
wheels. A typical system containing an MCU,
an H-bridge and a motor is shown in
Figure 17-2.
The requirements for an MCU vary based
on the size of the motor and the required
efficiency. Most patient bed applications
require between 32 to 100 MHz, 16 to 156 KB
of flash memory, 2 to 64 KB of SRAM, a highly
accurate timer and the ability to synchronize
the timer with the analog to digital converter
(ADC). The requirements for an H-bridge
also vary, but most beds require a monolithic
power IC comprising control logic, charge
pump, gate drive and low RDS(ON) MOSFET
output H-bridge circuitry in a small surface
mount package.
Freescale offers a wide variety of products
specifically for motor control systems ranging
from digital signal controllers (DSC) to MCUs
and H-bridges. An ideal MCU and H-bridge
solution for a bed is an MCF51AC256 paired
with the flexible, low-power MC33926. In
Figure 17-1: Powered Patient Bed General Block Diagram
Powered Patient Bed
Infusion
Pump
Motor
Driver
Patient
Monitor
Power
Management
Infusion
Pump
Control
Patient
Monitor
Control
Other
Devices
Freescale Technology
Optional
some cases, depending on the complexity
of the motor system, a single DSC may be
sufficient to control the motor. Freescale’s
MC56F800x family is an alternative costoptimized
solution for real-time motor control.
17.3
Integrated Real-Time
Patient Monitoring
A powered patient bed must be equipped to
monitor the status of the patient and transmit
the data remotely to a nurse station. Typical
patient monitoring functions consist of blood
pressure monitoring, heart rate monitoring,
a pulse oximetry unit, ECG, blood glucose
meters and an infusion pump.
The modules shown in Figure 17-1 provide
extra features allowing health care providers
and the patient’s relatives to offer comfort to
the patient. Some of these modules include a
tilt accelerometer and motor driver to control
the bed’s tilt, powered wheels to facilitate
movement of the patient to different areas
of the hospital, USB and Ethernet ports to
provide connection with a PC or the hospital
network, VoIP gateway to provide direct
communication to the nurses’ station, and an
LCD screen and keypad for user interface.
17.4
Integrated Tilt Control
The tilt control module is used mainly for the
safety and comfort of the patient. Although
hospital beds are often maneuvered in many
directions and in some cases, in an urgent
manner, the safety of the patient must be
paramount at all times. Electronic sensors
can be used to monitor the tilt of the bed and
provide an alarm if the bed is at an unsafe
angle. Furthermore, the tilt control module is
used to position the patient in the bed at the
most ideal angle for the patient’s comfort.
This is the most prevalent use of the tilt
control module.
Accelerometers can be used to measure
both dynamic and static acceleration. Tilt is
a static measurement where gravity is the
acceleration being measured. Therefore, to
achieve the highest degree of resolution of
a tilt measurement, a low-g, high-sensitivity
90 Medical Applications User Guide
UART
UART
UART
LCD
Controller
LCD
Display
MCU/MPU
SPI
USB
MII
Keypad or
Touch
Screen
Wireless Comm
IEEE ® 802.11x Wi-Fi ®
10/100 Ethernet PHY
CAN
XSCVR
Accelerometer
CAN Bus
CAN
XSCVR
Bed Tilt
Control
Motor
Driver
Bed Tilt
Motors
Nursing
Station
Wired
Network
CAN
XSCVR
Wheel Motor
Control
Motor
Driver
Wheel
Motors
VoIP
Gateway to
Public Phone
Network
Pressure
Sensor
CAN
XSCVR
Pump
Control
Motor
Driver
Pump
Motors