Improving reliability of Force Sensing Resistors for a Pressure Ulcer Prevention System

Improving reliability of Force Sensing Resistors for a Pressure Ulcer Prevention System

Langereis, Jordy (TU Delft Electrical Engineering, Mathematics and Computer Science)
Veldhuijzen, Duco (TU Delft Electrical Engineering, Mathematics and Computer Science)

French, P.J. (mentor)
Mastrangeli, Massimo (mentor)

Delft University of Technology

Electrical Engineering

2018-07

Pressure ulcers develop when skin is subjected to a mechanical load over time, resulting in tissue damage. Within a wide range of patients, pressure ulcers occur, often accompanied by severe pain.
This is resulting in a prolonged hospital stay. The company Momo Medical is developing a sensor plate
with the goal of preventing pressure ulcers by using sensor data attained from the patient laying on
his bed to guide the nurses, whom are responsible for re-positioning the patients. The sensor plate
is placed between the mattress and the frame and has eight force-sensing resistors (FSRs) and two
piezoelectric sensors to detect the static and dynamic behaviour of the patient. This data is processed
by an algorithm to detect a patient on the bed, the position of the patient, and how long the patient
has remained in the same position. At the moment the sensors are not reliable enough to detect the
position of a patient on the bed yet.
This bachelor graduation project focuses on the goal to improve the reliability of the sensors. The
group has been divided into three subgroups, where this thesis focuses on the improvement of the
reliability of the FSRs in the sensor plate.
First a state-of-the-art analysis was conducted to get a deep understanding of pressure ulcers, the
existing ways to prevent them, different sensors to measure pressure, the operation of a FSRs, and
lastly different ways to calibrate such a sensor.
Then a set of quick measurement on the sensors in the plate were done to get get an overview
of the current system and it’s cause of the unreliability. The biggest aspects were drift, hysteresis and
repeatablility. After that it was a clear that a reliable setup was essential and was therefore developed by
the use of a strain gauge load cell as a reference and a stepper motor for time efficiency. Both the output
of the strain gauge and the FSR needed to be amplified and converted by an ADC to retrieve a digital
output. The readout of the FSR, load cell and the control of the stepper motor were all programmed
and controlled on a Nucleo F103RB micro-controller. Then measurements were conducted on the
sensors, and the errors where visualised. Then first-order and third-order polynomial approximations
of the sensor output were used as calibration and it was resulting in a major reduction in errors.
Lastly this thesis consists of a recommendation and future work since the calibration setup was a
prototype and is a foundation for a setup in production but needs to be expanded to be able to handle
eight FSRs at the same time. Also duration tests need to be conducted to retrieve knowledge when
a sensor plate needs to be re-calibrated. Besides that the domes should be changed to avoid shear
forces, since this a big cause of the error in the FSRs in this application.

pressure ulcer
FSR
force sensing resistor
calibration
setup
decubitus

http://resolver.tudelft.nl/uuid:edfb085d-b432-468a-bd89-c1e1987835d2

2023-08-01

Student theses

bachelor thesis

© 2018 Jordy Langereis, Duco Veldhuijzen