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Abstract

This work deals with the design of a small kitchen appliance with a weighing function
integrated into a control loop in order to prevent unbalanced movement of the appliance
during operation. In this particular case, the weighing should be appropriate for larger
amounts (up to 5 kg) of ingredients. The size and shape of the load cells should be small,
similar to the existing rubber feet of the appliance.
Another requirement related to the target application is that small, low-cost load cells with
high electrical output signals are employed. In fact it was the lack of devices complying with
these requirements that fostered the development of the load cell reported in this dissertation.
The goal was to develop a load cell with a simple construction to cut down the costs. In
addition, the load cell should be composed of standard components that can be easily
assembled using environment-friendly technologies.
A hydraulic load cell complying with the above requirements was conceived and modelled
with the finite element method (FEM) prior to assembly and testing the samples in order to
select the most suitable components and their dimensions. The requirements of a simple
construction and standard components that can be easily assembled led to the decision to use
hydroformed metallic bellows, which are usually made of a variety of materials, like stainless
steel, phosphor bronze, brass, and Monel (nickel-copper alloy). In our case phosphor bronze
(CuSn8) was selected. This material has excellent resistance to corrosion and is relatively free
from creep, drift and hysteresis, and can be easy soldered. The fluid that fills the internal
space of the hydraulic load cell is in contact with all the metallic parts and the membrane of
the silicon pressure sensor. Therefore, the fluid must be good electrical insulator; it must be
chemically unreactive and non-abrasive; and it must have stable physical properties over a
broad temperature range. A silicon fluid was chosen because of the direct contact with the
unprotected silicon pressure-sensor die.
Based on the performed simulations, a series of prototypes was implemented. A 0.4-MPaabsolute
silicon pressure-sensor die MS7904A and MS7204A from Intersema Sensoric was
selected as the sensing device for measuring the pressure of the fluid inside the hydraulic load
cell. The samples were characterized at 10 °C, 20 °C, 30 °C and 40 °C. To achieve more
accurate and repeatable tests, a special measurement device for characterising the hydraulic
load cells was built. The whole procedure for the exercise and the measurements of the test
samples was controlled and measured in the LabVIEW environment.
High response of the output voltage to the applied load has been obtained. It ensures good
noise immunity and gives it an advantage over conventional strain-gauge load cells. The main
characteristics were measured in accordance with the OIML R 60 recommendation, and the
obtained results fulfil the requirements imposed by the target application.