An Integrated Portable Device for the Hand Functional Assessment in the Clinical Practice - Biomedical Engineering Systems and Technologies - page 81

current injection in the patient's hand. The touch board is based on the MSP430F2013
MCU, managing the reading of the capacitance associated to 8 key-shaped sensible ar-
eas on a PCB. The layout of the board is designed to easily accommodate both left and
right-handed exercises and the sensor shape allows to find a comfortable hand position
taking care of different hand sizes and deformities. This sensorized aid provides over
an I2C bus, whenever required, the current status of the keys in a single byte: the inter-
pretation of the data in the light of the exercise to execute is up to the main processor
firmware.

Signal Conditioning. Given the nature of the involved signals, which are slowly time-
varying, it is possible to operate at rather low sampling frequencies, with consequent
benefits in terms of real-time bounds for the signal processing algorithms. In order to let
the signal processing algorithms running on the MCU work with an adequate time res-
olution, a sampling frequency of 150Hz has been chosen. The analogue interface block
is essentially composed of four non-inverting, active low-pass filters, implemented with
an operational amplifier (TLV2375) and a single pole RC net. The value of its cut-off
frequency has been set to about 48Hz to exploit the filter as anti-alias with guard band
of about 25Hz under the Nyquist frequency, also limiting the 50Hz mains noise. The
outputs of the four filters are connected to as many different channels of the MCU ADC.
All the input stages have been especially designed in order to provide an adequate re-
sponse when the exercises are performed by a rheumatic patient, even if this limits the
operating range of the sensor.
Two different configurations have been employed. The first configuration is used for
the Tekscan FlexiForce sensor, which has been connected between ground and the op-
erational amplifier inverting input, making the stage a variable gain amplifier. Using a
fixed input, provided by a voltage reference at 0.5V, the output varies linearly with the
force applied to the sensor (between 0.5 and 3.3V). The second configuration, used for
the potentiometric sensors, has a fixed gain and a variable input voltage. The poten-
tiometric sensor is inserted in a voltage divider, with the wiper connected to the stage
input, so that the output value is proportional to the voltage present at the wiper.

init_MSP430

LPM0

check_BTooth filter

exercise code
recieved from host

LPM0

detection
algorithm

init_resources

send samples
to host

start signal
recieved

LPM0

stop?

processing

Fig. 3. Embedded control software flow diagram