As seen in the previous paragraph the reliability of the measure must be the highest as possible, hence to get repetitive measurements, the error introduced by different values of pressure cannot be tolerated. To solve this problem there are two possible ways: the first and simplest solution is to keep the pressure always to a constant value. In manually executed measurements this is not often feasible and the
operator may influence the testing process through unconscious variations in pressure. The second solution consists in compensate the measurement making it a function of the exerted pressure. A pressure compensation system has been then studied to allow the operator to push the probe on the measure point to a well defined and repetitive pressure. The block diagram of the compensation system is shown
in figure 5. A force transducer S is connected to an operational amplifier that provides an output voltage Vp proportional to the pressure on the tip.
Figure 5: Schematic of the pressure compensation system.
The force sensor operates on the principle that the resistance of a silicon implanted piezoresistors will increase when the resistors flex under any applied force. The sensor concentrates force from the application, through a stainless steel plunger, directly to the silicon sensing element. The amount of resistance changes in proportion to the amount of force being applied. This change in circuit resistance results in a corresponding mV output level. The pressure transducer provides precise and reliable force sensing performance using a piezoresistive
micromachined silicon sensing element. A low power Wheatstone bridge circuit design provides inherently stable mV outputs over the force range. The sensor has a typical sensitivity of 0.24 mV/g (0.20 mV/g min, 0.28 mV/g max) and supports an operating force from 0 to 1500g. The sensor performance has been evaluated and tested using a deadweight or compliance force in order to maintain operation within design specifications. The voltage Vp coming from the pressure compensation system and the voltage Vo coming from the acquisition circuit are then connected through a multiplexer to an analog to digital converter used as front-end of a microprocessor μP (see Figure 6).
Figure 6: Schematic of the final stages of the system.
The microprocessor is programmed to modify the value of Vo according to the value of Vp. A conversion algorithm is used to keep into account the transfer function of the force sensor in the dynamic of interest. As a first approximation, a linear function has been used but different algorithms can be analyzed. As far as the range of compensable pressures is concerned, two of the essential elements are constituted by the elasticity of the skin and by the diameter of the semi sphere. Usually a span of 50-300gr can be compensated allowing the operator to work in a linear range of pressures and maintaining the contact surface variation in a linear range. The status of the probe and of the pressure sensor can be at any rate checked by the application software. It is also possible to modify the pressure compensation
system parameters. Before performing any current acquisition the application software checks any possible residual pressure on the sensor. Figure 7 shows the complete electronic scheme of the electronics interface. This scheme and the current acquisition system are patented by Biophysics Research srl, Rome, Italy.