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4B15 Introduction to Bioengineering Electrodes and Transducers Lecture 4: Biomedical Measurements and Transducers 4.1 Introduction There are many physical measurements which are made in medicine. These can be either invasive or non-invasive. Measurements can be made without physical contact with the body, such as in the case of CT or MRI scans. Other measurements can be made with the aid of miniature transducers, such as an endoscopy or the measurement of intra-arterial blood pressure. Many measurements, however, are carried out in the form of blood tests, where the reaction of the blood samples to chemical reagents provide information on the condition of an organ or the efficiency of a bodily function. Such tests usually also involve the use of instrumentation to monitor the status of the reactions, such as the change of colour of a test strip. In general terms a transducer is a device which measures a physical phenomenon and converts the variation in this phenomenon into an electrical signal. This is so that the signal can then be conditioned and processed with the sophistication and precision offered by electronic instrumentation and equipment and then usually stored in electronic or hardcopy form for record and subsequent referral. 4.2 Common Physical Measurements Many physical parameters are commonly measured in both invasive and non-invasive settings. Among the most widespread are the following: Electricity - Surface Potentials: These are signals that can be measured from the surface of the body using electrodes and sereval examples have been mentioned already such as the ECG and the EEG as shown in Fig. 1. Fig. 1 Measurement of EEG and ECG 1 Pressure: Pressure can be measured externally as in the case of blood pressure measurement, where the gauge pressure in the compression cuff is measured as this is deflated to estimate the internal pressure in the artery as shown in Fig. 2. Solid-state pressure transducers are available for the purpose in a variety of ranges and accuracies. Miniature pressure transducers allow intra-arterial pressure measurement and catheterised insertion of transducers into the heart for long term monitoring. They are also available for other locations such as the eye. Fig. 2. Measurement of Pressure in Clinical Scenarios Temperature: As with pressure, temperature can be measured internally or externally. Sensors can be mounted on the body or inserted at suitable sites. They usually exploit the property of thermal expansion of a metal, or the differing rates of thermal expansion of two different metals. Examples of temperature monitors are shown in Fig. 3. Fig. 3 Examples of Temperature Measurement Transducers 2 Flow: The rate of flow of air into and out of the lungs is important in the diagnosis and treatment of diseases such as asthma and lung cancer as well as other pulmonary complaints. The flow of blood in narrowed arteries which have suffered stenosis or through damaged valves in the heart is also an important measurement, which can be accomplished through the use of miniature transducers that can be inserted through catheters. Fig. 4 Examples of Flow Rate Monitoring Strain: Damage to muscles, tendons and ligaments, as is common in sporting injuries, can be assessed with the aid of transducers which can measure the stress and strain which they have been subjected to and can help in monitoring the healing process. Fig. 5 Examples of Strain Measurement Acceleration: The technology behind accelerometers has improved much in recent years, allowing such transducers to become more compact in shape and size. These can be used to measure the forces placed on the body in high speed activities such as space travel or in general studies of walking gait or limb movement. They have also been used more recently to aid independent living among the elderly, where they have been used to monitor falls or unsteadiness in the balance of the elderly in their own homes. 3 Fig. 6 Examples of the Measurement of Acceleration 4.3 Typical Transducer Characteristics Many solid-state pressure transducers are constructed in the form of a Wheatstone bridge which is implemented directly on Silicon. The associated interfacing and signal conditioning amplifier and any other electronics needed can often be mounted on the same chip as is used for the sensor itself. An example of the construction of the Sensym SPX50 pressure transducer is shown in Fig. 7. This is fabricated as a piezo-resistive sensor in the form of a Wheatstone bridge, consisting of four ion-implanted resistors etched onto an integral silicon diaphragm which transform the shear stress due to applied pressure into an electrical output. The diaphragm is mounted and bonded over an access port where the input pressure is applied. The bridge is exited using an external supply voltage and can operate over a wide voltage range. Photo Fig. 7 The Physical Construction of a Solid-State Pressure Transducer An electrical equivalent circuit of the transducer is shown in Fig. 8, which can be seen to be that of an elementary Wheatstone bridge providing a differential output signal. With zero pressure applied to the transducer, the potential at both output terminals relative to ground is half the supply voltage. When input pressure is applied to the transducer, the potential at the positive output terminal increases from half the supply voltage, while that at the negative 4 Fig. 8 Equivalent Circuit of the SPX 50 Pressure Transducer output terminal decreases. The difference between these two potentials is a small signal voltage which is proportional to the applied pressure. The performance characteristics of the transducer are given in Table I. This table shows the parameters which describe how the transducer operates and the ranges which apply to the values of these parameters, due to manufacturing variations and the ambient conditions in which the transducer is used. Table I The Performance Parameters of the SX 50 Pressure Transducer Parameter Min. Typ. Max. Unit Pressure Range: 0 - 100 kPa Supply Current: - 2.7 - mA Full-scale Span: 75 110 150 mV Sensitivity: 750 1100 1500 µV/kPa Offset Voltage: -35 -20 0 mV - +48 - µV/OC -2400 -2150 -1900 ppm/OC - 4.65 - kΩ +690 +750 +810 ppm/OC - +0.2 +0.5 %FS Offset T. C.: Sensitivity T.C.: Bridge Resistance: Resistance T.C.: Lin & Hys Error: 5 Fig. 9 shows a plot of the transducer transfer characteristic, from which it can be seen that there considerable variation in the transducer properties due to manufacturing variation. non-linear characteristic output voltage sensitivity variation offset variation ideal linear characteristic applied pressure Fig. 9 Transfer Characteristic of the SX 50 Pressure Transducer The output voltage at the positive output terminal, pin 2, of the transducer is given as: ∆ ∆ ∆ ∆ where ∆RB is the change in the resistance, RB, of an arm of the bridge, which is proportional to the applied pressure and VB is the bridge supply voltage. The output voltage at the negative output terminal, pin 4, of the transducer is given as: ∆ ∆ ∆ ∆ The differential output of the sensor is then given by: ∆ ∆ ∆ ∆ 6 If a span sensitivity, S, is specified for the transducer as the output voltage in mV per unit of supply voltage, VB, per unit of applied pressure, p, that is as mVV-1kPa-1, then the ideal transducer output can be expressed as: In practice, there are large deviations from the ideal in the transducer characteristic, as can be seen in Fig. 9. The bridge output is not zero for zero applied pressure, but an offset voltage, VOS, exists which has an associated range due to manufacturing variations. That is to say, when zero input pressure is applied, the two output terminals of the transducer do not sit at a potential of VB/2, i.e. half of the bridge supply voltage, but can both differ slightly from this. This means that the differential output voltage measured between the two terminals may be some positive or negative value which is not zero. This modifies the output voltage to: where the offset voltage, VOS, is small in relation to the bridge supply voltage but can be quite large when considered in relation to the differential output voltage associated with the pressure measurement range. In addition, most transducers have a large manufacturing variation in the sensitivity of the diaphragm and consequently there is a large variation in the span sensitivity from one transducer to the next. This means that the parameter S has a large variation so that the full-scale output voltage from the transducer for a given bridge supply voltage varies considerably. In addition, both the offset voltage and the span sensitivity vary with temperature and consequently have associated temperature coefficients. This means that if nominal values of these parameters are quoted at a given temperature, T0, they will vary as the temperature deviates from this value, so that they must be treated effectively as functions of temperature. In this case the transducer output is given as: where T0 is the reference temperature at which nominal parameter values are quoted T is the actual operating temperature VB is the bridge supply voltage p is the applied input pressure VOS0 is the nominal value of the offset voltage S0 is the nominal value of the span sensitivity α is the temperature coefficient of the offset voltage β is the temperature coefficient of the span sensitivity 7 To complicate matters further, the transducer characteristic has some degree of non-linearity over its range of operation and deviates from the ideal straight-line output voltage vs. pressure relationship as shown in Fig. 9. Finally, properties such as hysteresis and ageing effects can also give rise to measurement errors. 4.3 Transducer Calibration and Compensation These sources of error can only be only be counteracted by passing the output of the transducer through a signal conditioning amplifier as shown in Fig. 10. Fig. 10. Signal Conditioning of a Silicon Pressure Transducer The offset voltage is cancelled by adding an equal and opposite cancellation voltage, VOC, in the conditioning amplifier. This is done in the first instance at the reference temperature, T0. The variation in span sensitivity is counteracted by adjusting the gain of the amplifier at the reference temperature to give the required maximum output voltage for full-scale input pressure. The bridge supply voltage can also be used for this purpose. The temperature coefficient of the offset voltage is compensated for by ensuring that the equal and opposite offset cancellation voltage has a temperature coefficient of the same magnitude as that of the offset voltage. The bridge supply voltage can also be made to vary with temperature and must have a temperature coefficient which is equal and opposite to that of the span sensitivity. When this is implemented, the output voltage from the conditioning amplifier is given as: ∆ ∆ ∆ ∆ where: 8 ∆T = T - T0 is the deviation in temperature from the nominal reference value γ is the temperature coefficient of the offset cancellation voltage δ is the temperature coefficient of the bridge supply voltage and the other parameters are as above. In order to obtain full cancellation and calibrate the output voltage to be independent of temperature requires: ∆ ∆ ∆ ∆ This requires: ∆ ∆ ∆ ∆ ∆ If the second order term in ∆T2 is ignored as negligible then full compensation requires: ; ; More accurately if the first two conditions are satisfied we need: ∆ ∆ ∆ ∆ ∆ ∆ Since ∆T cannot be taken as zero this requires: ∆ so that: → ∆ These values are only calculated for design purposes and perfect matching for compensation is achieved during the calibration process. A simple calibration procedure is: • • • • • • Set the ambient temperature to the lower value ( this may be the reference temperature T0 but does not have to be) Set input applied pressure to zero Adjust the nominal value of the offset cancellation voltage, VOC0, to get an output voltage of zero from the conditioning amplifier Set the applied pressure to its full-scale value Adjust the gain, G, or the nominal bridge supply voltage, VB0, to bring the output of the conditioning amplifier to the specified full-scale value Raise the ambient temperature to its higher value 9 • • • • Set the applied input pressure to zero Adjust the temperature coefficient of the offset cancellation voltage to obtain zero output voltage from the conditioning amplifier Raise the applied input pressure to its full-scale value Adjust the temperature coefficient of the bridge supply voltage to bring the output voltage of the conditioning amplifier to its full-scale value. 10