Whenever accurate measured values are required in an application, the advantages of digital sensors, compared to analog instruments, become obvious. Here, digital sensor means a sensor with an integrated analog-to-digital conversion, which uses a digital interface to transmit the measured value (e.g. CANopen, Profibus, USB) with the pressure value transmitted as a numeric value. An analog sensor, however, has no built-in analog-to-digital conversion and transmits its signal as an analog current or voltage signal, e.g. 4-20 mA or 0-10 V.
Advantages of digital sensors
Therefore, in applications where high accuracy is required, such as in test stands for propulsion technology, it is advisable to use digital sensors. Doing this avoids the further sources of error that exist in analog instruments, over and above the signal conditioning, as a result of the analog signal transmission. By the deformation of a diaphragm under a pressure load, a resistance change occurs in the resistance bridge fixed to the diaphragm. This change in resistance is converted into an electrical signal, amplified and transformed into a standard signal. The compensation of the sensor-specific errors (zero error, span error, non-linearity) is also made through analog circuit technology, for example, resistance networks. With digital sensors, however, the electrical signal of the resistance bridge is directly converted into a digital value and the subsequent compensation is instead made mathematically in a microprocessor. Here, depending on the required accuracy, non-linear errors of any order can be compensated and accuracies to 0.05% can be achieved at low cost. By using a C, an active temperature compensation can also be made, eliminating any temperature error within a defined temperature range. This compensated digital signal now exists in the pressure transmitter as a numerical value and then can be output via any digital protocol. During the onward transmission of this digital pressure signal, it is now immune to interferences which might cause a further deterioration in the accuracy.
Initially, the analog front end of both sensor principles is adversely affected by environmental influences such as temperature fluctuations, EMC, etc. However, in the case of the digital pressure transmitter, the pressure signal will no longer be influenced by external effects after the AD conversion. In the case of the analog signal chain, even the internal compensation is subject to possible temperature effects due to passive components. The output driver that generates the standardised output signal (e.g. 4-20 mA or 0-10 V) is also constrained by a variety of external influences such as cable length, input impedance of the signal evaluation, temperature, EMC, etc. Anyone who has already tried to evaluate an analog sensor signal with high precision will also know the problem of signal noise. Even in the unpressurised state, the evaluated signal is not fixed at 4 mA but fluctuating within a particular range, e.g. 3.985 to 4.007 mA. This is mainly due to environmental influences which the signal cable picks up, acting as an antenna.
For further processing, this analog signal value must be digitised, if only for visualisation on a display or as a control variable for a controller. For example, I/O channels of programmable logic controllers (PLCs) or external A/D modules are used for this. These components are also subjected to environmental factors that have a negative impact on the accuracy of the measured value acquisition. Thus, these A/D evaluation modules also have a specified accuracy, which is the best to determine an analog signal. This means that here the inaccuracy of the sensor itself is further added to by a deterioration in accuracy at the A/D module. This error through the A/D conversion, in turn, is also temperature dependent and, at the limits of the operating temperature range, will become even larger.
Accuracy explained
In the case of digital signal transmission, the overall accuracy of the measuring chain is influenced solely by the inaccuracy of the sensor. Following A/D conversion within the sensor, the pressure is available as a numerical value. This can be adapted through a microprocessor to any digital BUS signal. This adaptation has no influence on the accuracy specification. Nor is the transmission of the digital signal subject to any influences that would degrade the accuracy. So, with the example of CANopen as the transmission protocol, cable lengths of 1000 m are possible without any effect on the accuracy of the pressure signal. Moreover, there is no additional error at the signal evaluation end. There, one finds a digital BUS master which reads the digital values from the BUS and forwards them to the appropriate software/process control element. This all takes place with a digital numerical value, unaffected by any environmental influences.
For the sake of completeness, it should be mentioned that strong EMC interference may also affect digital signals. If pulse-shaped interference is superimposed, it could happen that what was originally a 0 arrives in the master as 1. However, this again shows the advantage of using microprocessors that can detect and correct these errors with their built-in intelligence.
During transmission, algorithms built into the sensor and the PLC ensure that transmission errors are detected. For this, by using cyclic redundancy checks (CRC), checksums are calculated from the measured values and for any discrepancies the measured value will be discarded and requested again. To some extent, it is also possible that the correct measured value can also be calculated from the transmitted checksum and thus the transmission errors generated can be corrected. This then saves retransmission and the associated loss of time.
Many manufacturers now operate on the basis of identical component strategies when configuring digital sensors the compensated pressure value is then ready to be transformed back to an analog standard signal through a D/A conversion. In the context of the comparison of analog and digital sensor technology, this is almost the worst case. Here, the digital, compensated, ‘clean’ sensor signal is converted back into an analog value that can be distorted by the effects of temperature, quantisation errors and other disturbances. Thus it makes sense, once the effort has already been made to process the sensor signal into a digital one, that it should be transmitted digitally to the PLC, eliminating further sources of error.
After comparing the basic construction of the measuring chains, we should now also look at the advantages using a calculated example of an accuracy assessment. Pressure sensors are available in both digital and analog versions at reasonable prices with accuracies of up to 0.1%. This accuracy should be used as a baseline in both cases.
In the example of the analog signal chain, there can be an error in the order of 0,1% just along the path of transmission. High contact resistance at the connection points in the case of 0-10 V signals or the superposition of electromagnetic interference (e.g. in the vicinity of pumps or motors or other potent sources of interference) can be the cause of these effects along the transmission line.
Low-cost analog input modules offer resolutions in the range of 10 to 14-bit and possess a basic accuracy of 1%, for example. Of course, this error is then added to the error of the sensor! With these specified accuracies, however, it only covers the accuracy at the reference conditions – if one moves outside of these reference conditions, further errors are accrued. Typical values here are in the range of an additional 1% temperature error over the entire temperature range.
Even the highest quality analog input modules, with up to 24-bit resolution, still have inaccuracies of 0,1%. And still, with these modules, additional temperature errors must be taken into account – although these are very low, they can still be in the range of 10 ppm/°C. For a module that can be used in the range -40 to 125°C, this would constitute an additional error of 0,165% over the temperature range.
Purely mathematically, the two cases are represented as follows:
Low-cost analog input module: 0,1% (pressure transmitter) + 0,1% (transmission path) + 1% (analog input module) + 1% (analog input module temperature error) = 2,2%.
High-quality analog input module: 0,1% (pressure transmitter) + 0,1% (transmission path) + 0,1% (analog input module) + 0,165% (analog input module temperature error) = 0,465%.
The estimation of the digital signal chain, however, turns out to be significantly simpler.
Here, the basic accuracy of the pressure transmitter stands (0,1% in our example), and there are no additional error influences in the onward signal path, so the measured value which is used in the evaluation process actually exists with an accuracy of 0,1%.
Comparing the costs
Digital systems also offer benefits on the cost side. The additional costs for sensors with digital interfaces have decreased in recent years. In the example of the pressure transmitter, there is already a supplier of complete pressure transmitter families where one can choose directly between analog and digital output signals at no extra charge. The cables required to transmit digital signals are quite expensive to purchase when compared to their analog counterpart, though in the case of a Bus system, only one line is required. An analog signal transmission cable is required per measurement point, so in total, the wiring for the digital system can be more cost-effective.
However, the lion’s share of the cost is in the signal evaluation. High-quality A/D modules with, for example, 8 analog inputs and 16-bit resolution, come at a price of around €2000. This means an additional cost of €250 per measurement point.
Bus masters for the common fieldbuses are in the range €200-500, irrespective of the number of required measuring points. In most automation systems, one or more fieldbuses are already in use, so to some extent, the cost of the sensor evaluation is already accounted for, since these can be attached to the existing Bus.
In summary, digital measuring chains exhibit their strength in applications where there are multiple measuring points and a secure and accurate transmission of measured values is needed. Particularly in the example of engine test benches, that run for considerable periods, 24 hours, 7 days a week, in an environment where elevated temperatures and also strong EMC interference prevail, it is recommended to use a fully digital measuring chain.
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