Advantages of Digital Transducers
Any measuring device that presents information as discrete samples and that does not introduce a quantization error when the reading is represented in the digital form may be classified as a digital transducer.
According tothis definition, for example, an analog sensor such as a thermocouple along with an analog-to-digital converter(ADC) is not a digital transducer. This is so because a quantization error is introduced by the ADC process.
A digital processor plays the role of controller in a digital control system. This facilitates complex processing ofmeasured signals and other known quantities, thereby generating control signals for the plant of the controlsystem. If the measured signals are available in analog form, an ADC stage is necessary prior to processing in adigital controller.
There are several advantages of digital signals (or, digital representation of information) in comparison to analog signals.
1. Digital signals are less susceptible to noise, disturbances, or parameter variation in instruments because data can be generated, represented, transmitted, and processed as binary words consisting of bits, which possess two identifiable states.
2. Complex signal processing with very high accuracy and speed are possible through digital means (Hardware implementation is faster than software implementation).
3. High reliability in a system can be achieved by minimizing analog hardware components.
4. Large amounts of data can be stored using compact, high-density, data storage methods.
5. Data can be stored or maintained for very long periods of time without any drift or being affected by adverse environmental conditions.
6. Fast data transmission is possible over long distances without introducing significant dynamic delays, as in analog systems.
7. Digital signals use low voltages (e.g., 0–12 V DC) and low power.
8. Digital devices typically have low overall cost.
These advantages help build a strong case in favor of digital measuring devices for mechatronic systems. Digital measuring devices (or digital transducers, as they are commonly known) generate discrete output signals such as pulse trains or encoded data that can be directly read by a digital controller. Nevertheless, the sensor stage of a digital measuring device is usually quite similar to that of an analog counterpart.
There are digital measuring devices that incorporate microprocessors to perform numerical manipulations and conditioning locally and provide output signals in either digital form or analog form. These measuring systems are particularly useful when the required variable is not directly measurable but could be computed using one or more measured outputs (e.g., power=force. speed).
Although a microprocessor is an integral part of the measuring device in this case, it performs not a measuring task but, rather, a conditioning task.
For our purposes, we shall consider the two tasks separately. When the output of a digital transducer is a pulse signal, a common way of reading the signal is by using counter, either to count the pulses (for high-frequency pulses) or to count clock cycles over one pulse duration (for low-frequency pulses).
The count is placed as a digital word in a buffer, which can be accessed by the host (control) computer, typically at a constant frequency (sampling rate).
On the other hand, if the output of a digital transducer is automatically available in a coded form (e.g., natural binary code or gray code), it can be directly read by a computer.
In the latter case, the coded signal is normally generated by a parallel set of pulse signals; each pulse transition generates one bit of the digital word, and the numerical value of the word is determined by the pattern of the generated pulses. This is, for example, is the case with absolute encoders. Data acquisition from (i.e., computer interfacing) a digital transducer is commonly done using a general-purpose input/output (I/O) card, for example, a motion control (servo) card, which may be able to accommodate multiple transducers.
