A mechatronic system can consist a wide variety of components, which are interconnected to perform the intended functions. When two or more components are interconnected, the behavior of the individual components in the integrated system can deviate significantly from their behavior when each component operates independently.
Matching of components in a multicomponent system, particularly with respect to their impedance characteristics, should be done carefully in order to improve the system performance and accuracy. In this chapter, first we shall study basic concepts of impedance and component matching.
The concepts presented here are applicable to many types of components in a general mechatronic system. Discussions and developments given here can be quite general.
Nevertheless, specific hardware components and designs are considered particularly in relation to component interfacing and signal conditioning.
When components such as sensors and transducers, control boards, process (plant) equipment, and signal conditioning hardware are interconnected, it is necessary to match impedances properly at each interface in order to realize their rated performance level. One adverse effect of improper impedance matching is the loading effect.
For example, in a measuring system, the measuring instrument can distort the signal that is being measured. The resulting error can far exceed other types of measurement error. Both electrical and mechanical loading are possible. Electrical loading errors result from connecting an output unit such as a measuring device that has a low input impedance to an input device such as a signal source.
Mechanical loading errors can result component.
Impedance can be interpreted either in the traditional electrical sense or in the mechanical sense, depending on the type of signals that are involved.
For example, a heavy accelerometer can introduce an additional dynamic load, which will modify the actual acceleration at the monitoring location. Similarly, a voltmeter can modify the currents (and voltages) in a circuit, and a thermocouple junction can modify the temperature that is being measured as a result of the heat transfer into the junction. In mechanical and electrical systems, loading errors can appear as phase distortions as well.
Digital hardware also can produce loading errors. For example, an analog-to-digital conversion (ADC) board can load the amplifier output from a strain gage bridge circuit, thereby affecting digitized data.
Another adverse effect of improper impedance consideration is inadequate output signal levels, which can make the output functions such as signal processing and transmission, component driving, and actuation of a final control element or plant very difficult.
In the context of sensor-transducer technology it should be noted here that many types of transducers (e.g., piezoelectric accelerometers, impedance heads, and microphones) have high output impedances on the order of a thousand megohms (1 megohm or 1 MW=1×106 W).
These devices generate low output signals, and they would require conditioning to step up the signal level.
Impedance-matching amplifiers, which have high input impedances and low output impedances (a few ohms), are used for this purpose (e.g., charge amplifiers are used in conjunction with piezoelectric sensors).
A device with a high input impedance has the further advantage that it usually consumes less power (u2/R is low) for a given input voltage.
The fact that a low input impedance device extracts a high level of power from the preceding output device may be interpreted as the reason for loading error.