26 Dec 2017
To understand how to design sensors into larger systems, one must understand what kinds of transformations need to occur between the phenomenon being sensed, the transducer and the sensor output which reports the measurement to the larger system of which the sensor is a part. Although there are many types of transducers, which operate based on an equally wide range of physical principals, we will not be spending too much time on this aspect of sensing. This series of articles will focus more on the commonalities in the electrical interface to such devices, and the subsequent means of deriving a useful measurement.
While the terms sensor and transducer are often used somewhat interchangeably, for the purpose of these discussions, we will define the transducer as the physical device that converts the sensed phenomenon to an electrical property, and the sensor as the system that contains the transducer and whatever electronic and software functions are required to deliver a report of the sensed quantity. In some cases, the transducer is the sensor – for example, a temperature transducer such as a thermocouple converts temperature directly into a measurable (but very small) voltage, and can also be considered a complete sensor – although it becomes more useful when electronics are added to provide amplification and other signal processing functions.
There is an enormous range of transducers and ways of implementing sensors with them – far too many to ever cover comprehensively in any work. One tool, however, that can be useful for bringing a bit of structure to this menagerie is the signal chain.
Most electronic systems of any complexity have two properties which can help both in the design process, and subsequent understanding of that design:
Hierarchical structure. Even moderately complex electronic systems are typically not designed as monolithic random collections of parts – when one is, it is often a sign either of bad engineering or an engineer looking for 'job security'. In most designs, components that implement a distinct function are usually clustered together into a 'functional block', with clearly defined inputs and outputs. Even a relatively simple design will often comprise a number of these functional blocks. More complex designs will often comprise a number of larger, more functional blocks, each of which may be repeatedly broken down into simpler blocks until you reach the component level. The primary reason for designing this way is to manage complexity – and it does not take much unmanaged complexity to make a system incomprehensible. A hierarchical design approach allows you to hide the details of the system in contexts in which they are not immediately relevant. For example, being able to define an amplifier as a 'block' which has a specified gain and bandwidth allows you to design that amplification function into the system without immediately having to perform the transistor-
Explicit Causality – While there are circuits which contain hidden feedback loops which may blur the issue of what causes what. design at the level of functional blocks relies heavily on explicit and well-
The 'signal chain' representation of an electronic system is based on the use of both hierarchical abstraction and explicit causality. A signal chain defines a system as a group of top-
While there are many possible ways of realizing an electronic interface to a transducer, at a very high level, there are a couple of functions that are commonly used, illustrated by the sensor signal chain shown below.
Excitation. While some transducers convert some of the energy available in the sensed phenomenon into electrical voltage or current, many do not. In these latter types of devices, an electrical excitation source (voltage or current) is needed to make the transducer operate and output a detectable signal.
Filtering and Bandwidth Narrowing. While a transducer's output will hopefully contain the signal you are interested in measuring, it will also to some degree contain signals that you don't care about, and would rather not see. These extraneous signals include components resulting from both external interference sources and intrinsic noise sources. If these extraneous signals are of comparable magnitude to (or larger than) the signal of interest, this can make it difficult to accurately measure the true transducer signal. As Lewis Branscomb famously noted, "God loves the noise as much as the signal" [Branscomb 1985] – meaning that nature's indifference between the two can make fishing the signal out of the noise one of the more challenging aspects of sensor system design.
Signal Processing. Once you have a signal that is of usable magnitude and reasonably noise-
In addition, in some systems, you may need to 'massage' the signal to get a meaningful measurement. For example, thermistors (a type of temperature transducer) have a very non-
Reporting – Does a sensor that does not report its measurement really sense? We are not going to leave this one to the philosophers, but flatly state that if a device does not report some kind of measurement, it may be something (a piece of modern art?) , but it isn't a sensor. A sensor needs some means of transmitting its measurement to the outside world, or at least to the larger system of which it is a part.
Digitization – This 'block' performs the function of converting a continuous analog signal into a discretized digital signal – a numeric value -
Note that the sensor signal chain presented here is more of a conceptual guide to thinking about what things need to be done in order to use a transducer than an iron-