W.J. Devenport and S.R. Edwards
Last Modified December 22nd, 2006
The purpose of Instrumentation Lab is for you to learn about some of the basic instrumentation used to operate modern test and measurement systems used in aerospace and ocean engineering applications. Such systems comprise a mixture of analogue electronic devices, like sensors and actuators, with digital data acquisition and control.
An analogue device is one that uses the level of an electrical signal (often the voltage) to represent the value of a physical quantity being measured (say displacement or temperature), or being applied by an actuator (say force or movement). A good example of such a device is a pressure transducer. The transducer has a diaphragm that moves when the pressure changes. The diaphragm is wired up as part of an electrical circuit. When the circuit is supplied with power (voltage from an electrical power supply) the transducer provides an output voltage that is proportional to the pressure. By careful design such a transducer can be made to respond to different pressure ranges, to respond very rapidly (up to hundreds of thousands of times per second) or to be very insensitive to other environmental conditions (like temperature). Like the pressure transducer, all sensors and most actuators are analogue devices.
A digital device (usually a computer) is one in which the physical quantity is represented as a number. This is fundamentally different. Think about entering a pressure in Pascals into an Excel worksheet. The number you enter is displayed as an arrangement of pixels representing the digits, and stored in the computer's memory as a series of binary 1's and 0s. Nowhere is the pressure you typed in represented directly by voltage, current or other electrical quantity. Representing a measurement, or control quantity in digital form is highly desirable, however, since then we can bring the enormous speed of computers and the flexibility of programming to our measurement or control system.
To make a modern measurement or control system work then we need devices that can convert the analogue voltage signal produced by a transducer to a series of digital numbers, and that can convert a similar string of numbers back into the voltage signal needed by an actuator. These devices are called, respectively, the Analogue to Digital (A/D) Converter and the Digital to Analogue (D/A) converter.
So, what is the Aerospace and Ocean Engineer's responsibility in all this? We do not design computers, transducers, actuators or A/D converters. Instead we put together these things to make measurement and/or control systems that perform the tasks we are interested in - like measuring pressure in a fluid flow, exciting a structure with a prescribed disturbance, or measuring that structural response. Putting together a system like this requires skills in several areas.
On the analogue side we need to know how to connect electrical equipment, and how to use basic electronic instrumentation so that we can confirm whether things are working here. For example, it would want to connect that pressure transducer up to a voltmeter or oscilloscope before we connect it to our A/D converter, to make sure that its output really is varying with the pressure as its supposed to. Likewise, we would want to connect some known analogue signal (from a function generator) to an actuator to make sure its working correctly before we connect it to the computer. Note that such checks are in no way redundant if you want to end up with a measurement system that actually works.
On the computer side we need to understand what the A/D converter does, what its limitations are, and what control we have over it. We need to be able to program the computer and A/D converter to make the measurement we want, to analyze, display and store it. Indeed, the computer is so powerful that deciding what analysis we want to do, and understanding how to do that, becomes a significant task in itself. Giving you a start on this knowledge and hands on experience of its application is exactly what Instrumentation Lab is about.
The Instrumentation Lab sequence is divided into 5 periods, supported by a series of lectures, and 5 manual chapters. Instrumentation lab periods are held once every two weeks. Most of your work during labs will be done in teams of 2 or 3. During the first period you learn how to use analogue instrumentation. In the second period you get to use this knowledge to perform an experiment to determine the dynamic response of a simple structure. In the third period you are introduced to A/D converters, and the programming of digital measurements using the LabView programming environment. LabView is the de facto standard for most digitally based measurement and control used in engineering and science. The fourth period introduces you to spectral analysis and its programming in LabView. Period 5 is where you get a chance to use these new digital measurement and analysis capabilities to further study the structural dynamics experiment of period 2.
Your work is assessed through homework assignments and the electronic logbooks you generate from your experimental work in periods 2 and 5.