Introduction to measurement and data acquisition

This site is devoted to useful measurement. The art of measurement of physical values by electronic methods is covered in adequate detail for the novice to understand the principals and gain confidence by practical experience.

While measurement can be a goal in itself, it is generally undertaken to further one's knowledge about a "system". A "system" can be an ecosystem, a manufacturing process, a biological organism, the climate, an aircraft's flying characteristics, a domestic air-conditioning system, drug effectiveness, pollution impact ... The list is never ending and covers every imaginable interaction in the physical and perhaps even the mystical world. Knowledge is power!

Systematic or not systematic? - that is the question

In attempting to understand "what is going on" in a system, there are several ways to approach the problem. One can be systematic and rational by postulating a hypothesis, identifying the inputs, outputs and states of the system and then proceed to prove or disprove the hypothesis. This is done by measuring as many inputs, outputs and states as possible, letting the system "run" for a while, collecting, collating and analyzing the data and testing for a fit against the original hypothesis. If necessary modify the hypothesis to better the fit with the data.

Alternatively, one can adopt an intuitive approach. This may not lead to a full understanding of the system but may lead to an understanding of aspects that are important to you. A hypotheses may not be formulated. The intuitive approach will ensure the measurements you think are important are made. These may or may not be optimal, but will probably increase understanding. Sometimes exciting serendipitous discoveries are made in this way.

Know your system, know what to measure

Most physical systems can be categorized into one of two system types:- open loop or closed loop. The term loop refers to a feedback loop where a system's output may be feedback to the input in some way that influences the output. An open loop system has no feedback so its output is entirely dependent on its inputs, how the inputs are processed and time.

A closed loop system does have feedback and is more complicated. The feedback can produce a stable system by automatically correcting for "errors" in the output. Feedback can also produce instability, with the system oscillating in what may appear as an unpredictable manor. Closed loop systems are far more interesting than open loop systems - and more common in both the industrial and natural world.

The reason to identify the type of system is to ensure that you measure all the parameters - inputs, outputs, feedback and states - that are likely to be the system drivers.

Measuring equipment

A bewildering range of measuring equipment is available. To make the right choice is not easy and requires at least a basic understanding of the technology. Less technical knowledge is required for "turnkey" or "out of the box" solutions to specific problems however, these tend to be less versatile.

Sensors and transducers

Sensors are devices that convert a parameter such as temperature into another parameter such as voltage that is readable by a measuring device. Usually a sensor is the raw detecting device operating on some fundamental physical principal. When electronics are added to the sensor may be referred to as a transducer. The electronics is generally needed to excite, amplify, compensate, linearize, filter and interface to other equipment.

Smart sensors

Add a microprocessor to a sensor, stir in a little software - the result is a smart sensor. These devices will become increasingly common as the enabling technology becomes more affordable. Linked in a network these devices will progressively replace centralized measuring systems.The IEEE Std 1451 standard development has contributed a lot in this area, particularly with its TEDS (Technical Electronic Data Sheet). The new IEEE 1451.4 may give the standard a new energy.

Meters

Traditionally, a meter is a display device employing a moving coil attached to a needle used to indicate the value of a single parameter. These have been largely replaced by digital displays that still display a single parameter value. However, the simplicity of a meter and the ability read its output at a glance will ensure that it remains popular.

Data acquisition (DAQ) computer peripherals

A wide range of products are available in this category. There common attribute is that they require a computer to manage their operation.

Internal plug-in DAQ cards

These are cards that fit into the expansion slots of standard IBM compatible desktop personal computers. Typically, an external breakout box is needed for sensor connection. The inside of a computer is an electrically noisy place, so great care needs to be taken to minimize noise pickup.

PC cards (PCMCIA)

These small cards plug into the PC Card socket of note book computers although adapters are available for desktop units. Their small size has lead to compromises in their performance compared to internal plug-in cards. They are also regarded too fragile for reliable industrial and field applications.

External DAQ boxes

These are measuring devices connected to one of a computer's communications ports (RS232, Ethernet, USB, Firewire etc.). Because they represent the best DAQ solution, especially with the coming of the fast communications ports, these products will begin to replace both the internal plug-in cards and PC Cards .

Distributed DAQ systems

These are an extension of the External DAQ Box concept by allowing multiple boxes to be networked to a computer. Such a network can be wide area, or localized. The connecting network may be slower and less deterministic than short range networks such as Firewire and USB.

Stand-alone DAQ boxes and data loggers

The Stand alone DAQ box is an extension to the external DAQ box in that it can operate without the assistance of a computer. Readings are stored internally and can be downloaded into a computer when convenient.

Data loggers are a category of stand alone data acquisition devices. They are typically low power and are designed for remote unattended operation. They may be programmed or "setup" from a front panel or by connecting a computer. A data logger may have from one to hundreds of channels.

Personal monitoring assistant (PMA) new!

This is a new class of easy to use equipment. They are hand held measuring devices that may come with built-in sensors. They are highly programmable via downloadable applications and provide the user with a small graphical user interface.

Specifications

Understanding product specifications is half the battle! Help is provided here.

Links to manufactures of relevant measuring equipment can be seen here.