InTech Home01 March 2003
Isolating confusion
Control and measurement signals make or break the process.
A stray signal can cause system inefficiencies, product contamination, and even system failure. Isolating signals has become a top priority.
There are several methods available for isolating signals. Each has advantages and disadvantages when measured against the performance, size, and cost characteristics of an ideal isolation solution.
All isolation devices, regardless of their design or technology, perform the same function: They interrupt common mode currents while preserving information flow. Isolation also permits separating (and accurately measuring) small normal mode signals that ride on large common mode carrier (unwanted) signals.
In a perfect world, an isolation circuit would allow information to flow across an isolation barrier with no signal degradation, no power consumption, no size or cost penalty, and infinite protection against common mode voltage differentials and transients.
This type of isolator doesn't exist but does serve as a benchmark to evaluate isolation technologies.
In industry today, one common method used is no isolation at all. Down the road, this leads to replacement of parts, circuit boards, and constant maintenance and/or troubleshooting.
Isolation can be analog or digital. Both analog and digital isolation can use components such as transformers and optocouplers for the separation of unwanted signals. These components integrate into different types of isolation circuits depending on the cost, efficiency, and real estate.
DESIGN ENGINEERS ARE TURNING
A common analog isolation technique uses transformers in the circuit design based on amplitude modulation. This type of a design uses a chopper circuit to chop the analog waveform into positive and negative segments and inverts every other cycle at very high frequencies.
The resulting signal then passes across a transformer and goes through reconstruction, demodulation, and filtering.
Although this method is popular with low-cost isolators, it does introduce errors into the control system. This method is common with isolated PLC cards or controller cards.
Design engineers are turning to digital technologies in order to improve accuracy, improve system functionality, and reduce cost. Advances in digital isolation technology provide better accuracy and higher resolution at lower costs.
The digital circuit converts the analog signal to a digital format that is then passed through an isolation barrier to a microprocessor or digital signal processor that evaluates a large number of measurement criteria and passes the accurate signal form along.
Selecting isolation technology depends on cost, size, and performance. Optical isolators provide the lowest cost but high isolation levels. Magnetic isolation provides high accuracy and efficiency but costs more.
Manufacturers of electronic components and signal isolators use different types of isolation techniques, depending on the form, function, and cost of the device. In some cases a combination of technologies is used.
The industry trend in high-performance signal isolators is to provide high functionality and accuracy by using a variety of components in each module to make the highest quality isolators and converters.
—Davis Mathews, Phoenix Contact Inc. (www.phoenixcon.com)
