Analog signals have been part of industrial control systems for a very long time. The two most common signals are 0-10V (“voltage”) and 4-20mA (“current”), although there are a wide variety of other voltage and current protocols. These signals are called “analog” because they vary continuously and have theoretically infinite resolution (although practical resolution is limited by the level of residual electrical noise in the circuit).
Measurement sensors typically provide analog output signals, because these electronic circuits are well-understood and the designs are relatively economical to produce. But that doesn’t mean it’s easy to design and build a good-quality analog sensor: in fact it is very difficult to engineer an analog signal that is highly linear over its measuring range, has low noise (for high-resolution), is thermally stable, (doesn’t drift as temperature changes), and is repeatable from sample to sample. It takes a lot of careful engineering, testing, and tweaking to deliver a good analog sensor to the market.
In today’s digital industrial control world, however, analog is looking more and more like a relic from another time. Yes, analog still has some important advantages; among them are universality (analog signals are very common on sensors and controllers), moderate cost (of sensor and input point), and straightforward troubleshooting (for example, with a standard electrical multimeter).
But in the era of industrial control networks, the weaknesses of analog are becoming more and more apparent. Analog is susceptible to ambient electrical noise that can disrupt the signal, which drives the need for shielded cables and careful attention to proper grounding practices (which drives installation cost). Understanding of proper analog wiring is becoming a lost art as we all have grown accustomed to robust digital signals in our daily lives. And today, we want automated diagnostics without the need to send someone poking around in control cabinets with a multimeter.
In networked control systems, handling analog signals becomes cumbersome in comparison to connected digital architectures. It is often necessary to run shielded cables back to analog input cards or distributed analog slice I/O, thus failing to take full advantage of the digital network. Oftentimes, analog ports are available on a machine-mounted network interface block…but the high cost of double-ended shielded cord sets and the lack of standardized sensor pin outs and grounding schemes makes connectivity overly difficult to implement in this scheme.
So, what’s next after analog? Without a doubt, analog will be supplanted by digital. Whereas analog signals have infinite resolution, digital signals have finite resolution that is determined by a discrete number of increments. The number of increments is expressed as a particular bit depth, for example 8-bit gives us 256 increments, 10-bit delivers 1024 increments, 12-bit yields 4096 increments, and 16-bit results in 65,536 increments. You can calculate these numbers yourself: the number of increments is simply the number 2 raised to the power of the bit depth.
If you’re a regular reader of this blog, you have probably heard about IO-Link, an emerging non-proprietary standard in the controls industry. If you haven’t heard about IO-Link, I would certainly recommend reviewing some past posts on this blog and becoming familiar with the topic.
In my view, IO-Link is the first “universal” digital interface that has a real shot at displacing analog as the preferred interface for measurement sensors. Sensors with integrated IO-Link interfaces can be produced at moderate cost and can plug directly into an IO-Link port on an IO-Link distributed modular interface block. The interconnection cable is a simple, low-cost unshielded 3-wire double-ended cord set (since the digital signal level is 24V DC digital, cable shielding is not required).
The IO-Link digital sensor interface preserves all of the advantages of analog (simplicity, moderate cost, ease of troubleshooting) while eliminating the disadvantages (hassle of shielded cabling, noise susceptibility, difficulty interfacing to digital control networks). In fact, the IO-Link digital sensor interface brings along many important new benefits like bi-directional communication/queries, remote parameterization/calibration, and fault diagnostics.