By Steve Meyer || Analog inputs and outputs are generally associated with sensors. These in turn are the key to controlling real-world processes. So depending on the speed and resolution needed to measure events being controlled, analog inputs come in many performance ranges. Common sampling resolutions are 12 to 24 bit, and sampling frequencies range from kilohertz to megahertz.
Basic terminology: Defining analog in context
Understand that analog inputs are just signals, but analog controllers such as standalone PID controllers are actually intelligent subsystems. So in the context of this FAQ, “related controls” are in reference to the various intelligent peripherals such as robots and vision cameras frequently found in automated work cells. The interaction between these components is definitely different from normal I/O.
I/O interfaces for programmable automation controllers (PACs)
The interface of analog I/O to programmable automation controllers (PACs) can be as simple as it is in PLCs — a direct-backplane interface. However, other options exist. Because PACs are suited to distributed applications, I/O can be anywhere …
… and the network connection between controllers and remote I/O in these setups is part of the overall control strategy.
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Applications for programmable automation controllers (PACs)
Control systems that use programmable automation controllers (PACs) for complex or high speed analog signal processing depend on their ability to interface with intelligent processors to expand their capability to function reliably in extreme applications.
• In large process systems, the time base is very slow and network speed is not a factor.
• Other applications that involve high-speed signals such as testing of bearing vibration and radio-frequency testing have very demanding I/O requirements. Recent generations of PACs have addressed high-speed input signal interfaces using dedicated modules. These modules use FPGA technology to get very high-speed signal capture and preprocessing to prevent loading the PAC CPU.
Vibration signature analysis often requires inputs with high speed and resolution, and that creates a high-bandwidth dataflow. Structural testing using dozens to hundreds of such sensors is one of many applications commonly found that require this approach.
• One might assume that power-grid monitoring is simple. However, while the normal ac waveform is only 60 Hz, there is significant information occurring at the 15th harmonic. Therefore, sampling rates in excess of 2 MHz are needed to capture this information. What’s more, capturing, analyzing and storing that information requires a system many times faster.
• Vision systems require high bandwidth processing to capture images, resolve detailed features and update controllers in the low millisecond range. The vision task by itself requires a great deal of processing power, and is currently completely beyond the capability of a PAC.
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