Boost Your PLC System: High-Performance Analog Measurements Print

This article is part of the 7 Ways to Boost Your PLC System white paper series.


 We live in an analog world and we often find ourselves in need of high-performance analog measurements as we push the limits of automation technology. Can PLCs keep up?

New Technology for High-Performance Analog Measurements

Improving Your Control Application

It has been said that you can control only that which you can measure. As control applications become more and more sophisticated, you need to turn to higher-speed and higher-quality measurements. Unfortunately over the years, due to cost considerations and broad-based market needs, the PLC has become more and more tailored to general control applications with limited analog I/O and subkilohertz loop rates. There are a few axioms that all automation engineers know:

  1. Your outputs are only as good as your inputs.
  2. The faster you close the loop, the more precise and efficient your control.

However, due to the generalized functionality of PLCs, getting high-speed, high-quality measurements, such as dynamic signal analysis, high-precision voltage, and current measurements, is often challenging without using specialized PLC hardware.

NI CompactRIO programmable automation controllers (PACs) address these challenges by incorporating a flexible, high-performance real-time system and a highly flexible and reliable user-programmable FPGA in measurement-class NI C Series modules.


Figure 1. NI CompactRIO Family

NI offers more than 50 C Series modules. A variety of I/O types is available, including ±80 mV thermocouple inputs, ±10 V simultaneous-sampling analog inputs/outputs, 24 V industrial digital I/O with up to 1 A current drive, differential/TTL digital inputs with 5 V regulated supply output for encoders, and 250 Vrms universal digital inputs.

Because the modules contain built-in signal conditioning for extended voltage ranges or industrial signal types, you can usually make your wiring connections directly from the CompactRIO module to your sensors/actuators. In most cases, the CompactRIO modules provide isolation from channel-to-earth ground.

CompactRIO modules connect directly to reconfigurable I/O (RIO) FPGA devices to create high-performance embedded systems that deliver the optimization and flexibility of a custom electrical circuit completely dedicated to your input/output application. The RIO FPGA hardware provides unlimited options for timing, triggering, synchronization, and sensor-level signal processing and decision making.

Case Study: Optimizing Cold Rolling Mills in Europe

In a traditional, continuous, cold rolling mill, throughput can usually be increased by up to 20 percent. The mechanical parts of the plant are often in good shape because they are over-dimensioned, so only the motors, control logic, and motion control systems need to be replaced. This is a classic example of refurbishing. However, the biggest challenge is at the start of the production line, where the end of a steel coil needs to be welded to the start of a new coil.

Welding strips using incorrect parameters or insufficient control result in bad quality and the risk that the metal strip might break at the weld during pickling and or rolling. When the strip breaks, it springs back around the roll and coils around it. The strip then needs to be pulled back and recoiled as well as fed through the machine. This can lead to up to 72 hours of downtime. Therefore, better control over welding quality is very important – it needs to be performed quickly and reliably.


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Figure 2. Optimizing Cold Rolling Mills

The welding quality depends on several parameters such as the position of sheets and electrodes, pressure, and the rate the pressure is applied as well as the current. These parameters should match and be synchronized all together within a very short time (typically within a single 50/60 Hz period). The pressure is controlled by a hydraulic system, capable of delivering 30 tons of force, within 0,1 seconds to two cylinders. Because controlling the hydraulic system is a nonlinear process – the stiffness of the system depends on the position of the cylinder – a special control algorithm is needed.

The current, several thousand amperes, for the resistance weld should be controlled carefully as well. Thyristors are often used to control the flow of AC current. These thyristors can cut off the current flow only during the zero-crossing of the AC current. The required welding time is obtained by activating the thyristors at the right moment in time, within 50 µs accuracy. In the past, this has always been a challenge, along with turning the thyristors off, because it has to be synchronized with the power grid, where the large current draw also induces a disturbance.

Controlling the thyristor is more efficient using the CompactRIO system. With the FPGA, you can determine the correct moment to activate the thyristor within 10 ns. This control loop is difficult because the power grid contains high-frequency harmonics and disturbances. There is more involved besides measuring the AC cycle period and then determining the starting time to open the thyristor. This system acquires samples at a high rate and feeds the signals through a digital filter implemented on the FPGA to remove the harmonics and determine the real zero crossing.

Read the full case study

Additional Case Studies

How to Boost Your PLC System

Acquiring accurate and timely measurements is truly the foundation of any industrial monitoring or control system. If the input or feedback to your system is inaccurate, then the entire system will likely not perform up to expectations, no matter how sophisticated the analysis or control algorithms. Use NI CompactRIO PACs to add high-speed and high-resolution measurements that increase functionality and improve overall efficiency. For more information, please visit the following resources:

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