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Industrial sensors and field transmitters are an integral part of the factory automation and process control industries. Measurement accuracy and precision are critical to tightly controlled process loops, as well as for single-point measurement nodes. This article leads you through the design decisions—concerning the signal chain, power source, and interface—that can help or hinder you from getting the most out of your sensors and your system as a whole. You will see how to translate system demands into device specifications, and clearly understand the design decisions and tradeoffs involved in maximizing the impact of industrial sensor and field transmitter electronics in the process.
First, let's consider a typical factory floor. Most factory floors can be broadly segmented into a hierarchy of three levels: the enterprise level, the control level, and the field level as shown in Figure 1. The enterprise level is, in essence, the office network, the backbone of servers and PCs networked to log all of the data and perform higher-level functions. The control level takes us into nodes and clusters of programmable logic controllers (PLCs), human-machine interface (HMI) panels, and DCS/SCADA and similar systems. This equipment issues commands to, and collects the resultant data from, the lower levels, processing this into actionable information, which can then be used to define and drive the actions of the control loop. The lowest level in our hierarchy is the field level and it is perhaps the most critical level of the three, because the equipment in this level includes the sensors, transmitters, and actuators that perform the actual manufacturing tasks and relay back critical data.
 Figure 1. Typical factory floor hierarchy |
For example, in a pharmaceutical bottling plant, different constituent chemicals are mixed in the correct proportions to create a bottle of the latest drug. Every step in the process needs to be executed at the right temperature, pressure, and humidity conditions to ensure that the quality and characteristics of the resultant mixture are as desired. This requires the constant measurement of multiple variables, the timely transmission of data, and the instantaneous response of the equipment to the control signals. The various sensors, along with the field transmitters, are what allow any process to become a controlled process, and designing these systems to take measurements with a high degree of precision is critical to the functioning of any control loop.
Depending on the size of the factory and the complexity of the controls needed, the factory floor could have thousands of sensors. As such it is important to understand what a sensor looks like on paper to understand how it will act when put into your control loop. Figure 2 shows a snapshot of typical information from a sensor/field transmitter datasheet. While it can be hard to extract exact operational characteristics from the datasheet, it will give you a ballpark idea of how a device will perform, especially when compared to other sensors that might already be in use in the system. Datasheet information can be broken down into input, output, power, and isolation specifications. This example in Figure 2 is for a popular 8-channel temperature transmitter that can be configured in multiple ways, e.g., for 2- or 3-wire RTD, thermistor, or thermocouple inputs.
 Figure 2. Typical sensor/field transmitter datasheet |
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