UKM-SUM23-PG34-36_Layout 1 02/05/2023 09:37 Page 35
SENSORS
gain amplifier, and a sigma-delta ADC. It also provides the excitation currents for the RTD, meaning it can effectively replace five of the signal-chain components from the previous figure, significantly reducing the amount of board space required and enabling a sensor with a much smaller enclosure.
COMMUNICATIONS INTERFACE Most industrial sensors are designed to connect to a process controller using one (or more) industrial networks, including the many variants of field bus or Industrial Ethernet. This requires an application-specific integrated circuit (ASIC) to implement the selected network protocols. However, this approach has several disadvantages. Firstly, including a network- specific ASIC in the sensor design significantly increases cost, especially if the industrial networks are proprietary. It also limits the market for a sensor to those customers using that network. For the same sensor to work with different network protocols requires redesigning to include the necessary ASIC, which can be time-consuming, high risk, and expensive. Finally, the number and type of diagnostic features vary significantly by network type (with some providing none). Depending on the choice, it can be difficult for factory operators to identify and maintain sensors and any performance issues that arise once they have been installed in the field.
A better approach is to design a sensor independent of all industrial networks, thereby reducing development costs and broadening the potential customer base. This can be done using IO-Link, a 3-wire industrial communications standard that links sensors (and actuators) with all industrial control networks. In IO-Link applications, a transceiver acts as the physical layer interface to a microcontroller running the data-link layer protocol. The advantage of using IO-Link is that it carries four different types of transmissions: process data, diagnostics, configuration, and events, allowing sensors to be quickly identified, traced, and attended to if
Figure 2. A block diagram of an RTD industrial temperature sensor.
Figure 3. A voltage signal produced by a Pt100 RTD in response to increasing temperature.
a malfunction occurs. It also enables remote configuration - for example, if the temperature threshold for a process alarm to be triggered requires changing, this can be done remotely without needing a technician to step onto the factory floor. The MAX14828 is an example of a low power, ultrasmall IO-Link device transceiver. It is available in a (4mm × 4mm) 24-lead TQFN package and a (2.5mm ×
2.5mm) wafer-level package (WLP), allowing it to be easily integrated into an industrial RTD- based temperature (and other types of) sensor. The transceiver enables a sensor independent of the industrial network because it communicates directly with an IO-Link host installed at the process controller side, which manages communication with the interface ASIC as shown in Figure 6.
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Figure 4. An AFE implemented using separate discrete components in the signal chain. UKManufacturing Summer 2023 35
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