Design Con 2015

Product How-to: Analog integration drives factory integration

-December 07, 2013


Advances in analog and mixed-signal ICs have contributed greatly to unprecedented combinations of factory productivity, efficiency, quality, and safety. These ICs enable higher system performance, increasingly accurate measurements, robust communication, lower power, and higher levels of security. They are central to factory systems that act rapidly on large numbers of sensor-based measurements that precisely control material handling, processing steps, power and temperature, and operational safety.


When computational resources are integrated into these analog and mixed-signal ICs, the factory can deploy a distributed intelligence model. Factory data can now interface more readily with business-management systems. This integrated factory model, comprising many of the techniques and technologies described below, makes factory information available worldwide in real time to supply chain, sales, logistics, and senior management.


Distributed Intelligence

Integrated factories exploit distributed intelligence, pushing computational resources and decision making out of the control room and into widely dispersed process machinery. This shift in system architecture delivers data to management personnel in almost real time. It eliminates data-processing bottlenecks. It allows management to deploy more measurement and control nodes, and to act on significantly larger sets of process data.


The benefits of distributed intelligence to the factory floor can actually be summarized, be realized, in a single concept: uptime. Factories will reduce measurement-to-control feedback latencies; experience better reporting of process efficiency, throughput, WIP (work-in-progress) status, and machine utilization; and lower maintenance costs.


Benefits to the company, however, extend far beyond the factory floor. Integrated-factory systems provide up-to-the-minute workflow data which, when combined with supply-chain intelligence, allow operations to optimize their scheduling and inventory management. Worldwide sales and distribution management can act on precise and timely factory productivity data. Management can better optimize production-line balancing and capacity utilization.


These many benefits clearly depend on sophisticated software tools for optimal operation at the highest level of abstraction. But the software systems cannot deliver these results without accurate, efficient, and robust electronics to implement the sensor-based measurements, control functions, and energy-management tasks.


Integrated ICs Enhance Sensor Measurements

Mixed-signal ICs bring signal conditioning and digitizing functions closer to the points of measurement which are often widely distributed. Digitizing at a sensor node increases control-data quality by reducing noise susceptibility. Temperature measurements are among the most common process-control inputs. The temperatures of feedstocks and other raw materials, of processing environments such as ovens and tanks, and of machinery such as drive motors and molding machines, all impact manufacturing quality and system reliability.


Temperature measurement channels must provide open-load detection so that the system can force a safe control state when a temperature sensor or one of its connections is broken. A signal conditioner like the MAX31855, for example, will read a thermocouple that is, in turn, monitoring a processing oven’s temperature. In the event of any detected sensor fault, the signal conditioner will report an error code in the digital data stream so that the system can respond quickly and cut power to the oven and report a fault to the supervisory level system monitors.


There is, however, a challenge. Temperature sensors such as thermocouples and resistance temperature devices (RTDs) output voltage changes on the order of only 10s of microvolts per degree C. So, signal conditioners must accurately resolve small voltages corresponding to temperature measurements. If the signal conditioners are measuring RTDs, they must also provide an accurate excitation signal in the form of a low-noise current source. The RTD transduces the excitation current to a voltage that is proportional to its temperature. The excitation current is limited to relatively small values to prevent self-heating in the sensor due to I2R dissipation.


Integrating the sensor excitation, signal conditioning, digitizer, fault detection, and protection circuits in a single monolithic device greatly simplifies the design of measurement channels or channel clusters. Essentially forming single-chip measurement subsystems, mixed-signal ICs like the MAX31855 provide specified parametric performance from sensor to digital controller; they eliminate PCB layout complexity, reduce susceptibility to sources of electrical noise, and significantly improve functional density (Figure 1). In sum, analog integration improves system performance and process quality.



Figure 1: A complete thermocouple-to-digital converter includes cold-junction compensation and fault detection.


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