25 LIMS & Lab Automation
In comparison to an industrial protocol system, there are some shortcomings. A computer- serial-device connection probably means adding upfront coding. It’s likely that the entire system would take longer to respond to operational changes because of the added overhead of the computer operating system, and relatively slow COM port transfer rates. The lack of standardisation also means it can be burdensome to change system administrators, or transfer the custom application to any other process.
Complex Control Loops and Processes
Academic, aerospace, biotech and chemical research labs require great versatility, precision and repeatability to be valuable and effective in their fi ndings. Whether replicating human breathing through life-saving apparatuses, analysing chemical catalysts, or transfer-fi ltering a culture’s by-products for a pharmaceutical test, the stability of fl uid control is critical. Each of these examples requires precise control to ensure that the lab results are repeatable.
The control mechanisms that need to be in place to achieve this are myriad. Let’s consider the simple example of what happens when a storm front comes through your region. The barometric pressure drops, and this changes the baseline of every device in your lab that references gauge pressure. The systems that incorporate these gauge pressure devices must compensate for the change in pressure, and these resulting system changes may in turn reduce the stability of your fl ow control processes. Even this simple scenario reveals the need for closed-loop process control instrumentation that can continuously monitor changes in pressure and temperature in order to keep fl ow control processes stable.
Multivariate Digital Data Streams
A signifi cant advantage of using multivariate process control instruments that communicate digitally is that a single digital data frame can carry data from multiple process variables. The example below is a data frame from a fl ow monitoring instrument that features data for eight different process variables, in addition to date and time stamps and other data that supplement the measurement. Such a multivariate data frame can be obtained via simple ASCII commands at a frequency of about 20 Hz. Considering just the eight process variables, the effective transmission frequency of the data from this single device becomes 160 Hz.
Figure 3: Data frame with multivariate readings from an Alicat Scientifi c fl ow calibration device. Source: Alicat Scientifi c, Inc.
These older protocols provide communication with 64 to 250 individual devices per network.
More recently, a number of Industrial Ethernet (IE) protocols have been developed, including EtherCAT, Ethernet/IP, DeviceNet, Modbus TCP/IP and PROFINET. They are designed for fast communications, typically requiring as little as 30-100 microseconds (about 10-30 kHz) per data update cycle. They can operate over Gigabit Ethernet optical fi bre for lossless transmissions. Additionally, these protocols enable communications with a nearly unlimited number of devices on a single network, (except for EtherCAT, which has a cap of 65,536 devices). One thing to remember is that Industrial Ethernet data frames are larger, at a minimum 64 kb, compared to non-Ethernet industrial protocol data frames that can be as small as 1 kb. Although this greater size is included in the transmission fi gures above, the automation management system must be capable of managing these larger data packets from potentially thousands of devices.
In practice, however, Industrial Ethernet data transmission bandwidth is actually much slower for many industrial automation devices, because so many do not directly communicate over an IE protocol. In order to connect these devices to your IE network, you must use an intermediate adapter module that can translate the device’s proprietary communications language to an IE protocol. Even when used with a single process measurement device, the IE conversion slows data transfer to and from the device. When up to eight devices must share one translator, this can create a bottleneck in communications. To preserve the fast native speeds of IE protocols, it is best to choose fl uid control instruments that communicate over IE directly, without the need for intermediate translators. Eliminating translators also makes it much easier to update older IE networks, or to create new ones.
Complex data streams like the example above make individual instruments more effi cient, reducing the overhead of purchasing and maintaining multiple fl ow, humidity, pressure and temperature instruments for individual variables. This reduction of equipment naturally makes lab setups physically smaller and more easily managed.
Figure 5: Optimising speed in a digital network is often a concern.
Taking the leap to Industrial Ethernet Communications for Fast Multivariate
Process Control Ethernet/IP is one of the most rapidly growing Industrial Ethernet protocols, and its communications speeds are more than fast enough for most applications, provided your instruments do not require the addition of translators. If you are establishing a new IE network, Ethernet/IP is often a safe choice, as there is a large and growing catalogue of devices that are available with this protocol, with what is reputed to be the largest number of installed instruments in the world. Their peripherals are usually less expensive than those required for proprietary PLCs.
For typical users, the difference between analogue and IE speeds will be insignifi cant to the operation of a lab. However, the increases in accuracy and effi ciency that result from the elimination of noisy analogue signals can make a very positive impact indeed. The greater data throughput of an IE protocol can signifi cantly increase effi ciency for closed-loop control systems. This throughput is especially critical for fl uidic process control instruments that rely upon rapid feedback in order to adjust to changing process conditions.
Figure 4: The front display of an Alicat Scientifi c multivariate fl ow controller, demonstrates multivariate process data for mass fl ow, volumetric fl ow, absolute pressure and temperature. Source: Alicat Scientifi c, Inc.
Getting Faster Performance out of
Networked Digital Protocols The advantages of industrial protocols, compared to serial and analogue, are standardisation of requirements, ubiquity of appliances, and expandability. For larger networks, numerous industrial digital protocols are available, some of which have been around for several decades. Long-established industrial automation protocols include CANopen, DeviceNet, FOUNDATION Fieldbus, HART, Modbus, PROFIBUS and many others.
About the Author:
Andy Mangell is European Territory Manager for Alicat Scientifi c. He has spent his entire career in the fl ow and pressure control industries.
While Rube Goldberg contraptions are entertaining for the way they cobble together different mechanisms, they are memorable because they work. Because there is no single version of a laboratory, there is no one-size-fi ts-all solution for lab automation of fl ow and pressure control. For those accustomed to analogue signals, going digital may seem like distancing oneself from the process, but the increased variety of data, precision and effi ciency are compelling benefi ts. Which isn’t to say that a direct analogue signal has no place in automation, it’s just reaching a point of diminishing returns. Explore the range, and go with the best for your situation - from analogue to the most modern industrial protocols, there are a lot of choices that can work.
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