LIMS & Lab Automation


Automating Flow Control in the Laboratory: Options from Analogue to Industrial


Andy Mangell, European Territory Manager, Alicat Scientifi c


When setting up the automated components for controlling fl ow and pressure in your lab, you will have many things to take into account. Your considerations will be shaped by your end goal, and by the speed, methods, costs and complexity of the processes used to reach the goal. With unlimited funds, one could create a spectacular - albeit Rube Goldberg-esque - lab with every type of instrument, and it might tell you the deepest, darkest secrets of the universe. This is unrealistic, of course: labs are expected to adopt processes that produce the desired result, effi ciently and cost-effectively. Automated processes assist in accomplishing such goals. In most cases, the range of choices short-listed for consideration will depend on what is already used in the current lab, and what may make the test or process faster, more controllable and effi cient - and produce the most usable outcome, whether that be a physical change or data-based insight into a result.


Choosing a Road to Travel


Having decided on remote communications, the next consideration is, what form should that take? While it is certainly true that most organisations with previously installed communications protocols will want to continue in the same vein, networking and communications have undergone decades of revolution - a greater variety of installable instruments, cheaper networking hardware, or larger, faster networks may now be essential considerations. Who today would opt for dial-up speed internet, if high speed is cheaply available?


control process, the gauge pressure, input and output pressure, fl ow rate, species of gas, process temperature and even humidity each have interdependent effects. Tracking all the variables means that wiring complexity gets high.


The reality is that much of the instrumentation today uses digital signal processing internally, so converting those signals to analogue, and then possibly back to digital for analysis at a later point in the process, itself adds noise to the originally lossless digital signal. This may not be decisive, but it does eat into one’s measurement uncertainty budget.


Analogue communications may offer only limited control. In many cases, analogue communications permit only the most rudimentary commands, such as changing the setpoint of pressure or fl ow control. Given that many fl uid control instruments incorporate extensive programmability (such as being able to change engineering units or fl uid mix settings), this reduces the device’s native fl exibility and power in a process control application.


Quick Serial Options


Where the precision and versatility of digital communication is desirable, but the purchase of industrial PLCs for handling automation is beyond the wallet, instrument manufacturers offer drivers and even starter software kits. This may serve as a toehold entry to digital communications.


Figure 1: Device level ring connections for Ethernet/IP devices. Image source: Alicat Scientifi c, Inc. Going Analogue in a Digital World


Digital is not the only option. Although the technology environment is binary, in small scale systems with complexity equivalent to a benchtop seed-culture bioreactor, or automating the calibration of several gas chromatographs, analogue I/O is easier to confi gure. Its simplicity makes it an attractive option to many experimenters. If speed is paramount, the continuous data stream of an analogue connection remains the fastest method of communications available. In processes that are extremely timing-sensitive, that speed may be decisive. That said, this advantage is diminishing with the increase in digital protocol speeds.


On the downside, a potential risk in analogue communications is that of signal degradation. Signal degradation can lead to inaccurate readings, and can be caused by excessive cable length or external noise sources.


For some analogue installations, wiring complexity is very high, causing troubleshooting and maintenance issues, and raising costs. This comes of being able to read only one sensor, or signal, per wire. In a fl uid (plasma, gas or liquid)


Analogue Connections


Reading an output from an analogue signal can be as simple as using a multi-meter to measure the voltage or current output, and interpreting the scale to acquire your reading. Your readings will be proportionally slaved to the range of the device.


Consider a fl ow meter with a range from zero to one hundred litres per minute, confi gured with an analogue signal of 0 to 5 volts. A reading of zero volts is zero fl ow. At 2.5 volts the reading is 50% of the fl ow scale, and a reading of 5 volts is full scale fl ow (100 litres per minute). If your instrument has an integrated display screen, you can verify your readings during process setup with a visual check.


By connecting the device to a programmable logic controller (a PLC which can accept a 4-20 mA input signal), the PLC can be confi gured with a multiplier to interpret the analogue signal and provide the reading. By measuring current instead of voltage, you avoid the risk of voltage drop over long lines, which would entail re-ranging your interpretation of the scale.


Through serial communications, a common computer can be substituted for a proprietary PLC. It would provide data processing, monitoring of closed loop conditions, a human-machine interface, and data logging. Indeed, smaller process control systems can be managed using nothing more than ASCII commands over RS-485 and a common data management program like LabVIEW. Basic RS-485 networks, and even some RS-232 variants, can communicate with 26 or more instruments by assigning a unique letter-based unit ID to each instrument. For new users, it’s best to look for a vendor with driver and communications utilities prepared for download, written instructions and readily available tech support.


Figure 2: Multi-drop kits can be used to attach batches of fl uid control instruments through serial connections to a computer. Source: Alicat Scientifi c.


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