Incredible intelligence is being built into just about every device we touch. By using the information available in today’s smart/digital devices, combined with the communication capabilities of Ethernet-based digital communication protocols, the integration, start-up, maintenance and productivity of process systems such as biotechnology production equipment and bioreactors can be dramatically improved.
Mass flow control (MFC) instrumentation plays a critical role in biotech and biopharmaceutical processes, controlling the delivery of key process gases, including oxygen and carbon dioxide, as well as purge gases such as room air or nitrogen. Surprisingly, many process engineers underutilize the capabilities of digital MFC instrumentation. And, with the extra benefits offered by “smart” MFCs with Ethernet-type interfaces, such as faster communication speeds and the ability to acquire and provide real-time data through Internet and enterprise automation networks, the additional possibilities for enhancing biotech systems and processes are too often overlooked. With this article, we will examine:
- The evolution of process control instrumentation and communications from analog to digital
- The range of traditional digital and Ethernet-based communication protocols and their key capabilities
- Ways to enhance your biotechnology process systems and bioreactors by using the full breadth and depth of information available from digital MFCs
Digital communications evolution
If we look back at the dawn of electronic communication to the telegraph, we see the earliest limits on how much information could be sent and how fast. In 1841, when U.S. President William Henry Harrison died of pneumonia a month after taking office, it took 110 days for the news to reach Los Angeles via mail.
If you do the math on the number of letters carried, the number of words per letter and the time to get there, the data transmission rate is equivalent to about 6 bits per second. Today, we have data rates of 100 megabits per second (Mbps) and higher. And the density and complexity of information sent at those speeds is truly transformative.
Advanced communications protocols today enable ultrafast information exchange between devices. This is one of the fundamental changes driving the evolution of automation systems from analog to digital: giving automation systems greater ability to leverage the amount of information that can be communicated through input/outputs (I/O) or machine buses.
These protocols also enable sophisticated, real-time interaction and control between the programmable logic controller (PLC) or distributed control system (DCS) and digital instrumentation. This is crucial to taking full advantage of advanced control, diagnostic and alarm capabilities available on digital instrumentation.
Range of digital communications protocols
While some of the older technologies like 4-20 mA analog I/O are used (mainly for I/O purposes), most new systems use some form of digital communications for factory-wide monitoring and control. Furthermore, there is a trend toward Ethernet-based protocols like EtherNet/IP and EtherCAT.
There are multiple digital communications protocols in use across the automation market in general (see Table 1) and specifically in biotechnology process systems, including the following:
- FOUNDATION Fieldbus
- RS-232 and RS-432
One of the fundamental design decisions OEMs and systems integrators make, early in their equipment design process, is the communications protocol.
Advantages of “going digital”
No matter what digital communications protocol is selected, there are fundamental advantages provided by “going digital:”
- Powerful source of rich data from the MFC to PLC/DCS to enhance process control: Multivariable capabilities (flow, temperature, etc.), alarms, totalizer values, valve drive/position and other data
- Real-time diagnostics information is readily available: Facilitates installation, start-up and troubleshooting; supports predictive and/or preventative maintenance
- Increased device flexibility—such as devices with multiple calibrations—reduces spares inventory and maximizes uptime
- Simplification and cost savings with standardized cables: Off-the-shelf multidrop cables are more cost-effective vs. custom discrete wiring; multidrop system configurations simplify wiring requirements vs. analog point-to-point installations; simplified wiring reduces documentation level of effort and errors; flexible topologies offered by digital systems simplify future expansions
How digital protocols enhance device performance
Automation devices with digital I/O, regardless of the protocol used, can provide a myriad of information that is not available from a basic analog I/O device. A typical analog I/O device can provide a single process variable or, in some cases, two.
With a digital MFC, you can read the flow, totalized flow, temperature, valve drive and other variables simultaneously—and you can communicate that information to the PLC/DCS or other devices on the network for further action in real-time. You can take advantage of multigas capabilities and dynamic gas range switching by sending a digital command. For example, you can set up your system so that an MFC can be changed from a 25-standard-liters-per-minute (slpm) oxygen device to a 20 slpm CO2 device.
This capability can enable significant process equipment cost savings. For example, by taking full advantage of a digital MFC’s multigas/multirange capabilities, it is possible to reduce the number of equipment SKUs required by 90 percent, reducing inventory and simplifying purchasing.
Preventive maintenance and process quality are also enhanced by having a broad array of thresholds and alarms that can be set and monitored by digital MFCs. These can include:
- High-flow alarm and low-flow alarms
- No-flow indication
- Setpoint deviation alarm
- Valve drive alarm
- Temperature out of limits
- Totalizer overflow
- Internal power supply failure
- Valve drive out of limits
- Device calibration due
- Device overhaul due
- Internal diagnostic alarms
Alarms like these can help keep critical process systems on track. For example, it’s common in biotechnology operations to schedule regular device calibration at six-month or annual intervals, often to comply with regulatory requirements. MFCs with “device calibration due” and “device overhaul due” capabilities can alert operations personnel to these intervals, helping ensure they proactively schedule process equipment downtime to comply.
Alerts such as “valve drive out of limits” can help operators pinpoint the source of problems. If O2 pressure feeding a bioreactor chamber goes too low, it could be because a filter is clogged or a regulator is not working properly. Some digital MFCs can open the device's valve to try and compensate for that.
Eventually the valve will reach the limit of its ability to open and won't be able to achieve full-scale flow. The bioreactor’s PLC gets an indication of valve drive out of limits and takes advantage of that alarm before the bioreactor reaches that point. Further investigation of the system can be undertaken to determine the root cause of the issue.
Digital application: flow totalization in facility gas usage monitoring
A common application where accurate flow totalization is required is gas usage monitoring. In this example, several different systems or locations within a bioprocessing facility share a single gas source, which is a common system design. To account for usage or to allocate costs properly, the facility needs to monitor the amount of gas consumed by each user.
Typical installation: A typical installation for this application includes several flow meters plus secondary electronics with totalizer function cabling from each device connected to a central monitoring system (see Figure 2). The totalizer gets a flow signal from the flow meter, calculates the totalized flow and sends that value to the central monitoring system.
With this approach, the accuracy of the totalized flow may not be optimized. There may be some additional error due to resolution of the analog-to-digital converters (ADC) and signal noise. The user also needs to be sure the analog signals were calibrated properly and that they match the span and time units of the flow meters. Signal filtering, signal cutoffs, sample rates and sample period can also have an impact. All these factors could lead to improper billing or cost allocation. There are also additional hardware and cabling costs with this approach that could be avoided.
Digital application: bioreactor total flow measurement
Using digital mass flow meters: An alternate approach uses digital mass flow meters that calculate the totalized flow value internally. With this approach, no additional inaccuracy is introduced with a secondary calculation, and the need for digital-to-analog conversion is eliminated.
Understanding the total flow into the bioreactor, in real time and without the need for digital-to-analog conversion, is a key factor that process managers can use to optimize bioreactor yields (see Figure 3). The digital totalizer command provides real-time feedback to process and equipment engineers.
Considerations when choosing digital communications protocols: supported network topologies
It’s important to realize that when selecting the digital protocol, you will be designing a network in the machine: sensors, drives, devices like MFCs, touch screens and the machine PLC or DCS are all going to be networked together. There are several network topologies that can be implemented (see Figure 4):
Point-to-point or star networks offer easy setup, fast and reliable pathways (one can fail and the others keep working) and the ability to add nodes; however, this topology is not good for large networks. Also, this is the only topology for analog and RS-232 devices.
Multidrop networks can use either a bus or daisy chain arrangement. They offer easy setup but are vulnerable to disruption if there is a break in the chain. This is typical for RS-485, DeviceNet and PROFIBUS networks.
A ring topology can handle a lot of traffic and is easy to install, manage and troubleshoot. EtherCAT, EtherNet/IP and other Ethernet-based protocols support this topology.
The mesh topology, while the most complex (it’s the topology the internet uses), is extremely reliable because of its built-in redundancy: Adding devices actually improves data rates and reliability. It works well for wireless networks; however, in a wired environment, it can be expensive.
Along with a system’s network topology, digital protocol selection should also be based on the essential performance characteristics of each protocol. While there are many factors to assess for each protocol, the number of nodes supported, the throughput (or baud) rates and the message size are essential.
This information can help OEM design engineers and end-user engineering teams evaluate the real-time communications requirements for their systems, as well as how much/how dense the information each digital device needs to share in real time with the system controller. Using MFCs that support multiple digital and Ethernet-based communications interfaces can help ensure biotechnology OEM and end-user automation systems are equipped with state-of-the-art fluid measurement and control systems that offer the enhanced flexibility and efficiency required by this rapidly changing industry.
Steve Kannengieszer is the global marketing director, Industrial Business Unit, Brooks Instrument