by Sean O’Grady, Product Manager, Valve Terminals and Electronics, Festo
Pre-screened referenced products open up a whole new generation of automation solutions.
Communication is critical in the daily reality of an engineer. Every minute spent calculating design limits or researching material compatibility seems like it is offset by an hour in project meetings, customer calls or technical consultations with colleagues. But a whole other type of communication impacts our work everyday. Specifically, we’ll address the often frustrating and time-consuming task of getting control systems to communicate with analog devices. We’ll also explore ways that programs, such as Rockwell Automation’s Encompass program, helps engineers find solutions to these challenges.
Relay logic—simple and straightforward
Early machine control was accomplished by so called “relay logic.” Push buttons, limit switches and other sensors would trigger motor starters, solenoid valves and other electrical loads to automate basic machine control tasks.
Much like the gas pedal in a New York City taxicab, relays are either on or off. As requirements emerged for speed control, proportional pressure control, and other mechanical shades of grey, our relay logic control systems grew to include stand-alone, purpose built control elements suited to these tasks. Factories around the world filled acres of combined control cabinet space with thousands of electromechanical timers, PID loop controllers and their many and varied cousins.
While certain examples of these devices could be complex and esoteric, for the most part, getting them to talk to each other was simple and straightforward. The same wiring diagrams that had been developed to plan the original relay logic systems evolved to include these new devices.
As relay logic gave way to PLC logic, the relays and standalone control devices were virtually absorbed into the PLC itself. The wiring diagrams went from paper to screen, but were otherwise indistinguishable from the ladder diagrams that lend their name to our ubiquitous PLC programming language. Aside from the obvious discomfort that many a stubborn engineer expressed at having to embrace computers, this trend made the life of the engineer considerably easier. Sitting at your laptop and creating a timer module right out of thin air was a lot simpler than specifying, purchasing, installing and commissioning an electromechanical timer relay. The same could not always be said for other processes’ control elements that were trying desperately to be included in this new, digital world.
Changing the way things get done
Fast forward to the year 2013 and let’s meet an engineer by the name of Sam, who’s just been charged with the task of automating a system to help remove the shells from nuts in a large food processing operation. Sam’s customer has developed a machine that does a great job of crushing the shells of nuts without pulverizing the valuable nut meat inside. The machine uses a matrix of pneumatic cylinders that apply pressure to the nut cracking devices. The success of our process depends heavily on the precise control of the force being delivered by these cylinders.
As with most natural materials, the physical properties of the nuts being processed depends on a multitude of factors. Over years of careful experimentation, Sam’s customer has developed an extensive menu of pressure settings that correspond to specific combination of factors such as moisture content and temperature. Sam’s task is to develop a control system that can measure each of these variables in real time and adjust the cracking pressure accordingly.
With the majority of the mechanical engineering complete, Sam’s work will consist largely of finding the right sensors and regulators to apply to the system. After a few meetings and phone calls with vendors, Sam has a nice little pile of sensors and actuators that his technicians bolt to the customer’s machine. This brings Sam to the part of his job that is all about communications.
In days past, his process would look something like this:
1. Calculate flow/pressure and speed requirement for all connected actuators.
2. Specify a proportional pressure regulator that meets these requirements.
3. Design and implement plumbing to connect the regulator through control valves to the connected actuators.
4. Specify and install a compatible analog input/output card at the PLC or local field I/O location.
5. Wire the regulator to the analog I/O module.
6. Determine the scaling factor used by the regulator. (How many V/mA per psi or bar of pressure.)
7. Determine the scaling factor of the analog I/O module (SINT right justified, linear
8. Write code to convert the units used in the control program to the units required by the various scaling factors between control and the regulator itself.
9. Repeat step 6-8 for the associated pressure feedback that the unit sends to control (You remembered to specify a unit with integrated pressure feedback, right?)
10. Glower in frustration as your marketing counterparts tease your about your communication skills.
With the advent of modern, integrated proportional pressure regulators, the process looks something like this:
1. Calculate flow/pressure and speed requirement for all connected actuators.
2. Specify an integrated valve manifold that includes all required control valves, proportional pressure regulators and network adapter.
3. Skip steps 3-9 above, as everything is fully integrated.
4. If your application requires 65.4 psi, send the manifold the decimal value 65.4.
5. Sit there and watch as the unit sends the decimal value of 65.4 back to control to verify that it’s done what you’ve told it to do.
6. Walk down to marketing and ask them why it’s taking so long to come up with a new slogan for your new machine.
Simplifying with function integration
A new term has emerged for devices that simplify the integration of complex electromechanical function directly into the control architecture: function integration.
Function integration is a direct result of the culture of mutually beneficial communication that is fostered and supported by programs such as Rockwell Automation’s Encompass program.
Rockwell developed the Encompass program as a way of pre-screening and then referencing products that work in conjunction with Rockwell hardware and software systems. The idea is to speed up integration and remove the uncertainty of third party products on Rockwell Systems. By encouraging the players within the industrial control market to work together, they have created an environment in which the very nature of the communication has evolved. We’re now moving beyond old communication standards towards a bright new future in which PLCs and the devices that help them control modern industrial machinery have conversations that are easier for humans to understand, while at the same time communicating more detailed information about the subtler details of the application. In the “old” example listed above—steps 1 through 10—the engineer went through a great deal of work simply to communicate one piece of data from the PLC to the attached device. In our example, this data had to do with a commanded pneumatic pressure, but it could have just as easily been a speed command to a drive or any number of other control parameters. The true beauty of function integration comes from the realization of additional data that simply wasn’t available before.
To understand the potential benefits of function integration it’s important to note that the most fundamental change that has taken place is that we’ve converted a one-way transfer of information to a true conversation. In the “old” application, the PLC commanded a certain pressure. To verify that this pressure was reached, an additional pressure sensor, along with its associated input module, wiring, parameterization, scaling and so forth had to be commissioned. Not only do modern function integration devices integrate that return path directly into the installation, the fact that this two-way communication is available opens the door for a wide range of additional information communicated to the PLC. In addition to the command and feedback signals, the following data should be considered.
• Is the supply voltage within range?
• Is the control program asking for a setting that is outside of the approved range?
• Should the actual value be filtered at the device, to prevent the programmer from having to deal with transients in the user program? If so, what level of filtering is desired?
Other function integration modules can share a wide range of helpful information, such as:
• Hysteresis setting within the module
• Detection of short circuits within connected wiring
• Detection of broken wires within connected wiring
The list goes on and on. The journey into this world of function integration is just beginning. In a few years, specifying, ordering, installing and wiring stand-alone, external sensors and actuators will be an increasingly rare undertaking. Just as we’ve moved away from individually wired solenoid valves and motion controllers, in favor of fieldbus connected variants, we will also move away from stand-alone sensors and proportional control devices.
As with any change within our industry, there will be a learning curve for engineers and users, vendors and customers alike, but with the community of Rockwell Automation Encompass partners supporting this innovation, the learning curve, and associated benefits, can be accelerated.
Examples of Encompass function integration with Festo products.
VPPM Pressure Regulator – High performance proportional pressure regulation.
• The VPPM Proportional Pressure Regulator can be integrated directly in the Festo CPX valve terminal system, allowing direct communication to Rockwell hardware through EtherNet/IP or DeviceNet.
Valve terminal mounted pressure sensors
• Several variants of integrated pressure sensors are also available for the CPX valve terminal system
• Sense supply pressure from directly within the valve terminal, or sense up to 44 external pressure sources
Installation systems – Type CTEL and CPI
• Allows inexpensive decentralized installation of valve terminals and inputs to form a centrally mounted fieldbus device
• Plug and work simplicity
• Allows valves to be placed close to actuators, increasing speed and saving energy
Filed Under: Factory automation, Design World articles, Electronics • electrical, I/O modules, Networks • connectivity • fieldbuses, PLCs + PACs