Modern control valves offer communications, diagnostics
An old machine design engineer once sat me down and taught me everything he thought I needed to know about the science of pneumatic directional control valve selection.
While his approach seemed to have been unsophisticated to machine design, there was a certain elegance to it. For most machines, a detailed and exacting approach to the design of a pneumatic control system isn’t going to be the thing that sets your machine apart from the competition, so spend as little time on it as possible. Today, smarter pneumatic designs can reduce compressed air use by nearly half, save space, and save design time.
Modern market expectations require that each aspect of a machine’s design takes advantage of advanced technology, when it is appropriate. There are many advances in modern control valve design, and available best practices allow easier implementation without completing a master’s course in pneumatics.
Consider the following application: a design for a machine that requires 48 solenoid valves to operate a variety of small actuators. The machine is comprised of several cells along a 10-meter total length. The customer requires demonstrated energy savings relative to earlier iterations of this equipment. The customer also requires quick and easy diagnostic capabilities.
The basics of pneumatics
To match a valve to each actuator application, two basic questions need to be answered:
How fast does the actuator need to move?
This question will help in selecting a valve with the correct flow rate. A multitude of free, online tools can help with this calculation.
What needs to happen when power fails?
This question will help select the correct valve function. Each valve below will cause a cylinder to extend and retract in the same fashion, but each will behave differently when power is removed, such as in an emergency-stop or power failure situation. It’s important to select a valve that will provide the right behavior in this situation. For instance, it might make sense for a welding gun to auto retract or for a door actuator to be vented to allow it to be operated manually (see Table 1).
It should be noted that rod locks, pilot-operated check valves, and other devices allow for a wider array of functions.
As the design process moves from the actuators and connected valves to the valve manifolds, and finally to the control system, some architectural questions need to be answered. In the past, it was common to choose a valve family based on the worst-case scenario. The valve with the largest flow requirement would set the precedent for the entire valve manifold. In cases where it was possible to mix valve sizes on the same assembly, bulky, expensive adapter plates were required. Recent advances in valve manifold technology now allow the designer to mix and match valves with a wide variety of functions and flow rates on one manifold, as Figure 1 shows.
A dense constellation of valve functions on one manifold can help achieve a simple, space saving, easy to mount, and cost-effective design. The next task in the design process is to consider certain hidden costs with those benefits.
Two competing cost factors must be considered when selecting how and where to mount pneumatic valves. First, one manifold of 48 valves will always cost less than four manifolds of 12 valves each. Second, long tubing runs between valves and cylinders hide surprising costs. For example, let’s assume that all 48 actuators are 32×100 cylinders that cycle 30 times per minute for two shifts per day.
Traditional scenario: 48 valves are on the same manifold and run 8 mm tubing an average of three meters to cylinders. Per year, $14,270.88 is spent on compressed air.
Decentralized scenario: Four manifolds are installed, each with 12 valves. Since the valves are closer to the cylinders, shorter runs are used (500 mm) of smaller (4 mm) tubing to achieve the same cylinder speed. In this case, only $7,365.60 is spent on compressed air per year.
Each application is unique, but as with any such application, these costs must be considered. In the early days of fieldbus valve manifolds, the financial case for centralized valve manifolds was quite strong. Newer advances in network technology allow multiple manifolds to share one EtherNet/IP network (from ODVA) (or other industrial Ethernet protocol) node for surprisingly low cost. The market has produced a wide variety of these decentralizing technologies. Each has its own capabilities and limitations, but they are an effective tool in driving down the cost of a multiple manifold design like the one described above (See Figure 2.).
Pneumatic energy savings
The vast majority of pneumatic cylinders are applied in situations that require actuation in only one direction. Unfortunately, in many cases, the actuators are plumbed in a way that causes them to consume just as much energy retracting from the work as they do to actually accomplish the work. Referencing the same example as above, but using only one bar to retract the cylinders, air consumption would be further reduced from $7,365.60 to $4,922.88. This isn’t a new idea. For years, sandwich pressure regulators were available at each valve station that could accomplish this, but the additional capital investment wasn’t always clearly justifiable. A better solution supported by many modern valve manifolds is the option of simply supplying pressure to the valve manifold in a different way.
Figure 3 shows two valve/cylinder combinations. On the left is the traditional plumbing, and on the right is the modern alternative. Applying a lower pressure to the appropriate side of the valve reduces the amount of air consumption. In the past this was often impossible, as many valves required that pressure be applied at port 1 to supply the valve’s pilot circuit. In most cases, modern valves receive their pilot supply from a separate galley, adding flexibility to create a more energy efficient design.
The most recent important and transformative change within the industry has been the shift in thinking about machine safety. New machine safety regulations have started to replace rigid prescriptions for operator safety with more nuanced processes that take a broader view of worker safety. New regulations have proven to be effective at enhancing workplace safety and present machine designers with opportunities to integrate safety systems into normal cycle operations in a way that enhances productivity instead of impeding it. A growing number of common applications can now be solved by integrated electro-pneumatic products suitable for use in safety-related parts of control systems to the EN ISO 13849-1 standard (Safety of Machinery — Safety Related Parts of Control Systems — Part 1: General Principles for Design).
An example is the use of a "sensor valve" to switch the pilot supply to a manifold. It was common for legacy systems to completely exhaust pressure from a manifold, or even an entire cell, when an operator had to reach into a fixture to load a part. The time and energy expended to pressurize the cell after each load cycle was detrimental to productivity. A specialized valve/sensor combination can allow the control system to vent only the pilot air from a manifold, preventing the valves from shifting without wasting large amounts of energy. A provision is made to directly monitor the pressure, allowing a dual channel safety solution to be realized.
As machine builders and equipment suppliers become adept at integrating modern safety solutions into production machinery, this segment of control will continue to grow.
Diagnostics for pneumatics
Diagnostic capability is not new within the pneumatic world. High performance valve manifolds have included sophisticated diagnostic capability for many years. When applied in the intended way, a fault within a valve manifold will result in a clear and actionable message on the machine’s human-machine interface (HMI). In some cases, control engineers and programmers went as far as to e-mail or short message service (SMS) trouble codes to the appropriate maintenance resource. This level of integration has been relatively rare. In most cases, people only start digging through diagnostic tools to find the cause after a machine goes down.
With greater use of Ethernet-based protocols, more diagnostic data is available. On-board Web servers produce useable data that is much faster and easier to access. It is also more common for pneumatic equipment manufacturers to provide software for popular control systems that allow a programmer to more quickly acquire, decode, and use the diagnostic information that is presented by the pneumatic valve manifold.
The emergence of the IO-Link device network (from Profibus & Profinet International) is bringing diagnostic capability to applications that were previously served by hardwired manifolds. The unique point-to-point nature of IO-Link, coupled with its wide adoption, has translated into diagnostic capability at even lower price points.
The modern machine designer
Machine designers have come a long way since the days of "pipe size is the right size." Through better design, modern material, on-board intelligence, and communication, today’s machine designer is able to apply safe, efficient pneumatic automation systems that are capable of performance and operational reliability that simply were not possible only a few years ago.
Sean O’Grady is product manager of valve terminals and electronics, Festo. Edited by Emily Guenther, associate content manager, CFE Media, Control Engineering, email@example.com.
- Opportunities for machine designers
- New challenges for machine designers
- Benefits of applying modern control valves.
Do the benefits of modern control valves outweigh the challenges for machine designers?
More about EtherNet/IP is available from ODVA.
ISO provides more about the safety standard EN ISO 13849-1:2015.