By Mike Bacidore, Managing Editor
We have 10 different pneumatic devices on our machine, all controlled individually via PLC outputs and all located in close proximity to each other. We would like to reduce wiring by using a remote device that would communicate digitally to our PLC and that would control all of the devices via a bank of actuators mounted on a manifold. What factors do we need to consider in our search for the right remote actuator bank?
—from September ’08 Control Design
Answers
Do Your Homework
First, you must know if there are any critical cycle times between the motion of these actuators and other parts of the machine. A digital network will introduce a latency that might not have existed with the direct I/O. It can be 10-50 msec or more and depends on the number of devices on the network, the type of network and the amount of data being handled. If this is a concern, you have to do some homework on the available networks.
Your choice of controller usually will dictate a narrow scope of networks that it supports. You will likely want to choose the most cost-effective option that will work with your application. Things to consider include physical media costs, configuration costs and hardware costs.
Finally, look for items that bring down total cost of ownership. It likely would bring costs down by having the inputs of the cylinders return directly to the valve bank. Installation and commissioning should be improved. Also, look for diagnostic capabilities that reduce commissioning time and an IP rating that might reduce installation costs by removing or reducing a cabinet size.
Frank Latino, Product Manager,
Valve Terminals and Electronics
Festo
Many Factors in Remote Valve Bank Choice
Is cost an issue? You might want to reconsider making this change for only 10 outputs. Compare the cost of the current system with PLC and output cards vs. the cost of a PLC with scanner card and remote I/O devices. Taking into account only the cost of the hardware, making the change from discrete wiring to digital communication is more cost-effective for larger applications. To truly evaluate the full cost, you also should consider other factors. What is the distance from the PLC to the pneumatic devices on the machine? Running one digital cable over long distances is more cost-effective than running several individual wires to each output.
Do you need a highly skilled electrician to wire your machine? The existing solution means wiring each individual output back to the PLC, correctly marking each wire, ensuring no cross-wiring. This can take a highly skilled electrician several hours, if not days or weeks, to complete. Digitally controlled valves, on the other hand, are typically plug-in valves connected to fieldbus nodes using common connectors. Connecting these valve banks requires a lesser-skilled employee to simply connect a cable to a connector. Changing or installing the plug-in valves requires loosening a couple of screws. The valves make the electrical and pneumatic connection at the same time; therefore, a machine builder can improve the ease of installation, serviceability, and interchangeability with lower-skilled employees.
There are other bells and whistles you might consider that are available through digital communication. Are there any additional I/O needed? Are there devices other than pneumatic valves that might need discrete wiring? A single PLC scanner card can control thousands of I/O through fieldbus communication. Comparing this to PLC I/O cards, a builder would have to purchase multiple PLCs and I/O cards to use the same number of I/O that can be controlled via digital communication from only one fieldbus scanner card. Another often integrated feature of digital communication is feedback. Most fieldbus products on the market today have integrated diagnostic functions. These diagnostics allow a signal to be sent back to the PLC should an error occur. A few manufacturers of pneumatic valves have built-in diagnostics that can indicate the exact valve output that has failed—a feature that greatly reduces machine downtime.
If you decide to switch to digital communication, the first consideration is which communication protocol to use. This many times can be narrowed down by the type of PLC you use. Each PLC manufacturer has specific protocols that it promotes with its brand of PLC. That doesn’t mean you have to choose the PLC manufacturer's protocols, since many third-party suppliers offer interface cards for various protocols for several brands of PLCs. Do you want to simply drive a few outputs with one scanner card or want to add in higher-level features such as factory-level monitoring/networking or motion control? Some protocols such as AS-Interface are designed to handle basic bit-level turning on/off of outputs and reading inputs. These fieldbus protocols are typically easier to install and commission, while protocols such as Profibus are designed to handle factory-level monitoring, motion control and other high-end functions. These higher-level fieldbus protocols require more setup during installation and commission.
The final things to consider are the voltage and the actual characteristics needed for the valve bank itself. What voltage is on the machine? Keep in mind that fieldbus solutions are typically 24 Vdc. If you are using 110 Vac and are unable to make the switch to 24 Vdc, then relays with additional wiring will be required.
What are the sizes of the pneumatic devices you are controlling, and how much air flow is needed to control them? What is the tube or connection size needed?
Last, are there environmental factors such as IP ratings, temperature, vibration or air quality to consider? Some pneumatic manufacturers offer online tools to help size and specify custom valve banks to meet the application’s need. A customer can use these tools to input the specifications of the pneumatic devices. These tools will then tell them the flow needed for the valve bank and the tube size needed for the application.
Now you’re ready to configure the valve bank needed. Some pneumatic manufacturers offer online configurators that allow the customer to configure the exact valve bank they need, along with any fittings or accessories, and get a part number for the completely assembled and tested manifold.
Marchelle Forish, Senior Product Specialist,
Bosch Rexroth
Yes, You CAN
The best approach is to have the devices linked together via RS485 or CAN bus, subject to speed and response time, using a single, 4-conductor cable. Up to 128 devices can be linked together where a single master will act as the controller. The pneumatic devices and actuators will be slaves.
Karmjit Sidhu, Vice President
Business Development
American Sensor Technologies
Ask Questions to Find the Answer
To reduce wiring when controlling output devices such as manifold-mounted pneumatic valves via a PLC, it is accepted industry practice to employ a wiring scheme where the electrical information, i.e., which valve to turn on, is communicated serially over a single two-conductor cable from the PLC to the valve manifold.
Several components—PLC with a scanner/bridge and pneumatic valve manifold with a network node—are needed to implement a solution like this. The network node connects to the pneumatic valve manifold. The node interprets the information sent from the PLC via the scanner, so it can be used to control the individual valves.
If the only device controlled by the PLC is the pneumatic valve manifold, this will be an expensive solution. However, as the number of devices increases, this solution becomes more attractive.
The choice of protocol can depend on many variables, such as customer preference, PLC manufacturer, number of nodes, cycle time and distance from the PLC to the remote device.
Are diagnostics required? Diagnostics aid in troubleshooting and startup and can be a great benefit during normal operation. If diagnostics are required, are the customer's requests addressed?
During product selection, consider several factors, such as correct valve flow, response time, appropriate IP rating for the application, whether the complete package mees the IP rating required by the application or customer, and if the complete package meet any size constraints dictated by the application?
Joerg Hohler
Product Sales Manager
SMC of America
