Simulating machine, robot and process skid operation in software is the best way to test controller code, device settings and HMI configuration—short of actual connection to the completed system. And in many instances simulation is a more practical alternative to testing on an actual system for a variety of reasons ranging from the ability to prototype to the desire to maintain a safe environment by testing in software first.
When the complete machine, robot, skid or system isn't available for test, and sometimes even when it is, simulation can shorten the design cycle, improve performance and greatly reduce startup time. Table 1 lists some of the specific advantages of simulation.
Accurpress America in Rapid City, South Dakota, makes hydraulic press brakes and shears for fabricating sheet metal (Figure 1). Allen Guernsey, machine control development specialist at Accurpress, says the machines can be complicated. "Our current version of the PLC for the press brakes can handle 28 axes, and we can link machines together to work in tandem," he says. "Sometimes the press brakes and shears are parts of cells, and in these cases other machines arrive at the customer from different factories, so simulation is the only way to run the machines together prior to startup."
Simulation has reduced the R&D time. "Being able to watch machine movement in a simulated environment has enabled better PLC code because, in simulations, more fail safes can be tested without risking machine crashes," explains Guernsey. "For example, I can simulate hydraulic cylinder characteristics, introducing errors like a stuck cylinder. So far, I have not run into anything that I could not simulate."
Accurpress uses Beckhoff Automation controllers and TwinCAT automation software. "With TwinCAT, the simulation of machine control is fundamental to the software’s design," says Guernsey. The built-in simulation saves time and cuts cost because it’s not necessary to buy a separate software simulation package, learn how to use it and integrate with the controller and its software.
Dave Perkon, vice president of advanced technology at AeroSpec, a machine builder in Chandler, Arizona, also uses simulation software from robot suppliers. "Most of the top robot suppliers have simulation software," he says. AeroSpec designs and builds a variety of discrete manufacturing equipment, ranging from lean assembly stations to high-speed robotic assembly and handling systems (Figure 2).
"During the concept and quoting stage, simulation software helps keep a project moving in the right direction and keeps the customer engaged as their requirements come to life," notes Perkon. "If the actual equipment doesn’t meet the simulation results, it indicates the program needs more work."
And, once the system is designed, some simulators create code. "Although there are configuration differences, the end product from many of these simulation packages are directly downloadable software programming to the robot," Perkon explains.
Software Simulation for Safety
Phoenix Contact offers SafeConf safety configuration software that allows hardware simulation direct from the programming software. "When designing automation, it’s important to ensure the functionality of safety systems, most notably on machine startup and validation," says Zachary Stank, product marketing specialist. "Simulation allows you to walk through the startup process step by step before hardware is engaged to ensure all safety logic is functioning properly."
Richard Clark, third-party product support engineer at InduSoft, explains how its customers use HMI software simulation capabilities. "It’s common practice for machine builders using InduSoft Web Studio to create a simulated environment to approximate the actual process or machinery. The advantages of creating a process control or machine simulation is implementation in a safe environment without exposing workers to safety-related issues. It also doesn’t expose the production lines to potential disruptions or contamination while new technology is being introduced."
Challenges and Issues
Although simulation provides many benefits, it’s not without challenges (Table 2). Simulation packages aren’t perfect. "One of the most common challenges I’ve come across with various simulation packages is the simulation shutting down if there are any errors," says Tim Malyszko, director of operations at Malisko Engineering, a system integrator in St. Louis. "In other words, if a simulated device points to a tag that does not exist in the physical or simulated controller, the whole simulation dies." You also have to know your tools. "Simulation software isn’t very intuitive to use and requires a substantial learning curve," he adds.
Simulations aren’t always accurate. "There have been times when our actual robotic equipment did not meet the simulated robot cycle time," notes AeroSpec’s Perkon. "Adjustments to the robot motion profile, and adjusting approach and departure motion profiles, are usually required in the real world. Simulation doesn’t clearly duplicate the real world perfectly."
InduSoft’s Clark agrees. "The main challenges in creating or using simulated environments is accurately modeling and reproducing the operating conditions where the process, machine or robot will be finally integrated."
Bill Savela, marketing director for Delta Computer Systems, sums it up. "A simulator must provide results that are true to the actual hardware," he says. "While simulation can provide valuable information prior to installation, it’s not the real system. Tasks like final tuning, verifying sequence of operation and ringing out wiring are still required."
Simulations are becoming a major part of control system development. Many vendors, such as robot manufacturers and control software suppliers ,are building simulation capabilities into their systems because they realize the value it provides. As Daymon Thompson, TwinCAT product specialist at Beckhoff puts it, "Software and hardware suppliers in the automation industry are aware of the advantages and the power of simulation, and embrace it as core to machine design."
But simulation is not quite a walk in the park. Setting one up can be difficult. And if the simulation isn’t a very close match to a real system, it won’t be very useful.
Despite these difficulties, simulation benefits outweigh the challenges in many applications, particularly those where testing on live equipment isn’t feasible, safe or convenient.