When a robot slammed into a slide-mounted weld gun at Citation Global Manufacturing a few years ago, the now-defunct company’s mechanical engineer stared at the two devices for a few seconds, and said, “Well, that didn’t happen on paper.”The weld gun hadn’t completely returned to its home position before the robot tried to occupy the workspace, and so the robot didn’t have enough clearance to avoid a collision. Citation’s staff fixed their machine on the plant floor, but the real-world accident and resulting downtime could have been avoided if the weld gun and robot had been running earlier in a 3-D simulation.
“A good simulation and validation software could have flushed out this problem ahead of time, and determined if the right switch was in the logic, or if a cycle time needed to be increased,” says Mark Taylor, Global’s controls engineering manager at the time. “However, if all these parameters, including switches, wiring, and cycle times, aren’t accurately entered into a simulation, then users can have problems and downtime. Still, users need to know that they can use simulation to design problems out of their processes by using dynamic data flows that make their simulations as real time as possible.”
So, just when does a picture become real? At what point does a simulation become an operation? Logically, like judging people by their actions, an image might best be defined as real when it can do what real objects do. When its presence is felt and it can be used to act on its surroundings, it can claim membership in the real world.
Pictures are real enough, of course, but a cave painting can’t hunt an animal, a blueprint isn’t a roof over your head, and the coolest, most-detailed, highest-resolution CAD/CAM drawing can’t independently address an individual machine’s unique quirks and environmental wrinkles, just as a simulation can’t operate that machine’s overall application. Or can it?
Simulation is pushing ever further into the real world, and several software-based design tools have grown sophisticated enough to not only mimic very complex devices, but also incorporate unique test, PLC, and other data from individual applications before building the machines they need. Pre-configuration and pre-testing are being joined by “pre-operation.”
In fact, CAD has long had the ability to add application-specific data, but the level of programming required grows and quickly becomes prohibitive, as physical forces such as weight and friction are simulated. However, simulation is pulled into reality anyway. As it performs more prototyping duties, more bugs can be worked out ahead of time and more can be saved on modifications that otherwise would have to be done during or after construction. As usual, the trick is simplifying the programming, while maintaining enough computing power to achieve useful simulations.
For instance, when designing its new General Surface Mount (GSM1) pick-and-place machine for printed circuit boards, Universal Instruments in Binghamton, N.Y., needed to improve the assembly system’s head and Z-axis, and design mechanical elements that would support that head’s acceleration and deceleration rates, including a 10-ms settling time. Universal’s motion control engineer, Jim York, says his company used simulation software to wire together pre-defined function blocks onscreen, and model properties specific to GSM1’s servo drive, such as position, velocity, and current control, as well as all parameters affecting cable tension, including acceleration and deceleration rates, load mass, spring rate, and friction (See Figure 1).
FIGURE 1: VIRTUAL OBJECT WIRING
Universal Instruments used simulation software to reproduce real-world current, load distance, load torque, load velocity, and cable profiles of its GSM1 pick-and-place machine before construction.
“Component operating parameters were entered directly to the appropriate blocks through pop-up dialog boxes,” says York. “During simulation, we viewed the dynamics of the cable’s tension in plots and real-time graphs. As we entered known values for acceleration, deceleration, and mass, their effect on the tension could be monitored immediately. This allowed us to adjust the spring rate so the tension was positive at all times.
“Simulation allowed us to design GSM1 much faster than if we had assembled a breadboard and performed physical testing. Also, the GSM1 model provided a high accuracy, allowing us to examine signals that would have been too difficult to monitor in a breadboard. Because we could view the entire dynamic picture of the mechanical load, we designed the components to properly support the acceleration and deceleration rates of the vertical axis, ensuring tight control of the load while it moved up and down. And, by validating the design through simulation, we could identify the correct components before building the prototype, significantly shortening the design cycle.”