Motion controller can rejuvenate your machine
Your hydraulic machine’s productivity is falling, your downtime and maintenance costs are increasing, and your machine has the shakes. What do you do? You could buy a new machine, or you could upgrade your PLC for more performance and redo your control software, but there’s an alternative that you should look at: Consider adding an electro-hydraulic motion controller. (Updated Dec. 31 with more photos and motion-control diagram.)
One of our customers, a chemical company, had a machine that compresses rubber crumbs into bales. The machine had a cover weighing several hundred pounds that would close over the bale during every compression stroke. To keep machine throughput high, the cover needed to close at a high rate of speed—between 20 and 25 inches per second. Originally, the hydraulic cylinder that opened and closed the cover was operated by the PLC using an on/off hydraulic valve. To avoid the hydraulic shock that would occur every time the cover of the machine would close the original machine designers installed cylinder cushions. When it became obvious that the cushions were wearing out quickly, causing maintenance problems, we added a proportional valve controlled by an analog amplifier to enable smoothing out of the motion.
When better precision and smoother accelerations and decelerations were required, it made sense to replace the amplifier with a programmable motion controller that could interface directly with the proportional valve. Even though the cover could close and open just as quickly as before, the smooth motion profiles cured the maintenance problems and extended the life of the machine, while achieving the level of precise positioning that was required.
Offload the PLC
As an alternative in cases where more precise closed-loop control is required, why not just add a PLC or upgrade the one that you have, and modify your application software to do a better job of controlling the motion? To do precise motion control, a PLC would need to be able to execute the appropriate mathematical algorithms during its scan time, along with other functions that it is responsible for managing. It is hard for PLCs to do motion control, however, because they typically do not have instruction sets that are tailored for this function, and machine scan times can vary from scan to scan, making it difficult to deal precisely with feedback information.
For example, one machine used hydraulics to close the die on an injection molding machine that makes 20-ounce plastic cups. The machine used a control output from a PLC to control a hydraulic valve, but the PLC didn’t have the computing power to read a position transducer on the fly and change the valve setting to vary the rate at which the mold was closing, all within the PLC’s scan time. As a result, the mold, which makes 100 plastic cups at a time and weighs more than 100 pounds, would close with a bang, risking damage to parts of the machine and causing an imprecise sealing of the mold cavity and sometimes resulting in the production of poor-quality cups that had to be scrapped. To solve the problem, an electro-hydraulic motion controller was added to the machine and the PLC program changed to produce two commands to the motion system: an output that says “close die” and one that says “open die.” The motion controller was programmed to execute a profile that produces the appropriate acceleration and deceleration of the motion.
The motion controller, designed for high-performance calculation of motion algorithms, was an RMC controller from Delta Computer Systems of Battle Ground, WA. It has an instruction set tailored for closed-loop control of proportional servo valves, including the ability to use special gain terms in the control-loop equations called velocity and acceleration feedforwards that help optimize performance.
After we put the Delta motion controller in the system, keeping the old PLC, we were able to increase cycle rates and decrease shock. The large die sequence can now open and close smoothly in 1/3 second. This couldn’t have been done with the existing PLC. Since the motion controller has the performance to produce complex motion profiles, a simpler, lower-performance PLC can be used in many applications that might have otherwise required a larger, more expensive, high-performance PLC in the past. The motion controller can control position and force if the proper transducers are supplied.
Sometimes the performance of a machine is limited by the need to coordinate the motion of multiple motion axes. The machine’s output could be increased if the moving parts could move faster, but the old control system might not allow that. In another application, an old plywood veneer lathe included arms that held the ends of the log section (called a “block” in sawmill-speak), which needed to move in lockstep as the block was moved into place for peeling. Any lapse in coordinated control would allow the block to fall. With this in mind, the mill operator still wanted the machine to do its job more quickly.
Adding a controller to the lathe resolved the problem. Since Delta controllers can coordinate the motion of multiple motion axes simultaneously, the controller could move both arms of the machine, controlled as two discrete hydraulic axes, in synch without dropping the work piece. Additionally, through a process called gearing, the machine speed can change without losing the relationship between the axes in motion. This feature can also be very helpful for troubleshooting or responding to different production requirements or variability in raw materials.
Another sawmill application was a log kicker that needed to operate at higher speed. The kicker’s arms are rotated to push logs that have been de-barked and cut to length into the sawmill to be cut to the desired widths. One log is transported into the mill for each half-turn of the arms. The machine was previously controlled by an old PLC to move the arms at a uniform speed, but the mill’s operators saw that they could get higher throughput if they could swing the arms more quickly into position while they were not in contact with logs. Integrating a controller into the existing system added smooth accelerations and decelerations to the motion steps, and decreased cycle times, without changing the PLC or its software. This was possible because the RMC could communicate easily with the PLC—in this case using commands sent over an RS-232 serial interface.
Motion software support
Ease of use helps speed implementation. Some motion controllers require tedious programming with low-level machine instructions that make debugging and tuning of designs difficult. Other motion controllers are designed to be easy to program with high-level motion commands. So machine builders should examine the motion specs of a new controller and level of design support provided by the manufacturer. RMCTools by Delta Computer Systems includes helpful programs, such as the Curve Tool, that make motion profile development almost as easy as “point and click.”
A Tuning Wizard shortens the time to optimize motion controller systems. Delta’s tuning wizard automatically sets the motion controller’s position control loop gains according to settings input using the graphical interface.
As suggested above, adding a motion controller can improve controls of just the critical part of a machine. For this purpose, controllers are available that control a range of numbers of axes—from just one or two, up to eight axes or more.
If you’re plugging in a new motion controller, it makes sense to network it to the PLC. Ethernet network interfaces, such as EtherNet/IP and Profinet, are less expensive to install and maintain than old point-to-point wiring techniques and can enable new capabilities.
Gain motion controller programming experience using a motion simulator before trying the control program on the actual machine. Some motion controllers are supported with a built-in simulator that provides the ability to test and debug programs at a desk before you do a site test. The Delta controllers’ network interfaces can let you do remote monitoring and troubleshooting from your desk after the motion controller is installed. In the case where PLC replacement is needed, it may be more cost-effective to downsize PLC features and let the motion controller take a lot of what the PLC used to do. Several recently introduced low-cost PLCs are very effective for functions such as supervisory control, discrete actions, and alarming.
So if your old machine is starting to show signs of age, or it’s not keeping up with the newer machines available, consider adding new life with motion controller retrofit.
Perry D. Yoder is president of Oilquip Inc., a fluid power distributor and system integrator.