By Josef Karbassi • VP Division Automation • Piab AB
Industries including packaging, electronics, automotive, glass, and plastics are discovering the advantages of this low-cost option for vacuum systems.
Air-driven ejectors are gaining pace as the vacuum source for suction cup handling equipment on robots and special purpose machines, such as case/carton erectors, de-stackers or unloading equipment. The market for air-driven ejectors is flooded with several all-in-one ejectors promising added value through compact, lightweight design, intelligent integrated functions, energy efficiency, and sophisticated diagnostics/monitoring features. For the engineer designing a robot gripper tool, it isn’t easy to select the right ejector package. What is really important to consider when selecting the ejector? Which factors make a big difference in achieving a high degree of reliability (uptime) and productivity from the robot system or machine you are designing?
The important functions provided on an ejector can be summarized in three major categories: Functions to enhance uptime/reliability, functions to improve speed/picks per minute, and functions to reduce energy consumption.
Uptime—Most machine builders and robot integrators would agree that reliability of critical components in a machine/robot has great impact in creating a successful and profitable business. Having critical components repeatedly failing on the machine or robot—and the associated downtime—is a nightmare. An ejector unit is definitely a critical component for suction cup handling systems.
Speed—Besides uptime, machine or robot performance (often measured in picks per minute) is one of the main parameters that the end-user will measure and judge after installation. The performance of a vacuum handling system depends on the total system solution where ejectors, suction cups and accessories are included. However, one can safely say that the ejector plays a crucial role.
Energy consumption—A third area that is beginning to play an important role for end-users is energy usage. Several plants have stringent requirements to reduce energy consumption year by year and that adds pressure on machine builders and robot integrators as well. The new ISO 50001 energy management standard is growing in importance around the world and adds pressure to reduce energy. While ejector technology is an efficient way to create vacuum if used and controlled in the right way, a poor ejector nozzle design and inefficient part release function can easily consume three to four times more energy compared to the best in class models.
Important features
1. Ejector characteristics—The ejector nozzles can be designed for different purposes, such as to provide extra flow, extra deep vacuum levels or a good vacuum performance over a large supply pressure range. From a performance point of view, it can make a big difference in selecting the right characteristic for the application. High altitudes (3000-5000 ft) are another factor that influence performance of an ejector. A special nozzle characteristic is needed to achieve system reliability and performance at these higher altitudes.
2. Ejector efficiency—This is perhaps the most important factor. The speed (cycle time) will be determined by how fast the ejector can generate a safe vacuum level. If handling leaking materials with material variations (say, corrugated cardboard) the flow capacity at deeper vacuum levels is equally important to maintaining a high reliability. Initial flow capacity is critical in high-speed machines to secure a tight, quick grip with a bellows cup. Multistage ejector technology is 30 to 50% more efficient compared to corresponding single stage ejector technology with the same energy consumption. The only trade-off with multistage is a slightly longer ejector module. When using a single stage ejector, one must compensate with a larger ejector nozzle to achieve the same performance. That will cost in energy consumption, noise level, larger valves, and additional heat.
3. Blow-off/release efficiency—It is important to release the object as fast as possible, that it is as reliable as possible (meaning that vacuum in all cups breaks at the same time) and is as efficient as possible. In some applications, where an energy saving function can be used on the ejector, blow-off air typically counts for most of the air consumed in the cycle.
4. Valve construction and response time—For high-speed applications, with cycle times
5. Heat from valves and adaptive PWM—An issue with high-power DC valves is heat generation. Heat will reduce the lifespan of valves and surrounding components. By using PWM (pulse width modulation) on a DC valve, it is possible to use less power when the valve is holding and only increase power when needed to change position for a few msec. PWM can reduce power and heat generation from valves by more than 50%. However, there is a reliability risk with linear PWM technology. If the incoming voltage is very low, the holding voltage could end up too low and the valve can change position in an uncontrolled fashion. This can be avoided with adaptive PWM technology. The incoming voltage is measured and the adaptive PWM makes sure the holding voltage is always on the same level.
6. Allowed supply voltage span—Most valves used on compact style ejectors with integrated controls are specified for 24 VDC power supply with a small voltage tolerance. It is not uncommon that end users have poor control of the loads on the power supply they use. In order to minimize downtime, machine builders and robotic integrators should pay close attention to the allowed supply voltage range when selecting the unit and see if there are other features implemented to allow fluctuations in supply voltage. Adaptive PWM is a method that allows for supply voltage fluctuations.
7. Adequately sized vacuum hose—A common design mistake made by some manufacturers is insufficient sizes on the vacuum ports. A port that is too small means small diameters on the vacuum hoses. This cannot be compensated for with a larger ejector. An easy test is to start the vacuum ejector with all cups open; the vacuum gauge or digital vacuum meter should not show more than 3 in.-Hg for an open system. If so, there are major restrictions affecting performance.
8. Potential to place the ejector close to point of suction—Vacuum closer to the point of suction will always be the most efficient, fastest and most reliable solution. All-in-one ejectors are quite often large in size and relatively heavy. In most situations, they are centrally placed and serve several suction cups. Transportation of vacuum in long hoses will create flow losses and volumes to be evacuated. That has to be compensated by a larger ejector. All-in-one ejectors are available where the control section and ejector section can be divided so that the ejector can be placed directly at or very close to the suction cups. This will improve picks per minute and picking reliability.
9. Air saving function with adjustable hysteresis—An effective feature offered by many manufacturers is an automatic air saving function. This can save a lot of air/energy in suction cup systems for sealed materials, but can also be used when handling materials that tend to leak slightly. The ejector will stop when a set vacuum level is reached and the vacuum level is monitored. The ejector restarts if the vacuum level drops below the low-end of hysteresis (restart level). Micro leakages exist in all suction cup systems and leakage will increase if a suction cup is worn out. More leakage will make the ejector unit stop and restart frequently, speeding up wear on valves and other internal parts. This can also cause an annoying sound when the ejector goes on and off frequently. More sophisticated ejector units have an adjustable hysteresis for the air saving function. By increasing the hysteresis, the frequency of on/off can be reduced, protecting the valves. Highly intelligent units are able to determine the size of the leakage and can automatically turn off the air-saving function in case of a large amount of leakage.
10. Graphical User Interface and settings—All-in-one ejectors have a lot of functionality and features integrated into their design. Quite often, it is required that the customer understands how to make settings on the unit in order to obtain the desired functionality. A mistake in the settings can result in severe consequences. A common example is when users set the air-saving levels below part-present signal level to the robot/machine. That will likely cause dropped parts and increased downtime.
The advantages of air-driven ejectors over traditional blowers and rotary vane vacuum pumps are striking. Properly specifying and using these components will result in higher reliability, lower investment cost, reduced cost for energy and ownership, easier control and installation, and reduced size and weight.
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