Bob Buerkel, Product Manager
Vacuum and Pressure Sensors, Parker Hannifin Corporation
Many manufacturing processes use vacuum systems for pick and place operations. Adding pressure sensors with Auto Surveillance features into vacuum systems can reduce downtime, increase throughput, and enhance preventive maintenance programs.
Such sensors help reduce the annual cost of replacing vacuum cups by alerting a PLC when the Maximum Degree of Vacuum (MDV) decays. Even though a vacuum cycle may still reach part-on pressure, the peak degree of vacuum tends to decrease over time. Without monitoring, this peak degree will decline to the point of dropping parts or fail to reach part-on pressure.
Rather than wait for the next maintenance cycle, a sensor alert informs operators of a problem before system failure. An alert also lets operators replace only those cups causing leakage rather than change every cup during maintenance.
Getting a grip
A vacuum system includes all components connected to the vacuum generator or vacuum pump. Vacuum generators reach an MDV value based on specific compressed air inlet pressure and flow.
Part-on pressure is the ideal pressure for vacuum cups to lift and hold parts with a vacuum seal. It can be a programmed set point on a pressure sensor (H-1).
Part-off pressure occurs when the vacuum generator is on, yet the pressure is less than the required application pressure, and the cups are off the part. For the vacuum cups to grip a part with a seal, the vacuum pressure increases from part-off to part-on pressure. Pressure sensors typically have a response time of less than 2 ms when delivering inputs to a PLC for part-on signals.
To further improve response times, pressure sensors are available with two independent outputs. Output 1 can be programmed to anticipate part-on pressure. Output 2 (H-2) can be set for actual MDV. When the system pressure reaches part-on pressure, the degree of vacuum continues to increase. Output 1 signals the PLC to start the automation sequence. By the time the pneumatic valves and cylinders react, vacuum pressure will be at or above actual system design level. By anticipating this pressure, the automation process starts sooner, reducing overall cycle time. Vacuum generator manufacturers usually designate an optimum supply pressure and flow rate to create the Maximum Degree of Vacuum. Most systems never achieve this value because of changes in inlet pressure or inlet flow, loose fittings, deteriorated cup seals, or changes in product porosity.
Working with Auto Surveillance Mode
A pressure sensor with Auto Surveillance Mode has two open outputs.
Output 1 (H-1) is used for the part-on signal, prompting machine logic to advance to the next process step. Output 2 is programmed to the Auto Surveillance Output and monitors the Peak Degree of Vacuum (P-1) in every vacuum cycle. Because MDV can change during the life of the vacuum system, setting Output 2 at MDV is not always optimal. In this arrangement, the sensor functions as a failure protection indicator, monitoring the MDV in each automation cycle and alerting the system if the programmed peak degree of vacuum pressure is not reached.
Users can change the values of H-1 and P-1 with push button programming. P-1 can be automatically set to about 80% of Maximum Degree of Vacuum.
A successful vacuum cycle runs like this: the vacuum system is turned on and achieves part-off pressure. The vacuum cup array moves over the product, attaches, reaches part-on pressure, and seals on the product. Output 1 changes state, and the machine advances to the next step in the process.
Vacuum pressure continues to increase beyond the Peak Degree of Vacuum setting (P-1) and approaches the system’s Maximum Degree of Vacuum. Vacuum is then turned off and the cup array detaches from the product. The degree of vacuum returns to a level below part-on pressure.
In a successful cycle, the critical pressure is the Peak Degree of Vacuum (P-1). The Auto Surveillance Mode monitors the process to ensure that the degree of vacuum surpasses part-on pressure and exceeds the programmed Peak Degree of Vacuum.
In an unsuccessful cycle, the process is the same as above except that the vacuum pressure does not increase past P-1. The automation process continues, however, because part-on pressure was obtained. When vacuum is turned off, vacuum pressure decreases below part-on pressure. Because P-1 was not obtained during the vacuum cycle, the second output of the sensor changes to a passing state for 3s.
At this point, the MDV has decreased below the P-1 setting. This signal is a warning that vacuum pressure was compromised. Auto Surveillance Mode alerts the operator that the maximum degree of system vacuum is still above part-on pressure, the part is present, but pressure has decreased to an unsafe level.
If left unchecked, the system will continue to decrease in degree in vacuum, which may lead to dropped parts, increasing scrape rate and machine down time.
Sensors with Auto Surveillance Mode can be set to check degree of vacuum pressure after a specific number of elapsed cycles, such as 100. If P-1 is not obtained in 100 cycles, Output 2 can change to a passing state for 3s. If P-1 is obtained anytime during the 100 cycles, the pressure sensor resets the cycle count and starts monitoring the next 100 cycles. Extended monitoring allows the user to make sure that the failure to obtain P-1 isn’t a fluke and that it is, in fact, a persistent problem.
Filed Under: Factory automation, Data acquisition + DAQ modules, Sensors (position + other)