Closed loop flow control
February 2013
Valves, Actuators & Pump Control
Proportion-Air F-Series flow monitors are the heart of its new real-time flow control system and the electro-pneumatic controls can be electronically reconfigured to accept feedback from its F-Series flow monitors. This combination of components provides a complete flow control package for a wide spectrum of flow ranges. Because the feedback is real-time, the result is a series of flow controllers that respond instantly to change in command or system fluctuations. This fast response allows manufacturers and machine builders to use flow control in applications using high cycle rates. Real-time response allows increased cycle rates and thus higher productivity.
The servo valve and flow transducer are plumbed in a directional valve so that it can control the cap-end or the rod-end of the cylinder’s speed. Speed is adjustable on the fly with a command signal. Speed is visual unless an LVDT is used and velocity is monitored via a PLC: this is not common, but the capability is available. Repeatability is excellent with the flow control package. If the correct speed is achieved, with 5 VDC, for example, one can be assured that speed results with a 5 VDC command will be repeatable.
Ramped speed is easily achieved as well. So if a ramped command is given to a device, it will track that command with a ramped velocity. This is typically done when the user wants to throttle back speed just before reaching the end of the actuator’s stroke. Proximity sensors could be mounted on the actuator to the switch signal sent back to the PLC to start the ramp function.
Force is a concern for most engineers when this system is explained. Of course it is the force that moves the load attached to the cylinder. But force is not what drives speed, flow does that. So, users should pre-set supply pressure for the pressure required to move the heaviest load. The speed control package will manage the flow at the pressure set to ensure velocity is maintained. The best explanation is a heavy pressure vessel lid mounted so as to pivot like a dancer cylinder. The load changes as the lid is lifted and without control, the change in force past centre point causes the cylinder to want to run away. The flow controller helps keep runaway from taking place. If users need to adjust pressure on the fly to adjust force, the system would be able to deal with that as well: just account for the range of pressure.
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