In most industrial plants, these types of flow controls are typically done by a PLC. These same PLC’s usually take in all of the inputs of the process and compute all of the outputs to all of the devices in the process. The loads on these PLC’s can vary greatly and typically result in a long scan times that can adversely affect the PLC’s ability to provide the best PID control loop. This makes the control of the loop inaccurate, slow, and creates difficulty when trying to tune the loop to obtain the highest degree of control.
The best way to get around these issues is to use a dedicated local controller that provides the PID control loop for the Flow Rate Control. These types of dedicated local controllers can be integral to the equipment that is required for the task, having one or more microprocessors. The advantage of using a PID Controller that is part of the equipment is that it will be dedicated to only that task, using the knowledge and expertise of the manufacturer to obtain the optimal control parameters and PID tuning, thus optimizing flow control.
One great example of this type of device is the CentriFeeder with Integrated Control Valve, or ICV. This device uses the principles of measurement of the CentriFlow Meter, and combines it with the robust design of the ICV to provide both accurate Flow Rate Measurement and accurate Flow Rate Control. The ICV is a slide gate style control valve, which uses an AC, low rpm, brushless servo and an integral linear position sensor, which provides feedback for the blade movement and location or position to within 1 part in 1000. It is mounted directly to the measurement unit, providing a single, ready to install, Feeder unit.
The Feeder itself is much more accurate than most on the market, but it is the dedicated PID control loop within its electronics that make the system whole. The CentriFeeder Electronics is the heart of the Feeders capabilities, and uses integral microprocessors to compute the loop output based on the Flow Rate Measurement of the unit and the position location of the blade of the ICV. The customer is able to interact with the system through a color touchscreen HMI, The customer’s PLC can send the Set Point (Setpoint) to the unit over a 4-20mA signal or via a communications protocol like Ethernet/IP, ProfiBus, or DeviceNet. Additionally Flow Rate, Totalized Weight, Valve Position, Percent Open, Start and Stop, and a host of other values are available over the communications protocol. Other features like a Plot Screen for observation and tuning, Virtual Flow Stop, and Auto Delay make the system user friendly and product friendly.
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