Rate by weight is the measurement of a material that enters into or dispenses from a vessel over a given period. In a continuous feed, a loss-in-weight closed loop feeder controller works in conjunction with a screw- or auger-based feeder, vibratory feeder, conveyor, belt feeder, variable valves, or a pump to precisely measure dry bulk solids (or liquids using a pump).
New intelligent closed loop feeder controllers are delivering unprecedented accuracy in manufacturing operations where tiny amounts of material can make a huge difference to the finished product. For example, many plastic extruders use a feeder controller to mix in small amounts of color. Too much or too little color variation may be cause for quality control to reject finished goods so precise control is required. Intelligent controllers are predictive and learn from past material feeds, determining the quantity of material in motion or when material compacts or there are voids. An intelligent controller retrofitted to an existing feeder can make it more accurate or add features. A standalone intelligent controller is typically a fraction of the price of purchasing a new feeder system.
Rate Controller Overview
To determine a gravimetric mass flow, divide variable weight by a time constant (ex. lb/min). This loss-in-weight flow rate is controlled using closed loop PID (proportional, integral, & derivative) control. In essence, the controller continually adjusts the feeder based on a flow setpoint, using an analog output for speed control. The PID controller calculates an output percentage based on the parameters P - proportional, and I – integral (some controllers do not use a D - derivative coefficient). The rate calibration process can set the values for P and I automatically. D (derivative) can be set to 0. The output percent is determined by the formula:
P*[(Setpoint - ActualRate)/MaxRate + I * (integral of the proportional term)]
The feeder can be automatically refilled (gain-in-weight) while simultaneously delivering the desired amount of product at the desired rate (loss-in-weight).
While monitoring the quantity of material dispensed, the controller saves the data internally or delivers the data to PLCs, PACs, PCs, or DCSs help meet inventory or statistical requirements, over a variety of communications networks, such as EtherNet I/P, Modbus, Profibus, ControlNet, and so on. This makes integration with popular manufacturing control systems such as Allen Bradley seamless.
Adding Intelligence to Rate Control
An intelligent loss-in-weight rate controller may have many features not found in a turnkey system. A few of these features may increase the efficiency and lower the total cost of operations of a feeding system.
Access to Weight Data: An intelligent rate controller can automatically send weight data to a control system such as a PLC or PAC, making that data an integral part of the manufacturing process.
Built-in Web Browser: An intelligent rate controller with a built-in web server makes it easy to control the system anywhere, anytime on the manufacturing network, even from a remote location using the IP address of the system. This makes it easy to perform system diagnostics, monitor the system, or to change configuration.
Rate Exception Control: Almost all feeders vibrate during operation. An intelligent rate controller can not only ignore normal machine vibration, they can recover from an accidental knock or “exception” event that is outside of normal operation by temporarily going into an open loop state and returning into closed loop status almost immediately. Because the intelligent controller retains the weight data for in motion material during these events, it can account for the event in the feed and retain operational accuracy. These events can be as simple as unexpected material shifts when a void collapses or someone accidentally placing a toolbox on the feeder.
Automatic Feeder Refill: In open loop control, the control action is independent of the "process output" (or "controlled process variable"). In practical terms, it automatically applies accurate and responsive correction to a control function such as refills. This allows for feeder automatic refills (gain-in-weight) while simultaneously delivering the desired amount of product at the desired rate (loss-in-weight). In other words, the feeder does not have to stop while being refilled, reducing downtime and reducing the cost of operation.
Alarms: The rate controller sets off an alarm or shuts down the process if the limits exceed or are under set points for rate tolerances or weight limits. Timers in the instruments help determine how long the system can run in an “out of tolerance” condition before an alarm or shutdown occurs. This prevents accidental shutdown or triggering of the alarms due to momentary spiking or noise on the signal.
Electronic Calibration without Test Weights: Calibrating a load cell or system of load cells in a scale, informs the instrument what output to expect at two or more points on the “unique” curve of the load cell or load cell system, so it can accurately report the weight applied across the entire capacity range of the scale. However, traditional methods of calibration involve bringing the feeder down and using test weights to calibrate the scales.
True two-point, three-point, and five-point automatic rate calibration produces high feed accuracy without bringing down the feeder to calibrate the load cells or scales with test weights. Instead, the rate controller stores electronic calibration curves for each load cell in the system set at the factory. These calibration curves allow the system to accurately calibrate the load cells or scale electronically and use only small verification weights.
Easy to Add Recipes: Intelligent rate controllers can store hundreds of different system configurations for different batches or continuous feeds. The operator configures each ingredient with a name, rate setpoint and other parameters to deliver the exact amount of material for the specific process, whether working in batch or continuous mode. You can configure and store more than 100 ingredient names from either the front panel or the web interface. Some “black-box” controllers require a technician every time the plant needs to add new materials or recipes. Intelligent controllers make it easy to add new recipes and ingredients using simple menu controls or web interfaces.
In addition, mapping allows the setup of certain monitoring and control activities to meet process requirements without programming. Whether the input is digital weight data sent to a display or a signal used to trigger a command to a PLC, mapping is often the easiest way to achieve the desired result. Mapping is more flexible than the typical I/O addressing used in a PLC. If the controller has redefined I/O default mapping, the operator can use these or set up custom mappings to meet the unique requirements of a particular application.
Manual Mode: When the operator needs to take over, a convenient manual mode allows manual speed control, with switch to automatic control at the touch of a button.