In Dispensing, rate control of a raw material or finished product takes place multiple times during a production run.
In Batch operations, multiple feeds of different raw materials and quantities take place in each batch that is completed during a production run. Feeding these products simultaneously into a common hopper or vessel with the same feed duration could act as a mixing operation.
What is Material Feed Rate Management?
Managing the feed usually includes at least six stages in a material feed:
1) Ensuring the right conditions are met to start the feed
2) Starting the feed
3) Measuring and comparing the Rate of Change to the rate set-point during the feed
4) Controlling the feeder speed to maintain the rate set-point
5) Automatically refilling when the refill set-point is reached
6) Reporting feed data (target rates, gross weight, feed total weight, etc.)
There are several challenges that need to be considered to successfully manage a material feed, and in the real world process conditions don’t stay constant from one feed product to the next. Flow rates vary due to levels in vessels, material consistency, flow, environmental conditions, characteristics of materials, and pump or feeder conditions. Secondly, communication timing between a distributed field instrument and a PLC or DCS is usually not very deterministic, so the time between one read cycle and the next read cycle can vary for many reasons. Variations in the length of time it takes to send, update or collect data from buffers, and the number of programming loops that have to be processed in one program cycle can change from one cycle to the next. There are timing problems whether data is being used to compare real time weight against a rate set point, or whether an output is being used to control a valve, motor, conveyor or vibratory feeder. Lastly, during Loss-In-Weight feeds there are refill cycles to consider. Maintaining material totals and the target flow rate during refill is a must.
One thing to note: the higher the feed rate, the more likely a process change will affect the outcome of the feed result. It doesn’t have to be much. A percent in feed rate or a few milliseconds communi-cations delay, and an error will be introduced if the feed control is not set up to take care of it.
Measurement and Control Techniques
Stabilize the flow rate – using this technique is usually only possible in liquid or slurry applications. Such as in color kitchens for example. Color Kitchens are used to mix up batches of different dyes for fabrics. In this case, each of the raw material vessels are pressurized to a constant pressure to negate the effect of gravity and maintain a constant flow rate no matter what the level (height) of the liquid is in the vessel. The controller is sent a rate set-point and after that as long as the pressure remains constant it never has to change. This is a costly solution though, and only applies to some materials.
Move the set-point comparison as close to the process and measured weight as possible – a deterministic set point comparator (speed and repeatability) plays a big part in controlling a successful material feed result. Moving the rate set-point comparison out of the PLC or DCS into a dedicated scale instrument enables the opportunity for a more deterministic comparison that is closer to the process.
Use historical data from the current feed rate and control the flow rate close to the rate set-point—this is called “adaptive multi-speed feed control.” Until recent times this has been the most common method of addressing problems. The process flow rate is constrained by gravimetric force, so that any overall change in flow rate is reduced to a minute amount, thus reducing potential error in the feed. The set point rate values are sent to the controller and then the rate of flow is adjusted using the differential from the current rate to adjust itself to the best value. The problem here has always been the cost to install extra vessels, isolating the system on a scale, valves and/or pump speed controllers to maintain control. Secondly, there is a cost to manufacturing operations as well because during product run controlling the feed rate affects and lengthens the batch cycle time. This increases operational line efficiencies on every shift, every day, every week, every quarter and every year. It all adds up.
Adapt for the flow variation in real time during every feed -- this method is called “rate control.” First, an algorithm is used to capture the current Rate-of-Change (ROC), the value based on the current flow, and a second algorithm is used to predict the change needed to the current feed based on real time flow rate changes monitored during the feed.
Move the field instrument into the PLC chassis – this technique takes care of simplifying or shortening the “communication chain” that the data has to travel to be sent to or read from the PLC or DCS. This technique increases determinism and helps to simplify integration time.
Use the field instrument for total control and share data. I/O and control with the PLC using communications.
Use a controller that has self-calculating set-point outputs – this technique offers the fastest method for delivering the control signal to the actual motor or valve, PLC or DSC input. Again increasing determinism and simplifying integration time.
Providing a unique opportunity to share resources and automate your system to any level required. Use extra I/O already in the PLC or DCS backplane.
Use a controller that has built-in automatic or manual refill cycle management – this technique offers the best method of managing this problem at the local controller or PLC plug in module level.
The more demanding your application is, the more likely you need to use multiple techniques to optimize the material feed management operation/results.
Hardy offers instruments that deploy various combinations of the above mentioned techniques. These instruments when coupled with Hardy’s load point solutions offer differing mix of performance, capability, and cost.
Dispensing/Loss-of-Weight: Batch or Continuous
In Loss-In-Weight (LIW) operations the scale is installed on the vessel or container the material is being moved from. LIW feeds are used in Dispensing and Batch processes. Supply bins and feeders in loss-of-weight applications are supported by load points to weigh the material being dispensed into receiving containers or flows. Feeders are included on that load point or platform scales to calculate control for material as it is dispensed. Hardy sends control levels to variable discharge gates or motor controllers to control the rate-of-flow of material from the feeder while calculating the rate of loss for the ingredient. This rate is a range of 0-100% established during the rate calibration setup. The Hardy controller performs automatic refills for the supply bin or feeder when programmed weights are reached. It can also start auxiliary operations, supply alarm, status, shutdown indicators, and has a flexible configuration to fit many applications.
Instrumentation and Control:
Hardy’s instrumentation for controlled dispensing includes weight and rate controllers. They can be standalone systems, connected to host computers, or programmable logic controllers (PLCs). Total net weight can be tracked with Hardy’s totalizer feature, which keeps track of the total amount of net weight dispensed. Bar graphs give a visualization of the amount being dispensed or totalized.
High resolution, fast update rates, plus the adaptive capabilities offered in Hardy’s controllers allow for precise rate control required in differing processes. Example: if one hundred pounds of flour is to be added to the process at a certain rate, the scale needs to be able to be read in tenth of pound increments. The scale weight feedback allows the controller to calculate the ROC and control the rate of material entering the bag. Hardy controllers also have the ability to adapt for material bulk density changes. The flexible configurations allow you to match the controller to the application.
Filling is increased using the motor speed control capabilities of Hardy’s controllers. The rate is controlled using PID technology and continuous monitoring of the rate to insure the material fed is meeting the require flow rate. The ability to vary the rate as a ratio of another flow allows the process speed to vary as required.
Various alarms are monitored continuously and can be set to notify operators. For example, if the controller energizes the feeder and no rate of change is detected within a preset amount of time an alarm is set. If the feeder supply level shows a low amount of material, a refill is requested. If that refill fails and reaches a low-low weight in the hopper, the feeder stops and an alarm will signal. There is capability to notify upstream and downstream processes that will be affected with the stoppage of the feeder. Alarms are also set to notify the operator if the material falls over or under pre-defined rate tolerance ranges. The filling process can be configured to stop when an alarm goes off, continue at a preset rate, or continue as is. If a disturbance to the scale is detected a limit is set to the amount of correction is applied. This setting also allows for correction for product bulk density changes.
Hardy offers a broad line of process weighing instrumentation, sensors and scales for use in a variety of applications including mixing/blending, filling/dispensing, check weighing, rate control/monitoring, level by weight, and force measurement.
Industries such as Food & Beverage, Consumer Packaged Goods, Chemical & Petrochemical, Life Sciences, Mining & Minerals, Metals, and Building Materials all rely on Hardy products and industrial weighing expertise to provide timely, accurate data and control.
To find out more about our solutions or to ask for a quote, please contact Hardy at 800-821-5831 or visit www.hardysolutions.com.