Instructions for continuous dosing
General tasks of continuous dosing
Continuous weighing is being used in more and more areas of industry. The task of a continuous scale is to weigh products and feed them continuously into ongoing production. The weighing unit can be operated in different ways, be it in the form of force measurement, belt scales, bulk flow meters or differential dosing scales. These operating modes are compatible with the OPDENHOFF and STANDARD system solution possible.
The loss-in-weight feeder usually consists of a weighing hopper with load cell, a dosing unit with associated control - e.g. frequency converter - a storage hopper and a suitable control unit. The product flow is determined internally by the weight loss per time. Volumetric and gravimetric operation is an important component for controlling and regulating the product flow.
Volumetric operation:
Responsible for fast dosing into the setpoint range.
Gravimetric operation:
Responsible for precise adjustment and keeping the product flow constant.
The requirements for such a weighing principle are the...
... precise recording of the product weight
... Determining the flow rate of the dosing unit
... the fastest and most accurate possible adjustment of the product flow to the desired setpoint
Before the dosing process, a device characteristic curve for gravimetric operation is created from five points for control purposes. Five control values are set manually in steps from 0 to 100 % and the resulting product flow is noted in the commissioning program. The STANDARD system solution uses these five pairs of values to create a control value/flow characteristic curve. If a setpoint is entered and dosing is started, the STANDARD system solution uses the characteristic curve to calculate the approximate actuating value and passes this on to the loss-in-weight feeder. As a result, the loss-in-weight feeder doses quickly to the setpoint. As the device characteristic curve cannot 100% exactly reproduce the dosing characteristics, a small dosing difference will remain.
To compensate for this permanent difference, the system switches to gravimetric operation in an adjustable limit range around the setpoint. In this mode, a PID controller takes over the precise control. The controller records the current flow rate and compares it with the setpoint. If a difference occurs, the PID controller counteracts the difference. If the weighing container is almost empty, it is filled from a storage container. During this time, no weight measurement and therefore no product flow determination is possible due to the falling product. At the same time, the last set value reached is output so that the product flow remains approximately constant. After the settling time of the load cells, the system switches back to gravimetric operation.
Increasing the accuracy of the device characteristic curve: Record material characteristic curve
- Changes to the conveying properties
- Product-specific determination of the material characteristic curve
- When the weighing hopper is full: output of an average output value
- Recording the product flow at 90, 70, 50, 30 % of the nominal weight
→ Program calculates an adjustment factor for the device characteristic curve depending on the fill level.
Dhe illustration shows a non-linear dosing characteristic. Little product flow is achieved with small actuating values. However, above a certain speed (control value), the dosing unit achieves larger product flows. The STANDARD system solution offers the option of creating a device characteristic curve from 5 points. Take 5 points from 0 to 100 % control value and connect them together. This device characteristic curve shows considerable deviations from the original characteristic curve at some points. The result is that the set control value in this area leads to a considerably larger or smaller product flow. As a result, the PID controller compensates for a larger control difference and at the same time requires a longer settling time.
Facilitate the independent recording of device characteristics for new products
The OPDENHOFF system solution can record up to 20 points in order to simulate the device characteristic curve and minimize the control difference at the switching moment. The device characteristic curve must be STANDARD system solution be noted by hand. With the OPDENHOFF system solution the points of the device characteristic curve are automatically determined and saved, enabling continuous dosing.
The material characteristic curve can also be recorded in order to increase the accuracy of the device characteristic curve. The product in the weighing hopper presses on the dosing screw with its own weight. This means that the drive motor has to apply more force with a full container and the same control than with an almost empty container. As a result, a lower speed is achieved with the same activation of a full hopper than with an empty hopper. In order to integrate this change into the control system, a material characteristic curve is recorded which adjusts the device characteristic curve depending on the fill level. With the STANDARD system solution, this material characteristic curve is recorded manually. For this purpose, a mean actuating value (approx. 50 - 60% actuating value) is output for a full container. If 90 % of the actuating value is reached, this value is saved. This is repeated at 70, 50, 30, 15 % of the nominal weight. The changes in the flow rate are used internally to create a characteristic curve for adjustment. With the OPDENHOFF system solution this characteristic curve is determined automatically.
System solutions for continuous dosing at a glance:
STANDARD system solution
- Accurate weighing unit that can be easily integrated into the system via PROFIBUS
- Configuration in the SIMATIC S7 program gives the user the option of sending commands to the Siwarex via command numbers
- Settings in Siwatool are complex and cannot be made by inexperienced users
- Additional software is required to transmit device and material characteristics when dosing further quantities
- No device and material characteristic curve: everything is laboriously recorded by hand
- Runs continuously and therefore has only very short cycle times during operation, but if data records are updated, the CPU must load them with instructions from the STANDARD system solution
OPDENHOFF system solution
- The dosing program is used for the cycle time
- All data records and commands are integrated in the CPU
- Simple exchange of device and material characteristics possible when changing products
- The product name and product number are compared with the stored values each time the dosing in the transferred recipe is restarted
- Existing automatic loading of new product with existing device and material characteristics
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