People who search for "weight-per-meter controller" and people who search for "weight-per-meter control system" are looking for different things. The first group mostly knows it is missing a piece of equipment, and wants to know what the thing with the screen that you can wire up looks like, what it is made of, and how it connects into the line. The second group wants the whole scheme: where the measurement goes, who carries out the action, how the software keeps data, how the interfaces are modified. This article only covers the first.
Once the device itself is set out clearly, you will not be led around by terminology during a negotiation: you will know what is inside it, where its signals come from and what it drives, and which quotation item is blank.
First, tell apart three words that are easily confused
On the shop floor these three words are often treated as synonyms, and using them interchangeably has a direct effect on what you purchase and on where responsibility lies.
| Term | What it refers to | When a customer says it |
|---|---|---|
| Weight-per-meter controller | The device itself. A control device with an HMI that receives weight and speed signals and outputs adjustment commands | It has been decided to install one; comparing parameters, asking about interfaces |
| Weight-per-meter control system | The whole scheme. The controller plus weighing unit, actuators, software and interface modification, and commissioning | The direction is unsettled; wanting to know how the line is modified and who is responsible |
| Weight per meter | The physical quantity being controlled. The weight of cable per unit length, often written kg/m | When discussing quality problems, calculating material usage, or comparing constructions |
The relationship between the three can be summed up in one sentence: weight per meter is the controlled quantity, the controller is the device that does the work, and the control system is the device plus everything around it that lets the work get done. Buy only the controller while the weighing unit is not installed and the actuator-side interfaces do not work, and it will not hold weight per meter down once powered up; if you do not know how many signal outputs it can provide, the discussion stops at terminology. For the signal flow around the whole loop, how weight-per-meter control works gives a complete path.
What the controller body is made up of
Open the enclosure and a weight-per-meter controller is roughly four blocks, in the order the signal travels:
- Weighing and measurement unit: turns weight into a usable electrical signal. It may be integrated in the cabinet or be a separate scale body on the line, and it decides how steady the reading is.
- Computing and control unit: takes weight and haul-off speed once per fixed cycle, converts them into weight per meter, compares the result with the setpoint, and works out which way to adjust and by how much. The output is not "a single number" but an adjustment quantity with a direction, a magnitude and a target.
- HMI: the only part the operator can see and touch. Current weight per meter, reference wall thickness, screw speed, haul-off speed, extrusion output and status must all be present at once. On site, "Auto control active" is a status prompt: the loop is acting by itself, and at that moment the haul-off should not be turned by hand, or the manual change fights the adjustment the system is making.
- Output signal channels: send the calculation result out, normally as two channels, screw speed and haul-off speed, as either analog values or communication messages.
Of the four blocks, only the first two have to do with measuring and calculating correctly; the last two determine whether it can move anything. Most arguments concentrate on the first two, while problems most often appear in the last two.
Where on the line it is installed, and which signals it connects to
The position depends on the measurement point. When weighing the finished product, the controller generally sits between the extruder and the take-up machine; when weighing hopper weight loss, the measurement point is upstream at the feed end, while the controller stays on the operating side. The position is not complicated; what is complicated is the signal list.
Signals coming in: the weight signal comes from the weighing unit; haul-off speed comes from the haul-off drive or a speed encoder; line speed or a length-counter signal is also needed to cross-check the length side. Miss one channel and you are missing one known quantity: weight per meter is "weight divided by length", and without length all you have left is a pile of weight readings.
Actions going out: one channel adjusts screw speed, changing extrusion output per unit time; one channel adjusts haul-off speed, changing material use per unit length. Both must connect to somewhere that accepts an external setpoint. However finely the controller calculates, if the actuation end does not accept the signal, the closed loop is broken there. This is what decides whether it adjusts anything or merely displays.
What calculation it is actually doing
One cycle of calculation runs: sample, convert, compare, judge direction and magnitude, output, wait, recheck.
Conversion: weight divided by the corresponding length gives weight per meter. Weight per meter, reference wall thickness, diameter and material density are locked to one another: know any three and you can derive the fourth.
Take actual readings from the shop floor as an example: weight per meter 0.1413 kg/m, reference wall thickness 1.650 mm, wire diameter 17.50 mm, material density 1.42 g/cm³, wall thickness compensation 0.020 mm, screw speed 41.24 rpm, haul-off speed 60.013 m/min, extrusion output 508.70 kg/h, and the interface status "Auto control active". Density and diameter take part in the wall thickness conversion, haul-off speed and extrusion output determine the material used per unit length, and if any one of them changes, the weight-per-meter reading has to be reinterpreted.
Comparison: a deviation has only two directions, heavy or light, but the cause may be either of two things: too much extrusion output, or a haul-off that is too slow. The weight-per-meter figure alone cannot tell you which side it is; you have to look at screw speed and haul-off speed at the same time.
Deciding magnitude and output: a large deviation does not mean the correction should be made in one go. Extrusion is a system with thermal inertia and material residence time, and the screen reading reflects an action taken ten-odd seconds earlier; adjust too hard and the reading has not yet come back down before the system takes another bite, so it oscillates. The adjustment quantity is sent to the screw speed and haul-off speed channels together, and the two values sit side by side on site, so which one is moving is visible at a glance.
Recheck interval: after the output, the system waits a set time before sampling again. Too short, and it over-reacts to an adjustment that has not yet shown itself; too long, and it cannot keep up. This time depends on the line length and the haul-off speed, and it has to be tuned to the actual operating conditions. For why a reading swings, and how to tell normal adjustment apart from a loop that has not been tuned well, see troubleshooting weight-per-meter fluctuation.
A construction change is the step that most needs human involvement: the parameters are switched as a complete set, and "Spec check" on the interface is the closing action, after which the system may enter auto control. Wall thickness compensation deals with a fixed offset: material shrinkage and eccentricity make the actual wall thickness differ from the value converted from weight per meter by a constant amount, and an operator may enter a compensation value. Material usage statistics record the weight, length and change time of each reel, to be traced back with the process data.
Where its capability boundaries lie
Being clear about the boundaries is more useful than listing every function. The controller governs one quantity, the total amount of material used per unit length; it cannot govern:
- Eccentricity: the total is right, but the material is distributed unevenly around the circumference, so one side is thin. That belongs to the die head, the die and concentricity.
- Appearance defects: burnt material, porosity, roughness, bamboo-like surface, are related to temperature settings and screw shear, and are not the same matter as weight per meter.
- Changes in the density of incoming material: if the density of incoming batches differs, the same screw speed puts out a different weight. The controller only calculates from the reference value you give it; it cannot know by itself what a new material should be, and the reference value must be set again by hand.
In other words, it solves "is the total material usage steady", not "is the material evenly distributed, is the appearance good, has the raw material changed". The test: if the problem shows up as weight per meter shifting as a whole or fluctuating continuously during a run of the same construction, the controller is the right direction; if it shows up as one side being thin or the surface being abnormal, deal with the die head, the die and the temperature first. For telling these two kinds of problem apart, there is cross-explanation in a comparison of measurement methods.
A few things to establish clearly when choosing a controller
- What is measured, and where. Ask the other party to mark on your machine drawing where the sensor sits, what it measures and where the signal comes from, and whether cleaning needs it removed. An answer that only talks about terms such as "loss-in-weight" or "gravimetric" without talking about position is not complete.
- Which kind of "accuracy" it is. At least three measures must be kept apart: the measurement repeatability of the weighing unit, the floating band of the weight-per-meter reading after entering auto control, and the actual deviation of the finished product off the line. The three have different test conditions and statistical methods, so which kind the percentage on the quotation belongs to has to be asked about and written into the acceptance terms.
- Whether it can be connected to the existing extruder and haul-off. Confirm machine by machine the existing control scheme, the form of the setpoint signal, and whether the electrical control cabinet has to be modified. This is the precondition for the whole thing working, not an installation detail. For how interfaces and configuration are aligned, the questions to ask when selecting a model sets it out in more detail.
- Whether a construction change requires recalibration. A construction change may happen several times a day. Establish clearly how many steps the standard procedure has, who does each step, how long it takes in total, and whether the calibration can be completed by the operator independently.
- Whether the data can be exported. In what format the historical data of weight per meter, screw speed, haul-off speed and extrusion output can be exported, whether it opens directly in spreadsheet software, whether it is stored on the device or on a plant server, and for how long. A curve that can only be viewed and not taken away is of no use for batch traceability and customer complaint analysis.
If the other party cannot answer any of these five, the scheme has not landed on your line. If you already have the machine data, tell us the setpoint method of the extruder and the haul-off when you contact us, and the reply will be considerably more specific.
A few common misunderstandings
- "Once the controller is installed, the process needs no attention." It only adjusts two quantities, screw speed and haul-off speed; temperature, screw configuration, cooling and take-up tension are outside its scope. Total material usage is automated, and the rest of the process still needs people to manage it.
- "If weight per meter is steady, the wall thickness must be acceptable." Weight per meter is the total weight per unit length, and wall thickness is how that total weight is distributed around the circumference. Where eccentricity exists, weight per meter can be very steady and the thin side still fails; concentricity and the state of the die have to help. It equally cannot govern outer diameter and ovality.
- "If the displayed value is accurate, the finished product is accurate." The displayed value is converted from the reference wall thickness, the diameter, the material density and the wall thickness compensation. Take the density wrongly and however smooth the display is, it points the wrong way; this kind of "steady but offset" cannot be found by looking at the process reading alone.
- "Manual sample weighing can be abolished completely." We suggest keeping it, but the frequency can be reduced substantially. Manual sample weighing then changes role from the basis for everyday adjustment to checking whether the system has drifted, and doing it once a week or at a construction change is enough.
Frequently asked questions
What is the practical difference between a weight-per-meter controller and a weight-per-meter control system?
The difference is in the periphery. The controller is the device with a screen, with computing and with output channels; beyond it, the system also includes installing the weighing unit, modifying the interfaces at the actuation end, data storage software, and commissioning and training. Buying only the controller without dealing with the periphery means it will not hold weight per meter down once powered up. For what the whole scheme includes, see system composition.
Can a single controller be added to an old production line?
In most cases, yes, and the amount of modification depends on two interfaces: where the weight signal is taken from, and whether the extruder and the haul-off motor can accept an external speed setpoint. The former is usually dealt with when the weighing unit is installed; the latter is the real watershed: if it cannot be done, the controller can only display, not adjust.
Why does weight per meter still drift slowly after the same construction has been running for a while?
First tell apart whether it is the loop or the input. If the control loop has not been tuned well, the reading swings back and forth around the setpoint; if the reference side has changed, whether because the incoming batch density differs or the wall thickness compensation is unsuitable, it shows as a steady shift as a whole in one direction. The latter will not come back by itself: the reference value has to be set again or the compensation amount corrected.
What does the operator have to do at a construction change?
Switch the parameters as a complete set for the construction: target weight per meter, reference wall thickness, diameter, density, compensation amount, and the corresponding screw speed and haul-off speed baselines. After checking them, carry out "Spec check" on the interface; only then may the system enter auto control. Whether the operator can complete this set independently within a few minutes is something to establish on the spot when selecting a model.
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