A vessel weighing system initially operates stably during commissioning. As soon as the frequency converter for the agitator starts, however, the weight reading begins to fluctuate. At low motor speed, only a few divisions are visible, while at higher frequency the indication jumps much more strongly. Mechanically, the vessel remains stable, the load cells are installed correctly, and the signal changes even at constant weight.
A typical suspicion is then: the load cells are being disturbed. In many cases, however, the cause is not the measuring element itself but the electrical signal path between the load cell and the weighing electronics.
This is particularly critical with mV/V load cells. A load cell with a sensitivity of, for example, 2 mV/V produces only around 20 mV differential signal at 10 V excitation and rated load. At ten percent load, the signal is correspondingly only about 2 mV. Small induced interference voltages, potential differences or contact problems can therefore be significant in relation to the useful signal.
The obvious response is often: “Then we will ground the shield at one end only.” This rule can be useful in certain measurement setups, but it is not sufficient as a universal requirement for industrial weighing systems. Some measuring chains are explicitly designed for single-point shield connection. Other systems connect the cable shield at both ends via the sensor housing and evaluation electronics and require low-resistance equipotential bonding for this purpose.
The decisive point is therefore not to create as many or as few grounding connections as possible. What matters is a defined grounding, shielding and equipotential bonding concept for the complete measuring chain.
The most important rule is: The cable shield is intended to divert electromagnetic interference. It must not unintentionally become the equipotential bonding conductor between two machine sections. If relevant potential differences exist between the load cell, weighing structure, control cabinet and electronics, these must be controlled by a designated low-resistance equipotential bonding connection – not through the sensitive measuring cable.
Why mV/V signals are particularly susceptible to interference
Analogue load cells are often based on a Wheatstone bridge circuit made from strain gauges. The weighing electronics excite this bridge with a defined voltage. Under mechanical load, the ratio of the bridge resistances changes and a very small differential voltage is generated between the two signal lines.
Sensitivity is typically specified in millivolts per volt. With a load cell rated at 2 mV/V and an excitation voltage of 10 V, the output at rated load is:
2 mV/V × 10 V = 20 mV
At half load, this would ideally be approximately 10 mV. If the weighing system operates only within a small part of the rated-load range or uses several load cells connected in parallel, the changes to be evaluated are correspondingly small.
This is precisely where the challenge lies. A 4–20 mA signal has a comparatively large signal level and a defined current interface in relation to many electrical interference effects. The mV/V signal of a load cell, by contrast, is transmitted directly from a sensitive strain-gauge bridge.
The weighing electronics can evaluate differential signals with very high resolution and may include filters for certain types of interference. However, they cannot distinguish whether an electrical voltage originates from a genuine load change or from unfavourably coupled interference if both appear similar at the input.
A mechanically high-quality weighing system can therefore perform significantly worse than expected because of poor electrical design.
What the cable shield actually does
The cable shield conductively surrounds the sensitive signal conductors. Its purpose is to prevent electrical and electromagnetic interference fields from affecting the signal lines as far as possible, or to divert coupled interference energy in a controlled manner.
The shield is therefore part of the EMC concept. However, it is not automatically the protective conductor of the installation, nor should it be used as a substitute for adequately dimensioned equipotential bonding between metallic plant components.
This distinction is frequently blurred in practice. A cable may, for example, have a shield connected to an earth bar in the control cabinet. At the same time, the metallic sensor housing is electrically connected to the shield. If the load cell is installed on a large machine structure at a different potential from the control cabinet, a balancing current can then flow through this very shield.
The same conductor then suddenly performs two tasks: it is intended to shield against high-frequency interference while simultaneously equalising a potential difference between two parts of the installation. This combination can interfere with sensitive measuring signals.
| Connection | Main function | Typical problem if used incorrectly |
|---|---|---|
| Cable shield | EMC shielding of the measuring line | Balancing currents flowing through the shield |
| Protective conductor PE | Electrical safety | Must not be interpreted arbitrarily as the signal reference |
| Equipotential bonding | Minimise potential differences between system components | Excessive resistance causes balancing currents to seek other paths |
| Signal ground | Electrical reference of the measuring system | Uncontrolled connection to PE can create loops |
| Sensor housing | Mechanical protection and, depending on the system, part of the shielding | Undefined connection to the shield changes the grounding concept |
For stable weighing measurement, these functions must therefore be deliberately planned.
Single-point grounding: useful, but not a universal rule
With sensitive low-frequency analogue signals, a single-point shield connection is often used. The shield may, for example, be connected at the weighing electronics or at a defined functional earth and left without an additional second earth connection at the opposite end.
The main idea behind this is to prevent low-frequency balancing currents from flowing through the cable shield. If the sensor side and control cabinet are at slightly different potentials, a current can flow through the shield when there is a conductive connection at both ends. This current, together with the resulting electromagnetic fields or voltage drops, can interfere with the sensitive measurement.
A single-point shield connection can therefore be very effective in certain installations.
However, this must not be confused with the statement that the cable shield of every industrial load cell must always be connected at one end only.
At higher frequencies, the behaviour of the cable, shield and earth connections changes. A long shield connected at only one end can be less effective against high-frequency electromagnetic interference than shielding that is correctly connected at both ends with a low-impedance, wide-area connection.
In addition, some sensor manufacturers already connect the shield internally to the metallic measuring body. Even if the installer does not connect a visible earth wire at the sensor cable, an electrical connection may therefore already exist through the sensor housing.
The question should therefore not be:
“Where do I cut off the shield?”
but rather:
“How is the shield intended to be connected in the specific sensor, cable, junction box and measuring amplifier, and what does the plant’s equipotential bonding concept look like?”
When a shield connection at both ends may be intended
In industrial force and weighing measuring chains, there are systems in which the cable shield is connected both to the metallic measuring body and to the housing of the evaluation electronics.
Such an arrangement can create highly effective, almost continuous electromagnetic shielding of the complete measuring chain. However, this requires that no problematic potential differences exist between the sensor side and the electronics.
This is precisely why such concepts often include a separate low-resistance equipotential bonding conductor between the machine or weighing structure and the control cabinet.
The cable shield then does not need to carry any significant low-frequency balancing currents. It can perform its actual EMC function.
This point is particularly relevant in large installations. A vessel can be connected via steelwork, piping, motors and leakage currents to a different local potential than a control cabinet located several metres away. The fact that both are connected to PE somewhere in the system does not guarantee a sufficiently small potential difference for a high-resolution measuring chain.
An intended shield connection at both ends should therefore not be “improved” by spontaneously removing one shield connection. Likewise, a system explicitly designed for single-point shielding should not be grounded at a second point without a specific reason.
The manufacturer’s documentation for the load cell and weighing electronics is decisive.
Consistently separate shielding and equipotential bonding
Effective equipotential bonding ensures that metallic system components between which no significant voltage should exist are connected with low electrical resistance.
In a vessel weighing system, this may include the vessel or supporting structure, the load-cell mounting components, the machine frame and the control cabinet.
The mechanical structure itself may appear to be electrically conductive. However, bolted joints, paint layers, corrosion, bearings or elastic components can make this connection electrically much poorer than expected.
An additional flexible earth or equipotential bonding strap across a load-cell mounting assembly therefore performs an important function. Among other things, it prevents electrical currents from seeking a path through the load cell, its mechanical contact surfaces or the sensor cable.
This is particularly relevant in systems with motors, welding operations, electrostatic charging or possible leakage currents.
The equipotential bonding must be dimensioned in accordance with the plant and manufacturer concept. A thin cable shield is no substitute for a purpose-designed equipotential bonding conductor.
Why the weighing mechanics are electrically relevant
Mechanical and electrical design cannot be completely separated in a weighing installation.
A load cell is located between a supporting base structure and the structure being weighed. It is therefore positioned exactly where two large metallic components either make mechanical contact or may be almost electrically isolated from each other.
If an electric motor is installed on the vessel, leakage currents or high-frequency interference currents can flow through the vessel structure. Flexible pipe connections, elastic bearings or painted structural parts can additionally alter the electrical return path.
Without defined equipotential bonding, the current seeks the electrically most favourable available path. Under unfavourable conditions, this path may run through load-cell mounting components, shield connections or other measuring connections.
Industrial load-cell mounting assemblies therefore often have explicit earthing or equipotential bonding connections.
This relationship is also important for diagnostics. A weight signal that changes when a large motor is switched on does not necessarily have to be caused by electromagnetic radiation coupling directly into the sensor cable. A change in the local reference potential of the mechanical structure is also possible.
Route measuring cables in an EMC-compliant manner
Even the best shielding strategy can only provide limited protection if the load-cell cable runs for many metres directly alongside a strongly interfering cable.
Motor cables downstream of frequency converters are particularly critical. Their steep voltage switching edges contain high-frequency components that can couple capacitively and inductively into nearby cables.
Cables from servo drives, contactors, solenoid valves, heaters and powerful switched-mode power supplies can also cause problems.
Load-cell cables should therefore be routed separately from power cables wherever possible. A short crossing is significantly preferable to a long parallel run. If signal and power cables must cross, a large crossing angle is generally advantageous.
Inside the control cabinet, too, the sensitive load-cell signal should not be unnecessarily routed through cable ducts containing frequency-converter outputs simply because this is geometrically the shortest route.
The entire cable route from the sensor to the weighing electronics is part of the measuring chain.
Use twisted wire pairs correctly
A suitable load-cell cable does not simply consist of several arbitrary conductors inside a common shield.
Functionally associated conductors are typically arranged as twisted pairs. With a 6-wire load cell, this includes, for example, the two signal conductors, the excitation conductors and the sense conductors.
Twisting reduces the effective loop area between outgoing and return conductors. An electromagnetic field therefore tends to couple similarly into both conductors. Because the evaluation electronics measure the difference between the signal conductors, a common interference component can be suppressed much more effectively than asymmetrical interference.
| Wire pair | Function | Why twist them together? |
|---|---|---|
| SIG+ / SIG− | mV/V measurement signal | Maximum symmetry of the most sensitive signal path |
| EXC+ / EXC− | Bridge excitation | Reduces magnetic coupling into the excitation loop |
| SENSE+ / SENSE− | Measurement of the actual excitation voltage at the load cell | Symmetrical return of the compensation information |
| Shield | Surrounding electromagnetic shielding | Must not be used as a normal signal or equipotential bonding conductor |
This pair structure should therefore be retained during repairs or cable extensions.
A cable with the same number of conductors but without suitable pair twisting, capacitance and shielding is not automatically equivalent to a cable intended for load-cell measurement.
Consider junction boxes and cable extensions
In vessel or platform weighing systems, several load cells are often combined in a junction box. This introduces an additional point in the signal path where shielding, equipotential bonding and insulation must be continued correctly.
The shield must not simply end by chance at a cable gland. Nor should it be connected through a long, thin pigtail if the intended EMC connection requires wide-area shield termination.
The junction box itself may be metallic or insulating and, depending on the system, may include a defined shield terminal or EMC cable gland. The manufacturer’s system concept must therefore be implemented consistently here as well.
Moisture can make the problem even more severe. Reduced insulation resistance or leakage currents can create connections between signal conductors, shield and housing that are not present in dry conditions.
A fault that occurs only after rain, cleaning or high humidity should therefore not be regarded solely as a conventional EMC problem.
Frequency converters as a typical source of interference
Frequency converters are among the most common sources of interference in industrial weighing systems. The main problem is not the motor current itself, but the rapid electronic switching of the output voltage.
The steep switching edges generate a broad frequency spectrum. Interference can enter other circuits via electric fields, magnetic fields, common grounding connections or parasitic capacitances.
A characteristic fault pattern is a scale that operates completely stably when the drive is switched off but becomes unstable immediately after the frequency converter is enabled. If the interference additionally varies with motor frequency or load condition, the relationship becomes even clearer.
The first response should then not be to increase the digital filter time of the weighing electronics to the maximum.
The cable route, shield connections, equipotential bonding, motor grounding and frequency-converter installation should first be checked. Only once the electrical design is correct should the signal filtering required for the process be optimised.
Systematically diagnose interference
EMC problems are often difficult to identify because they occur only under certain operating conditions. A systematic diagnosis is therefore much more useful than randomly reconnecting shields and earth conductors.
A particularly useful question is what reproducibly influences the fault. If, for example, it occurs only when a specific drive is switched on, this already provides more information than simply observing a fluctuating weight value.
| Observation | Possible cause | Sensible check |
|---|---|---|
| Indication becomes unstable only when the frequency converter is running | EMC coupling or potential shift | Check cable route, shielding and equipotential bonding |
| Zero point changes when the sensor housing is touched | Undefined ground reference or shielding fault | Check housing, shield and equipotential bonding connections |
| Measured value changes when the cable is moved | Cable break, shield interruption or terminal problem | Mechanically inspect the cable and connections |
| Interference occurs after cleaning or in humid conditions | Reduced insulation resistance | Check junction box, cables and cable glands |
| Signal becomes worse after connecting two housings | Balancing current through newly created earth path | Investigate potential difference and equipotential bonding concept |
| Signal is stable at the sensor but unstable after extension | Unsuitable shielding or cable construction in the extension | Check extension cable and shield transitions |
An instrument for measuring potential differences can be just as useful here as the indication from the weighing electronics themselves. If a measurable potential difference exists between the weighing structure and the control cabinet, the sensor shield should not simply be used as an additional connection.
Instead, the equipotential bonding concept must be investigated.
Why a digital filter cannot repair a poor installation
Modern weighing electronics offer powerful digital filters. These can, for example, stabilise the signal from a vibrating vessel scale or provide defined suppression of mains-frequency interference.
These functions are extremely useful. However, they should not be used merely to make an electrically disturbed measuring point appear visually stable.
A strong low-pass filter can remove rapid interference from the displayed value. At the same time, however, it reduces the usable dynamics of the scale.
In dosing or filling processes, this can cause a real weight change to be detected too late. The control system switches off too late and dosing accuracy deteriorates.
A stable indication is therefore not automatically equivalent to an interference-free input signal.
The interference source or transmission path should first be eliminated as far as possible. Filtering should then be adjusted according to the mechanical and process dynamics.
Practical example of a vessel weighing system
A mixing vessel is mounted on four load cells and evaluated using weighing electronics. The vessel also has a variable-speed agitator motor.
With the agitator switched off, the indication fluctuates by only a few grams. After the frequency converter starts, however, the indication moves by several kilograms even though the actual vessel contents remain unchanged.
Initially, it is suspected that mechanical vibrations from the agitator are being transferred through the vessel structure into the load cells. A test at very low speed, however, shows that the weight disturbance already occurs before any significant mechanical vibration develops.
Further investigation reveals that the load-cell cable and motor cable run in the same cable duct for several metres. In addition, during an earlier modification, the load-cell cable shield was disconnected from the intended shield terminal in the junction box and connected to a housing screw using a long single wire.
There is also no defined additional equipotential bonding connection between the vessel structure and the control cabinet. Electrically, the two are connected only through the general machine structure.
The measuring point is therefore not “repaired” by stronger filtering but systematically rebuilt. The measuring and motor cables are separated wherever possible, the shield is correctly reconnected according to the intended system concept, and the equipotential bonding of the weighing structure is checked and, where necessary, established.
After the modification, the raw signal remains significantly more stable even with the frequency converter running. The digital filter of the weighing electronics can then be set back to a shorter time constant.
This improves not only the stable weight indication. The response time during dosing also becomes shorter again.
The example demonstrates the actual purpose of a good EMC concept: interference should be prevented as far as possible before software filters have to remove it from the measured value.
Check calibration after modifications
A change to the shield connection alone should not fundamentally alter the mechanical sensitivity of a load cell. Nevertheless, after work on the measuring cable, junction box or weighing electronics, it is advisable to check the measuring chain.
During a cable modification, for example, a signal or sense conductor may accidentally be swapped, contact resistance may be introduced or the excitation voltage may change.
With 4-wire systems, changes in cable length and conductor resistance can have a greater effect on the measurement than with a correctly evaluated 6-wire arrangement using sense conductors.
After major work, at least the zero point and span should therefore be checked using a known reference load or the specified calibration procedure.
For legal-for-trade or quality-critical weighing systems, the corresponding operational and metrological requirements must also be observed.
Define the EMC concept during the planning stage
Many EMC problems do not originate during commissioning but already during control-cabinet and machine layout.
If motor cables, load-cell cables and communication cables are initially planned in the same cable route in the CAD system, the installation can often be corrected later only with considerable effort.
The route of the load-cell cables, the position of the junction box, the continuous continuation of the cable shield and the point at which the shield is connected to the weighing electronics should therefore be defined during the planning stage.
It must also be clear how the metallic weighing structure is incorporated into the equipotential bonding system.
When several load cells are used, the electrical architecture of the junction box should also be defined. Shield connections, EMC cable glands and enclosure sealing are not minor accessory issues but part of the signal quality.
The choice between 4- and 6-wire technology, the maximum cable length and the position of the weighing electronics should likewise be decided at an early stage.
Later modification is possible, but a measuring chain that is correctly planned from the outset is generally more robust and easier to diagnose.
Common mistakes
Always connecting the shield at one end only
A single-point shield connection can be useful, but it is not a universal requirement for every load cell and every weighing electronic system. Manufacturer instructions and the EMC concept of the complete measuring chain must be taken into account.
Using the shield for equipotential bonding
The cable shield is intended for electromagnetic shielding. Relevant potential differences between machine and control cabinet should be controlled using a designated low-resistance equipotential bonding connection.
Connecting the sensor side and control cabinet without checking potential differences
An additional earth connection can create a new balancing current if potential differences are already present. Wiring should therefore not follow the principle that “more grounding is always better”.
Routing the load-cell cable together with motor cables
Cables from frequency converters in particular can generate strong electromagnetic interference. Long parallel routing should therefore be avoided wherever possible.
Connecting the shield using long, thin wires
For high-frequency interference currents, a long connecting wire has significant impedance. Where wide-area EMC shield termination is specified, it should be implemented accordingly.
Filtering only the indication
A digital filter can smooth an unstable value but does not eliminate poor equipotential bonding or electromagnetic interference.
Ignoring the internal connection of the sensor shield
With some load cells, the cable shield is already connected to the metallic measuring body. This must be taken into account in the grounding concept.
Failing to recheck the system after modifications
After changes to the cable, junction box or electronics, it should be verified that zero point, span and signal stability are still correct.
Suitable weighing technology at ICS Schneider
ICS Schneider Messtechnik offers analogue and digital load cells, force transducers, mounting units, weighing electronics, summing solutions and measuring amplifiers for industrial weighing systems.
The load cells include different designs for platform, vessel, silo, tension and compression applications. Analogue versions typically operate on a strain-gauge basis and therefore produce small mV/V signals, which means that mechanical installation, wiring and signal conditioning must be considered together.
For integrating analogue load cells into SIMATIC systems, the SIWAREX WP231 is available, for example. It handles bridge evaluation, weight calculation, calibration and diagnostics and can be used both within SIMATIC S7-1200 and as a standalone weighing electronic system.
For applications with higher dynamics, the SIWAREX WP321 is available, among other solutions. In dynamic force and weighing applications, it is particularly important not to confuse EMC interference with an actual rapid mechanical load change.
The IM2 strain-gauge display and evaluation instrument has a direct input for load cells or strain-gauge sensors with typical sensitivities from 1 to 4 mV/V and integrated bridge excitation.
As a typical industrial load cell, the SIWAREX WL230 SB-S SA can be used for vessel, platform and comparable weighing applications.
For reliable system design, not only rated load and accuracy class should be specified. Cable length, 4- or 6-wire connection, existing frequency converters, spatial cable routing, number of load cells, junction box, control cabinet design and the existing equipotential bonding concept are equally important.
Force, weighing and displacement measurement technology at ICS Schneider
Further reading: Extending load-cell cables – correctly consider 4- and 6-wire technology
Further reading: Simulating an mV/V signal – correctly testing load cells and strain-gauge sensors
Conclusion
The output signal of an analogue load cell is small. With a sensitivity of only a few mV/V, even at rated load only a few millivolts are transmitted. A clean electrical signal path is therefore essential.
The statement “always ground the load-cell shield at one end only” is too simplistic. Single-point shield connection can help prevent unwanted shield currents caused by low-frequency potential differences. Other measuring chains, however, are explicitly designed for continuous shield connection through both the sensor and electronics housings.
The decisive factor is therefore not to confuse shielding with equipotential bonding. The shield protects the measuring signal path against electromagnetic interference. Equipotential bonding ensures that no problematic potential differences develop between the mechanical structure, sensor and control cabinet. If these functions are cleanly separated, the cable shield does not need to carry balancing currents.
Cable type and routing are equally important. Twisted signal pairs, continuous shielding, defined shield transitions and spatial separation from frequency-converter and motor cables significantly reduce the likelihood of interference.
Strong digital filtering, by contrast, should be used only at the end of the optimisation process. It can reduce the required process dynamics and may conceal an installation problem instead of eliminating it.
Anyone who plans the load cell, mechanical structure, equipotential bonding, measuring cable, junction box and weighing electronics as one complete measuring chain will not only obtain a more stable indication. The measuring point will also become more reproducible, easier to diagnose and more robust against later plant modifications.
FAQ on shielding mV/V load cell signals
Should the shield of a load cell always be grounded at one end only?
No. Single-point shield connection is a common concept for sensitive analogue measuring signals, but it is not a universal rule. Some manufacturers or measuring chains specify continuous or two-ended shield connection. The specifications for the load cell, cable and weighing electronics must be considered together.
Why can shielding at both ends cause problems?
If an electrical potential difference exists between the two ends, a balancing current can flow through the shield. With sensitive mV/V signals, this can interfere with the measurement.
Why can shielding at both ends still be useful?
A low-impedance shield connection at both ends can be highly effective against high-frequency electromagnetic interference. The prerequisite is a suitable plant and equipotential bonding concept.
What is the difference between shielding and equipotential bonding?
The shield is primarily used for electromagnetic shielding of the measuring cable. Equipotential bonding connects system components with low resistance so that interfering potential differences between them are minimised.
May the cable shield be used as the equipotential bonding conductor?
It should not be used for this purpose. If relevant balancing currents are expected, a suitable separate equipotential bonding connection should be provided in the installation.
Why are load-cell signals so sensitive?
With a typical sensitivity of 2 mV/V and 10 V excitation, the output signal at rated load is only about 20 mV. Small interference voltages can therefore be relevant in relation to the useful signal.
Why are signal conductors twisted?
Twisting exposes both conductors of a differential signal pair to a similar electromagnetic environment. Common coupled interference can therefore be suppressed more effectively by the differential input stage.
Can a frequency converter interfere with a load cell?
Yes. In particular, the rapidly switched motor cables of a frequency converter can generate electromagnetic interference that couples into nearby sensitive measuring cables.
May the load-cell cable be routed together with the motor cable?
Long parallel routing should be avoided wherever possible. Measuring and power cables should be routed separately.
What if signal and motor cables have to cross?
A short crossing is usually significantly preferable to a long parallel run. Where possible, the cables should cross at a large angle.
Is a metal cable tray sufficient as shielding?
It can improve the EMC situation, but it does not automatically replace a suitable shielded load-cell cable and a correct shield connection concept.
Can a junction box cause EMC problems?
Yes. Interrupted or unsuitable shield continuation, poor cable glands, moisture and corrosion can cause signal interference there.
Why can moisture affect the weight indication?
Moisture can reduce insulation resistance between conductors, shield and housing, thereby creating additional leakage currents.
Does stronger digital filtering help against EMC interference?
It can reduce visible fluctuations, but it does not eliminate the cause. At the same time, the response to genuine weight changes becomes slower.
Can the shield simply be disconnected at one end as a test?
For structured diagnostics, it should first be checked how the sensor and electronics manufacturers specify the shield connection and whether separate equipotential bonding is present. Random rewiring can create new faults.
Is the load-cell housing connected to the cable shield?
This depends on the specific load cell. In various industrial load cells, the shield is connected by design to the measuring body. This internal wiring must therefore be considered during planning.
Why is an earthing strap across a load-cell mounting assembly useful?
It provides a defined low-resistance electrical path between the mechanical system components. This prevents electrical balancing currents from having to flow through the load cell, bearing surfaces or sensor cable.
Does the system need to be recalibrated after changing the shield connection?
A correct change to the shield connection alone does not fundamentally alter the sensitivity of the load cell. Nevertheless, after work on the measuring wiring, it is advisable to check zero point, span and signal stability.
What information is important when planning an interference-resistant weighing measurement?
Important information includes the load-cell type, evaluation electronics, 4- or 6-wire connection, cable lengths, junction box, existing frequency converters and motors, cable routes, control cabinet design and the machine’s equipotential bonding concept.