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Load Cell Test Protocols and Calibrations

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In the Interface Load Cell Field Guide, our engineers and product design experts detail important troubleshooting tips and best practices to help test and measurement professionals understand the intricacies of load cells and applications for force measurement devices. In this post, our team has outlined some helpful advice for testing protocols, error sourcing and calibrations.

The first step in creating test protocols and calibration use cases is to define the mode you are testing. Load cells are routinely conditioned in either tension or compression mode and then calibrated. If a calibration in the opposite mode is also required, the cell is first conditioned in that mode prior to the second calibration. The calibration data reflects the operation of the cell only when it is conditioned in the mode in question.

For this reason, it is important that the test protocol, which is the sequence of the load applications, must be planned before any determination of possible error sources can begin. In most instances, a specification of acceptance must be devised to ensure that the requirements of the load cell user are met.

Typical error sources in force test and measurement are usually identified as being related to:

  • Lack of protocol
  • Replication of actual use case
  • Conditioning
  • Alignment
  • Adapters
  • Cables
  • Instrumentation
  • Threads and loading
  • Temperature
  • Excitation voltage
  • Bolting
  • Materials

In very stringent applications, users generally can correct test data for nonlinearity of the load cell, removing a substantial amount of the total error.  If this can’t be done, nonlinearity will be part of the error budget.

An error budget is the maximum amount of time that a technical system can fail without service level consequences. In force test and measurement, it is sometimes referred to as uncertainty budget.

Nonlinearity is the algebraic difference between output at a specific load and the corresponding point on the straight line drawn between minimum load and maximum load.

Nonrepeatability is essentially a function of the resolution and stability of the signal conditioning electronics.  Load cells typically have nonrepeatability that is better than the load frames, fixtures and electronics used to measure it.

Nonrepeatabillty is the maximum difference between output readings for repeating loading under identical loading and environmental conditions.

The remaining source of error, hysteresis, is highly dependent on the load sequence test protocol.  It is possible to optimize the test protocol in most cases, to minimize the introduction of unwanted hysteresis into the measurements.

Hysteresis is the algebraic differences between output at a given load descending from maximum load and output at the same load ascending from minimum load.

There are cases when users are constrained, either by requirement or product specification, to operate a load cell in an undefined way that will result in unknown hysteresis effects. In such instances, the user will have to accept the worst-case hysteresis as an operating specification.

Some load cells must be operated in both tension and compression mode during their normal use cycle, without the ability to recondition the cell before changing modes. This results in a condition called toggle, a non-return to zero after looping through both modes. The magnitude of toggle is a broad range. There are several solutions to the toggle problem, including using a higher capacity load cell so that it can operate over a smaller range of its capacity, use a cell made from a lower toggle material or require a tighter specification.

ONLINE RESOURCE: INTERFACE TECHNICAL INFORMATION

For questions about testing protocols, conditioning, or calibration, contact our technical experts. If you need calibration services, we are here and ready to help.  Click here to request a calibration or repair service today.

Force Measurement Solutions for the Construction Industry

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In the world of heavy machinery, the ability to protect these investments is critical to an efficient and cost-effective worksite. This is especially true in the construction industry, where any type of damage or disruption to onsite equipment can significantly delay project timelines and cost a construction company hundreds of thousands of dollars, or more.

Protecting equipment is important in the industry; however, the safety of people is paramount. Severe failures of the equipment can be dangerous to machine operators. One way construction companies are protecting people and their material investments is through the use of force sensor technologies with Interface’s precision load cells, torque transducers, load pins, tension links and load shackles, as well as data acquisition instrumentation.

The use of force measurement is a growing trend in construction because companies realize that they can use force sensors to track performance data on a wide variety of heavy machinery. This data can inform machine operators when they were pushing the machines past their respective limits.

Applications of Force Measurement Products Used in the Construction Industry

One of the key use cases of force sensors used in the construction industry is on heavy machinery attachments. Construction sites frequently utilize a crane, which is used to lift large bundles of material such as wood or steel with a grabbing type attachment, or used to transport construction workers to large heights with a basket or platform attachment.

For cranes outfitted with a lifting attachment such as a claw, a tension sensor can be used on the pulley mechanism to measure the weight lifted by the crane. The tension sensor can provide real-time data to the construction crew to help monitor the lifting process and provide the operator with the information necessary to refrain from lifting weights that are too heavy for the crane to handle. If the claw arm lifts more than the crane is able to withstand, the attachment could break off, or worse, the crane could topple over.

Another example of a crane attachment that can benefit from a force measurement sensor is the basket or platform type attachment used to transport workers to great heights. In this use case, a rotary actuator between the basket attachment and crane can be outfitted with a pressure transducer. This type of sensor will help measure the force placed on the attachment point to help rotate the basket in multiple directions and provide force data to ensure the basket isn’t over-rotated or carrying too much weight.

The final example of sensor technology used in construction is with a smart clamp. This is a use case that can be seen in multiple industries, in addition to the construction industry. A smart clamp, or soft-touch clamp, uses a compression load cell attached to a gauged piece of metal on both ends of the clamp. The clamp attachment is often placed on the end of a forklift type machine and used to transport delicate materials, packages, and other materials.

The compression load cell works by providing data back to the operator, letting them know how much force can be used to grab the object without breaking it. This used case is often found in the consumer packaging industry but can also be applied to the construction industry when transporting delicate building materials.

For many years, construction companies used this type of equipment and heavy machinery without the use of force sensors, making it harder to keep the equipment and workers safe. Today, more companies that develop attachments and heavy machinery have begun exploring force sensors to optimize the use of these machines. This creates a safer, more efficient and cost-effective environment for construction companies and protects their workers.

To learn more about specific construction industry use cases, review our detailed application notes below:

Lifting Heavy Objects

Harness Durability Testing

Interface is engaging with a number of customers in these industries to develop solutions to keep equipment safe and performing at optimal efficiency. To learn more about how force sensors can be used to protect your investments, contact our specialized application engineers and representatives of Interface products and solutions.

Contributor: Dan McAneny, co-founder and sales engineer at Tritek Solutions, one of Interface’s sales representatives covering the Pacific Northwest.