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Shunt Calibration 101

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Calibration is a critical stage to ensure proper accuracy and reliability of any force measurement device. There are many ways to calibrate and different types of calibration. In the standards of maintaining our quality and precision requirements, Interface calibrates every test measurement device we manufacture including our load cells and torque transducers. Every device is shipped with the most detailed calibration certifications in the industry.

With our experience and expertise, we understand that sharing what we know is beneficial to our customers and partners in the test and measurement industry. One of the means by which we do this is through a series of technical white papers.  A popular white paper that was written years ago still stands the test of time, as it provides a deep dive on the topic of shunt calibration. Click on the title “Shunt Calibration for Dummies,” to access the full white paper.

What is Shunt Calibration?

Shunt calibration is a technique for simulating strain in a piezo-resistive strain gage Wheatstone bridge circuit by shunting one leg of the bridge. The bridge may be internal to a discreet transducer or composed of separately applied strain gages. The resulting bridge output is useful for calibrating or scaling instrumentation. Such instrumentation includes digital indicators, amplifiers, signal conditioners, A/D converters, PLC’s, and data acquisition equipment. Care must be taken to understand the circuits and connections, including extension cables, in order to avoid measurement errors.

Benefits of Shunt Calibration

The biggest reason to use shunt calibration is the flexibility and low-cost it offers the user. In this method of calibration, the bridge circuit is already there, and you don’t need to make and break cable connections to run it. This means that a shunt calibration can be applied conveniently and at any time during the test program. It is often used in situations where the user is calibrating control system equipment that will be communicating with a transducer or to confirm that the transducer is functioning properly.

Expected Shunt Calibration Repeatability in Modern Transducers

An important question that comes up regarding calibration is what type of repeatability can I expect from shunt calibration? Included below are the specifications outlining expected repeatability:

Procedure for a repeatability test performed:

  • 100 Klbf Load Cell specimen loaded in compression.
  • 12 test cycles of 4 mV/V hydraulically applied physical load and 1 mV/V Shunt Cal on two bridge legs.
  • Rb = 350 ohm, Rs = 88750 ohm, 20 ppm/°C, internal to load cell.
  • Measurements over 3 days.
  • Interface Gold Standard HRBSC instrumentation.

Results of test

  • Std Dev of physical load measurement: 0.004%.
  • Std Dev of Shunt Cal: 0.001% pos, 0.001% neg.

The topics that are illustrated in examples and discussion points for this white paper include:

  • Basic Bridge Circuit and Formulas
  • Resistor Examples
  • Tolerance
  • Cables
  • Errors
  • Permanent Zero Balance Shifts
  • Transducer Toggles
  • Instrumentation
  • Procedures for Repeatability in Tests

If this is a topic that of interest, download this technical reference guide for further exploration and calculation examples in shunt calibration.

As a leader in calibration services, Interface has an A2LA ISO 17025 accredited calibration lab located at the company’s headquarters in Arizona. Many depend on Interface for expert recalibration, which we recommend to do annually for optima maintenance. Our own calibration lab has the broadest capability and highest quality of calibration and repair services available. We understand the criticality of proper calibration and traceability and have the experience and expertise necessary to meet your exacting needs.

Additional Calibration Resources

Extending Transducer Calibration Range by Extrapolation

Additional Interface Calibration Grade Solutions

Gold Standard® Calibration System

This refreshed white paper is a tribute to the contributions of LaVar Clegg.

Vision Sensor Technology Increases Production Reliability

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In the product manufacturing process, repeatability, process control, and inspection are some of the most important factors in creating high-performance products. At Interface, reliability and accuracy are the two most important features of our renowned force measurement products. For this reason, Interface continues to invest in the highest quality test and inspection technology to ensure each product that leaves the production floor is of the utmost Interface brand quality.

In this blog, we will be discussing our investment in top-of-the-line vision sensor technology and systems that help us to effectively reduce waste, improve efficiencies in manufacturing, and guarantee proper performance.

For many years, Interface has relied on our employees for their keen visual inspection of products that come off the production line. These trusted employees are looking at a number of things, including: are wires installed properly and are the serial and revision numbers correct. These respected and experienced Interface team members can inspect hundreds or thousands of products a day, with a heavy reliance on the human eye.

Interface load cells must meet stringent quality processing production standards, along with our calibration requirements before they are shipped to our customers. Interface will scrap any product that does not meet the exact precision standards for performance. In fact, if the product fails, we will not utilize the product at all. We will start our manufacturing process over to ensure the integrity and quality is met, as is expected with any Interface branded force measurement product.

In a recent experience of solving for a wiring issue that could cost thousands in waste, we sought out an innovative way to approach the challenge. To solve the visualization and human dependency concerns, we invested in a Keyence Vision Sensor IV2 Series. This camera system allows us to reduce human error by programming the camera to pick up misaligned wiring and notify the user.

It works with advanced sensor technology that can identify production issues and will provide the user with a green light if everything looks good, or a red light if there is an issue. The system is helping to significantly reduce both inspection time and production line errors. The vision technology system is currently implemented on our MBS mini load cell line, one of our highest volume production lines. Since implementing the system, we have not seen a failure that caused any type of loss.

We are now looking into how we could use the Keyence Vision Sensor technology in other areas of production. For instance, we are testing how the camera could be used to read serial and revision numbers that are laser marked onto our products. Every once in a while, if the laser is misaligned or the user hits an extra keystroke, the numbers could be off, and the customer will have an error in their records. With the innovative visualization system, the camera’s sensors could read the serial numbers and let the user know if there is an error quickly.

To learn more about how Interface is investing in innovation and technology to improve our processes and production methods, check out the IQ Blog at www.interfaceforce.com/blog/.  You can also learn more about Interface’s 2019 Arizona Manufacturer of the Year Award here.

Contribution from Nick Siegel, Design Engineer, Interface