Designing with Multi-axis Load Cell Sensors

When designing for any application requiring the measurement of force, torque and/or acceleration, the main challenge for the designer is understanding exactly what will be needed from the sensor. Many times, off-the-shelf load cells are perfect for the job. However, the size, performance level, and the materials that go into the load cell must be matched to the level of use and the environment of use for a perfect fit. If the fit is not perfect to begin with, even the best system design will not meet its goals.

To ensure a robust system design using multi-axis load cells, a designer must first be able to answer these basic mechanical, electrical and instrumentation questions:

Mechanical

• What range of loads, magnitude and direction will need to be measured?
• What combination of loads will need to be withstood, even if not measured?
•  What diameter, thickness and weight load cell can be accommodated?
• Can your system provide adequate structural support for optimum load cell performance?

 Electrical

• Will your system require analog or digital output signals?
• How will you acquire, use and/or store output data?

 
Instrumentation

• What are your needs for accuracy, resolution and linearity?
• If your specific accuracy, resolution and linearity needs are critical, are you willing to accept trade-offs in price, lead-time and mechanical safety factors to meet those needs?
• Will the load cell be subjected to unique, demanding environments?

This last point about demanding environments is especially important. To compensate for extreme environments, special compensation circuitry may be required. If the work environments will include exposure to significant radiation, it can be wise to place electronics external to the load cell body. Exposure to extreme magnetic fields, such as where load cells are used near medical MRI equipment, requires that the load cell and all components be composed of only non-ferrous materials. For harsh, dusty, dirty work locations, it pays to use specially sealed load cells.

Designing for NASA
When you're tasked with designing an application for use by NASA, you know the requirements will be very demanding. Therefore, the following example-in which system designers (from Hamilton-Sundstrand Corporation, Boeing International Space Shuttle MSER Division and Oceaneering Space Systems) working on a project for NASA needed us to develop customized load cells-is a good illustration of assessing project requirements in light of the sensors needed to make it successful.

The project with NASA involved the attachment of a 40-ft extended rigid boom onto the existing 40-ft arm of the Space Shuttle so that an astronaut could safely stand on it to perform repairs. With this type of application, numerous issues regarding allowable torque came into question.

First of all, because every action has an equal and proportional reaction in the earth's atmosphere, when none of the restraining forces found on earth are present, even the most minute action can have wildly different results in space. Therefore, the project required that, when a crew member is secured via portable foot restraint to the end of this boom, he or she needed to be standing on a platform containing multi-axis load cells to accurately measure all reactions.

Getting back to the essential questions to be answered for a sensor-related project, the main issue on this project was torque. So the question that had to be answered was: What amount of torque is allowable in order to maintain stable arm position?

The integral load cells selected for this project had to be able to help the NASA team learn how to handle the physical movements of repair efforts, what type of tools to use, how to use them and how to adapt to any limitations of the extended boom.

The resulting system developed for this was called an Instrumented Work Site Interface (IWIF), which included instrumentation and a data recording device that interacted with the sensors to check load and relay the data collected in real time for analysis.

On the IWIF, the load cell sensor is positioned below the feet so that all the shock absorbing the human body performs happened before the load was measured. The intent was not to measure what the crew member was doing with his or her hands or against the surface, but to measure the amount of force being applied to structure of the platform and arm after the person's body had reacted and dampened the load being applied.

The original JR3 load cells selected were 5 inch round disks, approximately 1 to 1.5 inch in thickness. The final design used a 7 inch disk, 1.5 inch thick. This requirement was driven by the capabilities the designers required of the load cell. These load cells were made from titanium, which allowed the design engineers to increase their margins and factors of safety because of the material strength. Titanium also has very good properties for measuring strain, as it is flexible and returns to its original state.

Battery operated data recorders were used to record the measured strain and acceleration. Commanded via RF from the deck of the orbiter, these recorders could be activated or deactivated and directed to transfer data to a computer. The batteries in the data recorder served as the energizing source for the load cell accelerometers and the strain gages.

For more information, visit: www.jr3.com

 

How Load Cells are Designed Designing a load cell starts with the selection of a solid round from one of three possible materials: 2024 Aluminum, 15-5PH Stainless, or 6AL-4V Titanium. The diameter and thickness is chosen based on the desired bolt pattern and load rating. For example, JR3's 6DoF sensors range in diameter from 2 in. to 20 in. Force ratings range from less than 10 lb. to more than 25,000 lbs; moment ratings are from 2 ft/lb up to 150,000 ft/lb. Aluminum is the preferred material for load cells, due to weight and cost of machining, but high loads require using titanium or stainless steel. There are typically three or four load-carrying elements (strain rings) spanning between the top and bottom ends of the sensor. Either four or eight foil strain gages are bonded to each strain ring. The gages are attached to an electronics board within the sensor that amplifies the signals and transmits them out of the sensor (analog or digital).