Foil gauges were great. That was then.
Development work doesn’t have to incur excessive costs. Sometimes, basic tools from a hardware store and simple skills can do the job well.
Commercial Load Sensor
Recently, a customer asked me to design a load sensor for industrial use. This application can support high-volume production, making hundreds of thousands or even millions of units possible.
The customer needed a low-cost load sensor for a trailer hitch. I checked out a few strain-gauge sensors, but their prices were quite high. They often cost several hundred dollars each. Engineers meant these sensors for lab testing, not mass production.
The search begins
I only needed a few for my prototype, but spending hundreds of dollars each was too much. Also, there was an extra cost for electronics to change the micro-ohm output into a readable format. Foil strain gauges are good, but their technology is over 80 years old. After an extensive online search for a cost-effective force sensor, I finally found one.
A simple, inexpensive load sensor
It had several layers of Mylar with a conductive coating. As force increased, conductance went up while resistance dropped. I created a small bench-test model for this proof-of-concept design. I used washers, nuts, and bolts. See figure 1 below.
Figure 1 – Load sensor parts showing load sensor, washers nuts and bolts
The sensor was a solid disk, about 1 inch in diameter. To use it, I needed to cut a small hole in the center to slip the bolt through. Conversations with the manufacturer confirmed that this would not affect the sensor much. A voltmeter connects to the sensor’s output leads. Then, I tighten or loosen the nut while reading the change in resistance on the voltmeter. It worked without any flaws. See Figure 2.
Figure 2 – Assembled load cell.
Figure 3 – Trailer hitch prototype FEA model
FEA model
I needed a specific spot that would give a compressive load matching the stress on the structure. First, I created the finite element model. Then, I adjusted the contact elements. This change let the loads go through the structure and into the bolts and washers in the assembly.
Next, I applied a variable force using a bottle jack. I checked if the stress on each bolted joint matched the external load. This process was simple, and I identified several key locations. See figure 4.
Figure 4 – load sensor location determined by FEA model
I built a test stand for the load sensors and the structural assembly from the customer. They made it of lumber, nails, and some bolted connections. To apply the loads, I extended the bottle jack. This was a qualitative test.
It Worked!
Next, we confirmed several possible locations. The next step is to change the output signal into a voltage or current that matches the applied load. Pumping the bottle jack caused a clear change in resistance. The load sensor manufacturer provided this circuit assembly. My customer could also make one themselves.
The project stalled at the proof of concept testing stage, which was disappointing. It was interesting and a lot of fun. It needed electrical knowledge, mechanical and structural design skills. Also, it involved building a finite element analysis model with ANSYS.
Norman T. Neher, P.E.
Analytical Engineering Services, Inc.
Elko New Market, MN
www.aesmn.org