Design Techniques for Custom Sensors and Transducers

Many manufacturers are going to custom or semi-custom component designs for their applications for two reasons: to provide tighter specs based on the latest technologies and to control aftermarket sales for maintenance purposes. Here, a single line of components illustrates the design process, paralleling the continual need for increased performance and faster turnarounds.

At HBM, Marlboro, MA, design engineer Tyson Hatch is part of the company’s Custom OEM Sensor Team, applying CAD and FEA software to provide custom and semi-custom strain gage-based sensors, such as load cells and pressure sensors for the industrial OEM. For the purposes of FEA modeling, it is necessary to know how the device is going to be loaded. When asked how often he knows what the end system will be or what industry the device is aimed toward, Tyson candidly remarked, “Often the customer provides a general description of the application. This gives the designer an idea of what the sensor will be used for and the environment to which it will be subjected. At the very least a list of key specifications and envelope dimensions are given." For upgrades to existing products, there is less of a need for specific application information.

Blind designs are common in today’s market because of the high competitiveness of most industries, not to mention the competitive edge a company gets when using custom components over off-the-shelf components. Still, according to Tyson, handling these concerns is routine for HBM. “Sometimes it’s not necessary to know precisely how the final product is to be used. That way, your focus can be on the design elements stated by the customer," he says.

When working from spec sheets, a designer must focus first on key elements in the list or else the complexity of design rises sharply. The typical focus when designing strain gage-based sensors, says Tyson, is the load being handled versus the sensor output (mV/V voltage ratio) that variations in load must present to the system controls. This first handful of numbers provides important input to the engineer.

A quick evaluation of several material types lets the designer know which best suit the particular application’s requirements. Factors that govern what type of material to use include such things as the device’s operating environment, capacity vs. envelope dimensions, and, often, the allowable deflection of the device.

Material	Type 420 Stainless	17-4 Stainless	2024-T351 Aluminum	Titanium Alloys
Density	0.279 lb/in3	0.282 lb/in3	0.1 lb/in3	0.16 lb/in3
Hardness	51.5 (Rockwell C)	44 (Rockwell C)	75 (Rockwell B)	36 (Rockwell C)
Tensile Strength, Ultimate	264,000 psi	198,000psi	68,200 psi	131,000 psi
Tensile Strength, Yield	157,000psi	183,000psi	47,100 psi	120,000 psi
Elongation at Break	11.5%	15%	20%	10%
Modulus of Elasticity	29,000 ksi	28,600 ksi	10,500 ksi	17,000 ksi
Electrical Resistivity	5.5e-005 ohm-cm	7.7e-005 ohm-cm	5.7e-006 ohm-cm	0.000178 ohm-cm
Thermal Conductivity (at 100oC)	173 BTU-in/hr-ft2-°F	124 BTU-in/hr-ft2-°F	833 BTU-in/hr-ft2-°F	46.5 BTU-in/hr-ft2-°F
Applications	Machine tools, aerospace, automotive, medical industry	Aircraft fittings, gears, fasteners, chemical process equipment	Semiconductor, photonics, chip manufacturing, medical industry, missile parts	Space, nuclear, power generation, airframe, components, biomedical implants
Comments	Great for use where hardness is an issue for high loads, excellent corrosion resistance.	Good fabricating characteristics, high strength, excellent corrosion resistance.	Used where tight tolerances are needed in controlled environments, good machinability and surface finish capabilities.	For high stress environments in specialty applications high corrosion resistance.

Once the material is selected and the basic envelope dimensions are incorporated, the initial CAD model is created. Using PTC’s Pro/Mechanica, the FEA model is created and manipulated repeatedly — the engineer starts with a basic component design and runs through a number of iterations focused on overall specification requirements. Only creativity in the approach limits the possibilities of design at this point.

With the FEA work done, the data is used to create the model’s sensing section, a high stress region typically used to obtain the desired output, and related to the location of strain gage placement. “We essentially weaken a section of material to force bending in that location. We then fine tune this area to obtain the desired stress levels needed to achieve the desired output and performance," says Tyson. “FEA analysis provides us with valuable feedback on the changes made to the design so that we can optimize our efforts."

Working closely with customer engineers becomes critical at this point, because often the final sensor design must fit inside a piece of equipment that is in its early stages of design. A simple change in the shape of the footprint where the sensor is installed can make a difference in overall reliability and/or performance of the sensor.

Once the model is completed in CAD and FEA, all data is ported out to a machining center. Once the custom design is machined, assembled and tested for compliance, the second- and third-tier specs are tested and evaluated for proper linearity, hysteresis, creep error, and zero balance. Additional specs include excitation voltages, electrical connections — whether they should be shielded or not — and overload circuitry if necessary. Key specs, such as full-scale output and load parameters, are tightly controlled to provide the highest reliability and longevity.

Adjustments are made after testing to the CAD and FEA data. With software packages like Pro/E and Pro/Mechanica, the data is adjusted and ported out for machining much quicker than in the past. Slight design variations can be computer modeled to reveal any hidden structural, thermal, or other design limiting factors. According to Tyson, time and the number of iterations necessary to come to a final solution are reduced, with most designs, from start to finish, completed in less than a week.

A Sensor Team like HBM’s not only designs from the spec sheets, but also makes material and design suggestions along the way. “The end result is seldom exactly like the initial spec sheet," says Tyson. “We’ve been doing this a long time and often provide better solutions and tighter specs for our customers than what they initially requested."

—RM