by Randy Frank, Contributing Editor
Incredible shrinking sensors satisfy system requirements.
Many of the newest sensing technologies are based on microelectromechanical systems (MEMS) technology and have an unpackaged footprint of a few millimeters or less. However, it takes more than a small sensing element to have a sensor that works in tight spaces. The sensor must satisfy other system requirements. Sensors designed using MEMS technology readily demonstrate both size and systems aspects.
Designed for smart phones and other portable electronic products, InvenSense MPU-9150 is the world’s first 9-axis MotionTracking device. It uses a System in Package (SiP) design that combines two chips within a 4 × 4 × 1-mm land grid array (LGA) surface mount package. The MEMS-based MPU-6050 contains a 3-axis gyroscope, 3-axis accelerometer, and an onboard Digital Motion Processor (DMP) capable of processing the company’s complex 9-axis MotionFusion algorithms. The second chip, the AK8975, is a 3-axis digital compass that uses the Hall Effect to detect magnetic fields. Even with three integrated 3-axis sensors, the processor and software are required to provide the motion tracking capability.
However, other sensing technologies are being pushed into smaller and smaller form factors and for far more applications than smart phones. In fact, the sensing requirement and the target applications establish the sensors technology and form factor. A pressure sensor, magnetic encoder, optical encoder, torque sensor, linear position sensor and 2D code reader provide examples of other shrinking sensors and the added value they provide target products.
The package defines small
No matter how small the sensing element, the end package obviously determines the final footprint and application volume requirement. The term subminiature is a common description for pressure sensors in a smaller than usual package. Honeywell’s recently introduced Model S Series subminiature pressure transducers provide an excellent example.
The sensing technology in the Model S Series is four active bonded strain gauges in a Wheatstone-bridge design. Additional packaging details include a flush diaphragm manufactured from 17-4 PH Stainless Steel.
“This is one of the smallest of Honeywell’s line of pressure sensors,” said Brian Duffy, application engineering manager for Honeywell Sensing & Control. “Most are larger.”
The Model S series has two packaging options: the BP357 and BP358. With a 17.53-mm length and 12.7-mm diameter, the BP357 is the smaller of the two packages; for the BP358 it’s 19.05 × 14.22-mm diameter.
“The Model S Series of subminiature pressure transducers fit into tight spaces with little clearance, and can accurately measure pressure ranges from 100 psi to 15000 psi,” said Duffy. “Because of its design, it allows you to get very close to the media that you are measuring; hydraulic fluid, for example.”
Two different techniques are used to handle the signal conditioning. Temperature compensation is performed with temperature-sensitive components located inside the sensor. A small circuit board in the lead wire (approximately 1 × 0.087 in. thick) addresses the zero balance. This balance board does not have to be in the same temperature as the transducers.
Based on the smaller size, potential applications for these sensors include: down-hole tools for oil and gas applications with temperature extremes of –54°C up to 150°C that can be configured as a gauge type or differential sensor, as well as automotive testing applications such as transmission test stands, pressure brake testing, and hydraulic sensor testing.
Redefining the sensor
As a component-level contactless magnetic encoder, the RLS RoLin redefines what a system designer can do for position feedback measurements. With a footprint of 12 × 5-mm for the version that direct solder attaches to a printed circuit board (PCB), RoLin is a position sensor designed for very small devices or motion systems and is available packaged, or unpackaged for low cost linear or rotary precision feedback.
“RoLin’s small size and price allows linear, rotary or partial arc position feedback at the point of interest eliminating issues associated with long-chain mechanisms,” said Howard Salt, Business Manager, Encoder Systems and Group RLS Marketing Manager.
Designed as a low cost alternative to small but expensive optical encoders, the system consists of a readhead and magnetic scale or ring with electronics inside the readhead. Radial or axial reading of the ring is possible.
“RoLin has been aimed squarely at the emerging low cost, small size, linear angle and rotary motion markets which are becoming much more prevalent in robotic, AUV, drone, electric motor, manipulator, medical, instrumentation and optical control markets,” said Salt.
With partner company Renishaw, RLS offers more than an encoder in a standard form factor. A large part of the duo’s small sensor business is custom manufactured to specifically suit OEM’s size, resolution, communications and price targets. “RoLin is not the smallest linear sensor we manufacture, it is the smallest sensor we offer as a standard product,” said Salt.
Integrating encoders in linear motors
A postage-stamp size (26 × 12.5 × 16.5-mm) scanning unit from HEIDENHAIN addresses the encoding requirements of linear motor applications.
“For certain motion systems, excess weight is a parameter to avoid, especially for those motion systems that can position to less than 1 µm,” said Kevin Kaufenberg, Product Manager at HEIDENHAIN Corp. “By reducing the size of the scanning unit and glass, the weight of the LIF 400 series linear encoder has been decreased, ensuring compatibility to these fine resolution motion systems.”
The miniature incremental exposed linear encoders also target motion systems that are decreasing in size to accommodate even smaller footprint machines. “The LIF 400 series is roughly 1⁄3 the size of a standard 20-µm resolution encoder,” noted Kaufenberg. “So, it perfectly fits to linear motors and motion systems using air bearings.”
Sense torque in a small space
A smart miniature screwdriver (TAT200) from FUTEK enables precise torque measurements in limited space. While customers may have had some input to the sensor’s design, engineering and production teams provided an active role in defining product requirements as well. They use the TAT200 within several of the company’s own sensor assemblies.
“The Miniature Torque Screwdriver is an auditing/assembly tool used to ensure that the torque applied is always proportional and never excessive upon a fixture/screw/bolt,” said Ehsan Mokhberi from the FUTEK Applications Team. “This Reaction Torque Sensor measures 50 in.-oz and 100 in.-oz capacities; the entire sensor only weighs in at 3 oz.”
Using the TAT200 with FUTEK’s USB Solutions and SENSIT Software creates a laboratory tool for data logging and displaying the graphing measurements on the operator’s computer screen. Alternatively, the sensor can be used with the company’s IHH500 Handheld Digital Display as an industrial tool for production and quality control measurements.
Linear position sensing with membrane potentiometers
Novotechnik engineers addressed the need for linear position sensing in a narrow space with LFP Series membrane potentiometers.
“We saw a general need for a very low profile sensor on the scale of an open sensor element but protected from fluid and dust so it could be applied ‘as is’ in some applications,” said Ivan Masik, President of Novotechnik U.S. “The first application uses the sensor as part of a linear electrical actuator and measures the vertical position of a surgical table.”
The design uses a screw-style pin attached to the application that runs across the membrane. Pressure applied through the pin connects the sensing track to a conductive ink track printed on a substrate. Movement across the membrane changes the resistance. This approach leads to a narrow design and opens up new application possibilities.
Masik said, “Other applications that this thin profile sensor would be a great match for include power windows and seats in vehicles. The LFP Series is well suited for applications where linear movement of an actuator already exists, therefore a shaft mechanism to drive the potentiometer is not needed. A simple electrical connection is provided by a three-pin connector.”
Compact may be small enough
Depending on the amount of added functionality that is squeezed into the package, compact may be small enough for some applications. Keyence engineers have determined that this is important for greater applications of bar code readers. The SR-750 Series of Auto ID products reflects this design philosophy. With new and advanced decoding algorithms, the company claims best in class reading capability.
“The SR-750 Series has the power to stably read the most difficult direct part marks despite its compact body,” said Eric Krupiczewicz, Sr. Technical Marketing Specialist at Keyence. “The small size allows installation close to machinery such as automated drills or welding irons [and yet] read codes without adding an index point or process step.”
Integrated lighting, lensing, processing, and communication are all achieved inside the unit’s footprint. Built-in communications include EtherNet/IP, TCP/IP, PROFINET, FTP, RS-232c and Power over Ethernet as well as data editing functionality. The trick is the added capability does not increase the reader’s size.
“The SR-750’s volume is 50% to 80% less than competitors’ similar 2D code readers,” explained Krupiczewicz.
For sensors, smaller is better
Whether the size reduction results in a sensor that is the industry’s smallest, one that is smaller than the competition or one that is smaller than other products within the same company, a reduced form factor opens the path toward increased applications. Additional features and improved performance increase the likelihood of being designed into those applications.
FUTEK
www.futek.com
HEIDENHAIN Corp.
www.heidenhain.us
Honeywell Sensing & Control
www.honeywell.com
Keyence
www.keyence.com/usa
Novotechnik
www.novotechnik.com
RLS, a Renishaw associate company
www.rls.si
