Something Fishy – TR Series Demo

Something Fishy – TR Series Demo

Merit Sensor introduced a new product demonstration for their TR Series MEMS pressure sensor at Sensor+Test in Nürnberg, Germany (May 19-21, 2015) and Sensors Expo in Long Beach California (June 9-11). “Something Fishy” was the brain child of 3 of our very talented engineers, Greg Liddiard, BJ Minson, and Chris Peterson, who designed and built a demonstration that showed the excellent stability and accuracy of the TR Series pressure sensor and its harsh media compatibility.

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TR Series Pressure Sensor

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TR Series Pressure Sensor with Ferrule Port

 

 

 

 

 

 

 

 

 


In Q2 of 2014 Rick Russell, President of Merit Sensor, announced an engineering competition for all of the engineers within the Sensors division to come up with a trade show and exhibition demonstration. The winning entry was “Something Fishy” by Greg Liddiard, BJ Minson, and Chris Peterson. Initially the design was a large clear tank that would contain many mechanical fish using pressure sensors to sit at a specific level within the tank. Someone would then stir up the water in the tank and send the mechanical fish off in different directions and at different depths, and as soon as the stirring stopped the fish would go back to their specific levels within the tank. This design posed an issue though as we could only control the height of the fish in the tank but not where they sat on the horizontal axis. As a result the three engineers decided to simplify the design by using just one mechanical fish (aka “Squido”) in a vertical tank. By using the single fish in a vertical tank we were able to show the accuracy and stability better than had we used the large tank with many fish. In addition this also afforded the engineers to connect the Squido via Bluetooth to a tablet and write an app to be able to control Squido in real time with a real time reading of the pressure within the tank.

Photo Jun 11, 11 31 53 AM Photo May 19, 2 47 05 AM

The Wheatstone Bridge

Introduction to the Wheatstone Bridge

The heart of Merit Sensor’s pressure sensors is a Wheatstone bridge that is comprised of a group of four resistors on a silicon etched diaphragm. As pressure is applied to the diaphragm the resistors are stressed, changing their resistance.

In an ideal setting, all of the resistors would be perfectly matched and completely temperature independent.

In the real world, however, differences exist between the resistance values of each resistor. In addition, temperature also changes resistor values. The change to resistor values and the overall bridge output due to temperature is known as the Temperature Coefficient of Resistance, or TCR.

Many applications require that a pressure sensor operate independently of temperature. In these applications, the pressure sensor’s TCR must be compensated for.

There are two general methods for TCR compensation – passive and active.

In passive compensation, the individual bridge resistor values will need to be measured in order to determine values needed for the compensation resistors.

In active compensation, a microcontroller, signal conditioner or analog circuit records the bridge output across various temperature and pressure conditions and adjusts sensor outputs accordingly.

Bridge configurations

a. Closed – A bridge in which all resistors are connected (See Figure 1).

AN103 Bridge Configuration Options - AN103-001
Figure 1 – Closed bridge

In a closed bridge there is no way to measure individual resistors as there will always be influences from the other three resistors of the bridge.

b. Half Open – A bridge that is divided into two branches and connected at one end (See Figure 2).

AN103 Bridge Configuration Options - AN103-002
Figure 2 – Half open bridge

In contrast to the closed bridge, a half open bridge allows measurements to be taken for each resistor, which is a benefit if the sensor’s performance needs to be determined. A half open bridge also allows for either active or passive compensation to be added as needed.

A half open bridge requires an additional electrical connection.

c. Full Open – A bridge that is divided into two branches, which are open at both ends (See Figure 3).

AN103 Bridge Configuration Options - AN103-003
Figure 3 – Full open bridge

Similar to the half open bridge, the full open bridge allows for each resistor to be measured. In addition to being able to use either active or passive compensation, each half of the bridge can be powered and measured independently. This is beneficial as some signal conditioners commonly used in pressure sensor applications require two independent branches.
However, the full open bridge configuration requires an additional electrical connection beyond that required by the half open configuration.

Examples of Implementations

a. Closed – Since individual resistors can’t be measured in a closed bridge, a closed bridge can be used with active compensation or in an application where sensor output fluctuations due to temperature changes are acceptable.

Figure 4 depicts a closed bridge with active compensation.

AN103 Bridge Configuration Options - AN103-004Figure 4 – Closed Bridge with Interface device (Signal Conditioning ASIC, Microcontroller, Analog circuitry, Etc.)

One example of a suitable application for a closed bridge where temperature independence is not critical is a pressure switch, where the absolute pressure measurements are not as important as knowing you’ve reached a pressure threshold.

b. Half Open – Active compensation can be applied to the half open bridge as in Figure 4 shown above. Passive compensation can also be applied to the half open bridge as seen below in Figure 5.

AN103 Bridge Configuration Options - AN103-005

Figure 5 – Half open bridge with passive compensation

The implementation of a half open bridge with passive compensation in Figure 5 shows the added components as well as the extra electrical connection (Vin+) required to close the bridge. The additional resistors, as they are named, accomplish span, zero and output impedance compensation. These components need to be added after open bridge measurements have been taken at the required conditions.

C. Full Open – The full open bridge has a wide variety of implementations. In addition to being used as a full open bridge it can be used as a half open (Figure 5) or a closed bridge (Figure 4). Figure 6 is an illustration of how a full open bridge could be used for two functions – temperature and pressure.

AN103 Bridge Configuration Options - AN103-006

Figure 6 – Full open bridge with two functions

In this implementation, half of the bridge is being used as a temperature sensor and the other is being used as a pressure sensor. Only half the pressure output signal will be present because there is only the voltage swing of half the bridge. However, this allows for the added benefit of a means to measure actual die temperature. The temperature measurement will allow for a more accurate input for temperature compensation than an ambient temperature measurement would.

Choosing the Appropriate Configuration for your Application

The entire sensing system should be taken into consideration when making decisions about the bridge configuration. First, the user must decide if temperature independence is important and if it is, whether active or passive compensation will be used. If active compensation is chosen and a signal conditioner or other electronic device will be used, that device’s requirements must be met.

Use caution as devices with similar functions may have very different requirements.

As previously discussed, each configuration has its own benefits and drawbacks. The added electrical connections of a full open bridge add to the complexity of assembly but allow for more flexibility as well as the ability to more easily troubleshoot bridge issues.

Ultimately, the choice of bridge configuration should be based on a thorough analysis of the system.

 

 

 

 

 

 

 

Disclaimer Notice
Merit Sensor Systems produces high quality products that perform within the parameters of the data sheet. Typical pressure and temperature performance values are not tested 100% but they have been validated during qualification. Merit Sensor cannot guarantee that the product will function properly after mounting and post processing by the customer. It is the responsibility of the customer to test and to qualify the function of the Merit Sensor product in the final package. Customer is responsible for the required knowledge to handle product and Merit Sensor assumes no liability for consequential damages that may result in yield loss or field failures in the final application.
THE INFORMATION IN THIS DOCUMENT IS PROVIDED TO YOU “AS IS” WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, WHETHER EXPRESS OR IMPLIED. Merit Sensor Systems reserves the right to modify this document and assumes no responsibility for any failures that result from the use of this information. All information provided in this document is for illustration purposes only.

Pressure Sensors for Harsh Environments and Extended Temperatures

Many pressure sensor applications in rugged environments such as aerospace, automotive, industrial, and even medical equipment present developers with conflicting requirements that lead to expensive compromises. These sensors are often used to measure pressure, level, and flow of harsh fluids such as gas, oil, refrigerant, or other caustic solvents that can damage the sensor element. Extended temperature requirements offer additional challenges, even beyond compensation for accurate pressure readings. Automotive and aerospace specifications are especially stringent, with operating temperature ranges as broad as ‑40°C to +150°C. And these rugged applications typically have high reliability and accuracy requirements, since component failure can lead to product recalls or safety risk. To respond, equipment manufacturers rely on expensive ongoing maintenance and component replacement to work around the sensor’s inherent short lifespan.

The packaging of the sensor component is critical in solving this issue, but is a challenge that has, until now, eluded sensor manufacturers. Consider a typical use case. An automotive application such as gas or diesel fuel-line sensing requires a sealed sensor element that can be installed inside the fuel line to detect pressure changes that indicate a plugged fuel filter, which provides a feedback signal to the car’s computer to notify the driver. Similar requirements occur in airplane engine, gear, and valve controls, and in measurement and control of compressors or leak-detection systems in industrial equipment. While medical applications may not require the pressure sensor to operate in fluids as harsh as gasoline, even saline solution can be corrosive over time, and the cleaning and sterilization process often requires repeated contact with caustic chemicals such as bleach.

The problem is that the adhesives that are used to create the pressure seal and protect the sensor die and related circuitry eventually soften in harsh fluids. Once the seal breaks, the sensor circuitry is damaged, creating a common reliability failure that can be expensive if it causes a product recall or demands regular maintenance and replacement of the sensing subsystem.

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Figure 1. Sensor package showing back-side entry to protect electronic circuitry from harsh media. [Source: Merit Sensor Systems]

Extended temperature requirements increase the packaging challenge. While some new adhesives may withstand higher temperatures than was possible in the past, they still risk die detachment at pressures of 300 psi, and humidity can destroy the bond strength of most adhesives. There are exotic epoxies that can withstand some temperature and humidity extremes, but storage and application raise additional manufacturability issues, and these epoxies can impact accuracy of the sensing element in extended temperature applications.

Extended temperature requirements increase the packaging challenge. While some new adhesives may withstand higher temperatures than was possible in the past, they still risk die detachment at pressures of 300 psi, and humidity can destroy the bond strength of most adhesives. There are exotic epoxies that can withstand some temperature and humidity extremes, but storage and application raise additional manufacturability issues, and these epoxies can impact accuracy of the sensing element in extended temperature applications.

For a pressure sensor to perform well from ‑40°C to +150°C, it requires a stable MEMS element as well as stable packaging and manufacturing processes. But instability typically occurs because of differences in the thermal coefficients of expansion (TCE) of the MEMS die and the substrate on which it is mounted. Stainless steel might be considered an ideal substrate, but its TCE is much higher than that of silicon. As temperature changes, the metal expands and contracts while the silicon elements soldered onto it experience much smaller changes. The MEMS element reacts to the stresses caused by the TCE differences, triggering errors that look to the system like pressure changes—presenting system designers with a new reliability issue.

An innovative new pressure sensor packaging approach creates a eutectic die bond on ceramic substrate using a gold-tin soldering alloy for a hermetic seal even in harsh fluids, at high pressure, and at extremely wide temperature ranges. The ceramic substrate has a TCE that is close to silicon so there’s no significant thermal mismatch, and gold and tin are common soldering elements that hold up well to harsh fluids. While their high individual melting points can impact manufacturability, a gold-tin soldering bond with an 80:20 ratio creates an alloy with a much lower melting point, which improves manufacturability while still maintaining the benefits of both metals in harsh environments. And while this gold-tin solder is more expensive than adhesive, the cost differential is minute in comparison to the significant improvement in long-term reliability and maintenance costs.

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Table 1. Comparison of pressure sensor package types across harsh application requirements.

An additional element to consider in comparing sensor packaging approaches is whether the pressure media comes in at the top or back side of the sensor. If the pressure is on the top side of the sensor, the circuitry must be protected from shorts or corrosion, which is typically accomplished with a protective gel. But a gel that is stiff enough to withstand corrosive fluids and high temperatures can also be stiff enough to cause stress to the MEMS element, which, again, generates sensing errors. In contrast, back-side entry exposes only the silicon, glass, and the eutectic die attach to the pressure medium—and these elements have been proven to withstand these harsh environments.

Figure 2. Merit Sensor harsh-media, extended-temperature sensors are available with optional ferrule (left), polycarbonate pressure ports (middle) for radial pressure sealing and standard face seal (right). Solder pins are also available to simplify system design. [Source: Merit Sensor Systems]

System developers who require pressure sensing capabilities in harsh-media, extended-temperature applications have discovered that packaging is critical to improve product lifetime reliability and reduce cost of ownership. It is a challenge that has finally been solved.

Scott Sidwell is sensors engineering manager for Merit Sensor Systems. He can be reached at [email protected]. For more information, see https://meritsensor.com.