SENSOR CHARACTERISTICS (5)

17 Environmental Factors

Storage conditions are nonoperating environmental limits to which a sensor may be subjected during a specified period without permanently altering its performance under normal operating conditions. Usually, storage conditions include the highest and the lowest storage temperatures and maximum relative humidities at these temperatures.

The word “noncondensing” may be added to the relative humidity number.Depending on the sensor’s nature, some specific limitation for the storage may need to be considered (e.g., maximum pressure, presence of some gases or contaminating fumes, etc.).

Short- and long-term stabilities (drift) are parts of the accuracy specification. The short-term stability is manifested as changes in the sensor’s performance within minutes, hours, or even days. The sensor’s output signal may increase or decrease, which, in other terms, may be described as ultralow-frequency noise. The long-term stabilitymaybe related to aging of the sensor materials, which is an irreversible change in the material’s electrical, mechanical, chemical, or thermal properties; that is, the long-term drift is usually unidirectional. It happens over a relatively long time span, such as months and years.

Long-term stability is one of the most important for sensors used for precision measurements. Aging depends heavily on environmental storage and operating conditions, how well the sensor components are isolated from the environment, and what materials are used for their fabrication. The aging phenomenon is typical for sensors having organic components and, in general, is not an issue for a sensor made with only nonorganic materials. For instance, glass-coated metaloxide thermistors exhibit much greater long-term stability compared to poxy-coated thermistors.Apowerful way to improve long-term stability is to preage the component at extreme conditions. The extreme conditions may be cycled from the lowest to the highest. For instance, a sensor may be periodically swung from freezing to hot temperatures.

Such accelerated aging not only enhances the stability of the sensor’s characteristics but also improves the reliability (see Section 18), as the preaging process reveals many hidden defects. For instance, epoxy-coated thermistors may be greatly improved if they are maintained at +150°C for 1 month before they are calibrated and installed in a product.

Environmental conditions to which a sensor is subjected do not include variables which the sensor measures. For instance, an air-pressure sensor usually is subjected not just to air pressure but to other influences as well, such as the temperatures of air and surrounding components, humidity, vibration, ionizing radiation, electromagnetic fields, gravitational forces, and so forth. All of these factors may and usually do affect the sensor’s performance. Both static and dynamic variations in these conditions should be considered. Some environmental conditions are usually of a multiplicative nature; that is, they alter a transfer function of the sensor (e.g., changing its gain). One example is the resistive strain gauge, whose sensitivity increases with temperature.

Environmental stability is quite broad and usually a very important requirement. Both the sensor designer and the application engineer should consider all possible external factors which may affect the sensor’s performance.Apiezoelectric accelerometer may generate spurious signals if affected by a sudden change in ambient temperature, electrostatic discharge, formation of electrical charges (triboelectric effect), vibration of a connecting cable, electromagnetic interference (EMI), and so forth.

Even if a manufacturer does not specify such effects, an application engineer should simulate them during the prototype phase of the design process. If, indeed, the environmental factors degrade the sensor’s performance, additional corrective measures may be required. (e.g., placing the sensor in a protective box, using electrical shielding, using a thermal insulation or a thermostat).

Temperature factors are very important for sensor performance; they must be known and taken into account. The operating temperature range is the span of ambient temperatures given by their upper and lower extremes (e.g., -20°C to +100°C) within which the sensor maintains its specified accuracy. Many sensors change with temperature and their transfer functions may shift significantly.

Special compensating elements are often incorporated either directly into the sensor or into signal conditioning circuits, to compensate for temperature errors. The simplest way of specifying tolerances of thermal effects is provided by the error-band concept, which is simply the error band that is applicable over the operating temperature band.

A temperature band may be divided into sections, whereas the error band is separately specified for each section. For example, a sensor may be specified to have an accuracy of ±1% in the range from 0°C to 50°C,±2% from-20°C to 0°C and from+50°C to 100°C and ±3% beyond these ranges within operating limits specified from -40°C to +150°C.

Temperatures will also affect dynamic characteristics, particularly when they employ viscous damping. A relatively fast temperature change may cause the sensor to generate a spurious output signal. For instance, a dual pyroelectric sensor in a motion detector is insensitive to slowly varying ambient temperature. However, when the temperature changes quickly, the sensor will generate an electric current that may be recognized by a processing circuit as a valid response to a stimulus, thus causing a false-positive detection.

A self-heating error may be specified when an excitation signal is absorbed by a sensor and changes its temperature by such a degree that it may affect its accuracy. For instance, a thermistor temperature sensor requires passage of electric current, causing heat dissipation within the sensor’s body.

Depending on its coupling with the environment, the sensors’ temperature may increase due to a self-heating effect. This will result in errors in temperature measurement because the thermistor now acts as an additional spurious source of thermal energy. The coupling depends on the media in which the sensor operates—a dry contact, liquid, air, and so forth. A worst coupling may be through still air. For thermistors, manufacturers often specify self-heating errors in air, stirred liquid, or other media.

A sensor’s temperature increase above its surroundings may be found from the following formula:

   ΔT° =V² / (ξνc+α)R       (25)

where ξ is the sensor’s mass density, c is specific heat, v is the volume of the sensor, α is the coefficient of thermal coupling between the sensor and the outside (thermal conductivity), R is the electrical resistance, and V is the effective voltage across the resistance. If a self-heating results in an error, Eq. (25) may be used as a design guide.

For instance, to increase a, a thermistor detector should be well coupled to the object by increasing the contact area, applying thermally conductive grease or using thermally conductive adhesives. Also, high-resistance sensors and low measurement voltages are preferable.

18. Reliability

Reliability is the ability of a sensor to perform a required function under stated conditions for a stated period. It is expressed in statistical terms as a probability that the device will function without failure over a specified time or a number of uses. It should be noted that reliability is not a characteristic of drift or noise stability. It specifies a failure, either temporary or permanent, exceeding the limits of a sensor’s performance under normal operating conditions.

Reliability is an important requirement; however, it is rarely specified by the sensor manufacturers. Probably, the reason for that is the absence of a commonly accepted measure for the term. In the United States, for many electronic devices, the procedure for predicting in-service reliability is the MTBF (mean time between failure) calculation described in MIL-HDBK-217 standard. Its basic approach is to arrive at a MTBF rate for a device by calculating the individual failure rates of the individual components used and by factoring in the kind of operation the device will see: its temperature, stress, environment, and screening level (measure of quality).

Unfortunately, the MTBF reflects reliability only indirectly and it is often hardly applicable to everyday use of the device. The qualification tests on sensors are performed on combinations of the worst possible conditions. One approach is 1000 h, loaded at maximum temperature. This test does not qualify for such important impacts as fast temperature changes. The most appropriate method of testing would be accelerated life qualification. It is a procedure that emulates thesensor’s operation, providing real-world stresses, but compressing years into weeks.

Three goals are behind the test: to establish MTBF; to identify first failure points that can then be strengthened by design changes; and to identify the overall system practical lifetime. One possible way to compress time is to use the same profile as the actual operating cycle, including maximum loading and power-on, power-off cycles, but expandedenvironmental highest and lowest ranges (temperature, humidity, and pressure). The highest and lowest limits should be substantially broader than normal operating conditions.

Performance characteristics may be outside specifications, but must return to those when the device is brought back to the specified operating range. For example, if a sensor is specified to operate up to 50°C at the highest relative humidity (RH) of 85% at a maximum supply voltage of +15 V, it may be cycled up to 100°C at 99% RH and at +18 V power supply. To estimate number of test cycles (n), the following

empirical formula may be useful:

n = N [ΔT_max / ΔT_test] ^2.5        (26)

where N is the estimated number of cycles per lifetime, ΔTmax is the maximum specified temperature fluctuation, and ΔTtest maximum cycled temperature fluctuation during the test. For instance, if the normal temperature is 25°C, the maximum specified temperature is 50°C, cycling was up to 100°C, and over the lifetime (say, 10 years), the sensor was estimated to be subjected to 20,000 cycles, then the number of test cycles is calculated as:

n = 20,000 [(50-25) / (100-25)]^2.5 = 1283.

As a result, the accelerated life test requires about 1300 cycles instead of 20,000. It should be noted, however, that the 2.5 factor was derived from a solder fatigue multiple, because that element is heavily influenced by cycling. Some sensors have no solder connections at all, and some might have even more sensitivity to cycling substances other than solder, (e.g, electrically conductive epoxy). Then, the factor

should be selected to be somewhat smaller. As a result of the accelerated life test, the reliability may be expressed as a probability of failure. For instance, if 2 out of 100 sensors (with an estimated lifetime of 10 years) failed the accelerated life test, the

reliability is specified as 98% over 10 years.

A sensor, depending on its application, may be subjected to some other environmental effects which potentially can alter its performance or uncover hidden defects. Among such additional tests are:

• High temperature/high humidity while being fully electrically powered. For instance, a sensor may be subjected to its maximum allowable temperature at 85–90% RH and kept under these conditions for 500 h. This test is very useful for detecting contaminations and evaluating packaging integrity. The life of sensors, operating at normal room temperatures, is often accelerated at 85°C and 85% RH,

which is sometimes called an “85–85 test.”

• Mechanical shocks and vibrations may be used to simulate adverse environmental conditions, especially in the evaluation wire bonds, adhesion of epoxy, and so forth.Asensor may be dropped to generate high-level accelerations (up to 3000g of force). The drops should be made on different axes. Harmonic vibrations should be applied to the sensor over the range which includes its natural frequency. In the United States military standard 750, methods 2016 and 2056 are often used for mechanical tests.

• Extreme storage conditions may be simulated, for instance at +100 and -40°C while maintaining a sensor for at least 1000 h under these conditions. This test simulates storage and shipping conditions and usually is performed on nonoperating devices. The upper and lower temperature limits must be consistent with the sensor’s physical nature. For example, TGS pyroelectric sensors manufactured in the past by Philips are characterized by a Curie temperature of +60°C. Approaching and surpassing this temperature results in a permanent destruction of sensitivity. Hence, the temperature of such sensors should never exceed +50°C, which must be clearly specified and marked on its packaging material.

• Thermal shock or temperature cycling (TC) is subjecting a sensor to alternate extreme conditions. For example, it may be dwelled for 30 min at -40°C, then quickly moved to +100°C for 30 min, and then back to cold. The method must specify the total number of cycling, like 100 or 1000. This test helps to uncover die bond, wire bond, epoxy connections, and packaging integrity.

• To simulate sea conditions, sensors may be subjected to a salt spray atmosphere for a specified time, (e.g., 24 h). This helps to uncover its resistance to corrosion and structural defects.

Reference Books About Sensor