Sensors Detect Metal Targets In Extreme Heat & Cold
For sensing applications exposed to temperatures from -25 degrees C to +180 degrees C, Baumer has introduced the new IFRH Series of Inductive High Temperature Sensors, a family of two-part proximity switches available in M8, M12 and M18 designs with chromium nickel steel housings.
Intended for flush-face installation, the IP67-rated IFRH sensors are designed to detect the presence of metal targets. With an overall length of just 30 mm, the M8 sensor is specifically designed for use in limited-space applications. Depending upon the housing size, nominal switching distance ranges from 1.5 mm to 5 mm.
An FEP jacketed cable connects the sensor’s temperature-sensitive processing electronics to the sensor head, protecting the electronics from damage. The sensors offer an operating voltage range between 10…30 VDC. These sensors are appropriate for use in aerospace, automotive, marine, PCB fabrication, food and beverage processing, oil processing, laboratory automation, rubber and plastics fabrication, chemical processing, packaging, print/graphics, textile production, and water treatment applications.
Designing with Semiconductor Temperature Sensors
July 13, 2009 by admin
Filed under Temperature
By John R. Gyorki
Editorial Director
Semiconductor temperature sensors are easy to use if you do your homework when matching the sensor with the application.
Temperature sensors are designed into every heating, ventilation, and air conditioning system. Automotive systems rely on temperature sensors for engine control. Many industrial processes require highly accurate and stable temperature measurements. Today’s computers and portable electronic devices have extremely high circuit density with no easy way for heat removal, which makes temperature monitoring a must.
All temperature sensor applications fall into the temperature measurement or thermal management category. The most widely used types of temperature sensors are thermocouples, resistance temperature detectors (RTDs), thermistors, and semiconductor temperature sensors.
Thermocouples are known for their wide temperature range and low drift. RTDs have the highest accuracy and stability, and good linearity. Thermistors are fast and have high sensitivity. However, all these sensors require a modest amount of interface circuitry and are difficult to use. Relative newcomers, semiconductor temperature sensors offer several benefits that other sensors cannot match.
Though their temperature range of -50oC to +150oC is rather limited, semiconductor temperature sensors have several valuable features such as excellent linearity, high sensitivity, and extremely small size. They are inexpensive; require no linearization, cold junction compensation, or signal conditioning; often combine several functions on one chip; and produce analog, logic, or digital outputs that can be interfaced directly to an analog measurement circuit, an analog-to-digital converter (ADC), a microprocessor, or a fan control. Measurement-system noise immunity is superior to other sensors since the output is already amplified or digitized inside the integrated circuit (IC).
The key to effective thermal management is knowing when to turn on the fan and when to pull the plug. Semiconductor temperature sensors, some as small as 115 by 63 by 40 mils, are smart enough to know when to do it.
Thanks to their small size, semiconductor sensors can be installed directly on PC boards, heat sinks, underneath high-power integrated circuits, and in small portable electronic devices. Typical applications include the fastest growing types of consumer products such as cell phones, PCs, PDAs, MP3 players, and automotive systems.
The sensing element
A semiconductor temperature sensor is an IC that combines a temperature-sensing element with signal conditioning, output, and other types of circuitry on one chip. It relies on the change of voltage across a p-n junction, essentially a silicon diode, in response to a temperature change to determine the ambient temperature. The bipolar IC substrate is designed to build p-n-p and n-p-n transistors, so in practice, the sensing diode is usually formed using a transistor with the base and collector shorted. The following equation shows the effect of temperature on the forward voltage of a silicon p-n junction:
VBE = VG0 (1-T/T0) + VBE0(T/T0) + (ηKT/q) ln (T0/T) + (KT/q) ln (IC/IC0)
Where:
T = temperature, oK (Kelvin)
T0 = reference temperature, oK
VG0 = bandgap voltage at absolute zero, V
VBE0 = forward voltage at current IC0 and temperature T0, V
K = Boltzmann constant, J/oK
q = charge of an electron, C
η = constant associated with a specific device
IC = forward current at temperature T, A
IC0 = forward current at reference temperature T0, A
A silicon bipolar diode (a.) is the temperature-sensing element. When biased with constant current, its voltage drop varies with temperature. When made as part of an IC, the diode is created by shorting the base and the emitter of a p-n-p transistor (b.). Switching between two current sources (c.) cancels the effect of the process-dependent saturation current on the accuracy.
However, determining temperature using this equation is not very practical since the voltage depends not only on the temperature, but also on the current and the device properties. To eliminate the effect of these variables, practical circuits switch the diode alternately to two constant current sources and determine the temperature based on the difference between the two forward voltages or their ratio:
Δ VBE = (KT/q) ln (IC1/IC2)
Where:
IC1 = forward current from first current source, A
IC2 = forward current from second current source, A
Frequently IC2 is selected to be 10 times greater than IC1. In this case, the thermal coefficient of the sensing element is approximately 200 µV/ oC.
Sensor types
Semiconductor temperature sensors come in three types determined by the output signal they produce: analog, logic, and digital. They also measure temperature in one of two ways: internally or remotely. When the measuring diode-connected transistor is built on the sensor IC substrate, the sensor measures the temperature of its own body. Sensors that have no measuring diode built-in connect to a remotely mounted sensing diode-connected transistor.
Analog Voltage and Current Output Sensors: The oldest type, analog output semiconductor temperature sensors, generates output voltage or current proportional to sensed temperature. They combine the sensing diode with the offset and amplification circuitry on one chip. The offset is needed because the output signal of the sensing diode is proportional to absolute temperature, oK. Since oC and oF scales are more common than oK, the signal needs to be offset to the new zero oC or zero oF point. Additional offset is needed to allow measuring negative temperature without using a negative power supply.
Since the voltage across the sensing diode changes only 200 µV per oC an internal amplifier is needed to boost the output signal in order to increase the system noise immunity. This makes the voltage output compatible with most analog temperature measuring and monitoring systems. Industrial applications that have high levels of electrical noise use current output IC sensors where output current is proportional to the measured temperature.
These sensors produce voltage or current directly proportional to the temperature measured by the sensing-diode-connected transistor. The offset and amplification circuitry allows positive and negative temperature measurements in oC or oF scales.
Logic Output Sensors: Many temperature control applications do not need to know what the current temperature is until it reaches a preset level–when some action needs to be taken. Logic output sensors designed for this type of applications work like a thermostat in our homes. In fact, these sensors are commonly referred to as thermostat sensors. They do not output the temperature value, but instead one or several logic outputs change their state when temperature exceeds (“hot” sensors) the preset value, or drops below (“cold” sensors) it.
The “hot” logic output senor changes its output state when the temperature exceeds the preset value using resistor RS: Resistor RH controls hysteresis.
Some devices have user-programmable trip temperature and hysteresis, which is needed to prevent chatter. The programming is usually done with resistors. Yet other devices have both the temperature and the hysteresis fixed. Logic temperature sensors are simple, inexpensive and are extremely easy to use. Typical applications include turning on fans and alarms, or setting an interrupt to signal the processor that the temperature limit has been exceeded.
Digital or Serial Output Sensors: A digital temperature sensor is really an analog sensor that contains an ADC and a serial output port. Sensor output options include an I2C, SMBus, or SPI interface that connects directly to a compatible microprocessor/microcontroller interface or a pulse-width modulation (PWM) output. In addition, many sensors have one or several comparators that drive either logic, open collector, or open drain outputs. These outputs are typically wired as an
interrupt for the microcontroller, an on/off line for a fan or alarm, or a hardware power shut-off control signal.
Digital sensors using the single-wire PWM format vary the ratio of high time to low time (duty cycle) of a square wave to represent the temperature value. The processor counts the duration of the high and low times and calculates the temperature based on the ratio. The clock frequency accuracy is not critical since PWM relies on the time ratio, not on the absolute duration.
The I2C, SMBus, and SPI interfaces allow bidirectional communications between the sensor and the microprocessor or microcontroller. The sensor sends the temperature value to the processor, and the processor uses the same interface to program sensor registers that control temperature limits and other functions.
The I2C and SMBus ports require two wires and are frequently used in computer applications. The SPI interface can be configured as a three-wire or four-wire port used most frequently with microcontrollers and in automotive systems.
Temperature sensing can be done locally or remotely. Most semiconductor temperature sensors have the diode-connected transistor on board and thus measure the temperature of their own body. Remote-sensing sensors have no sensing element on board and instead connect to a remotely mounted diode-connected p-n-p or n-p-n transistor. The temperature-sensing transistor can be located tens of feet away from the remote sensor IC. However, wires leading to the remotely installed sensing transistor require good shielding due to very low signal levels susceptible to noise.
PC thermal protection example
As computers (especially laptops and notepads) pack more computing power, thermal management becomes one of the most critical design issues. CPUs, graphics processors, and other highly integrated chips contain thousands of transistors, and their high clock rates increase dissipated heat due to switching losses. Simply running a fan is not a good option since fans produce audible noise and their service life is shortened when run continuously.
A typical digital temperature sensor combines an analog sensor with an ADC and a serial bus interface. The microcontroller can program additional logic outputs.
Today’s dense computing and portable electronic devices require sophisticated thermal management, and semiconductor temperature sensors provide this level of sophistication. These ICs combine a temperature sensor with additional functional circuitry and are frequently installed on a heat sink, directly under the microprocessor or a graphics chip. Some CPU ICs have a temperature-sensing transistor made on its own die that can be connected to the external remote-diode temperature sensor for monitoring.
An effective computer thermal management system can be built around a logic output temperature sensor with Low, High, and Control outputs. If the CPU temperature reaches the first preset point above the normal operating temperature due to high ambient temperature, exposure to heat, component failure, or blocked ventilation ports, the Low sensor output changes state and sends a signal to the CPU to reduce the clock rate to lower switching losses and heat dissipation.
Should temperature continue to rise and cross the second set point just below the maximum allowable CPU temperature, the sensor’s High and Control logic outputs change state. The Control output turns on the cooling fan and the High output commands the CPU to further reduce the clock rate. Should the temperature remain above the High set point after a certain amount of time, the computer will be powered down to prevent permanent damage. However, the most reliable shutdown mode bypasses the CPU since at this point the software might not be running correctly. Instead, an output from the sensor is wired directly to the shutdown input of the power supply.
An alternative approach uses a serial output temperature sensor where the CPU periodically polls the sensor and reads the actual temperature through a serial port. If the temperature reaches excessive levels, the processor will attempt to reduce the heat. If the temperature exceeds the absolute maximum limit, the sensor’s comparator output will directly shut down the power supply.
Things to watch
IC temperature sensors are not free from self-heating. Some devices have relatively high operating current, the main cause of self-heating, and this might become an accuracy concern when measuring the temperature of still air, for example. The thermal transfer between the sensor plastic housing and still air is not very efficient, so self-heating can cause the sensor-body temperature to rise above the temperature of the air. Since that sensor actually measures the temperature of its own body, the readings will be higher and thus reduce the accuracy.
Heat radiated or conducted through the printed circuit board by other components near the sensor can also affect the measurement accuracy, so when finding the best position for the sensor, the whole system needs to be carefully reviewed.
For more information go to:
Analog Devices, Inc.
http://www.analog.com
Maxim Integrated Products
http://www.maxim-ic.com
Microchip Technology, Inc.
http://www.microchip.com
National Semiconductor Corp.
http://www.national.com
Texas Instruments, Inc.
http://www.ti.com
Combination Liquid Level and Temperature Switch
May 13, 2009 by admin
Filed under Level, Liquid Level, Temperature
A liquid level and temperature switch was designed into one assembly to reduce the cost of a fluid system. Tempco can design a custom sensor to meet many applications such as industrial and commercial degreasers and hot liquid dispensing machines.
Liquid Level Sensor Options
The liquid level sensing can be obtained using magnetic floats and switches, or the conductivity method. The conductivity method uses the simple concept of the conductive property of liquids to complete a circuit and cause an external control relay to actuate.
Temperature Sensor Options
Thermocouples, RTD’s, thermistors and thermostat switches with an assortment of accuracies can be incorporated into the assembly.
Tempco Electric Heater Corporation designs and manufactures electric heating elements,
temperature sensors, temperature controllers and turnkey process heating systems.
Tempco
www.tempco.com
Micro-machined Silicon Pressure Transducers
March 25, 2009 by R. Nelson Parrish
Filed under Pressure
Omegadyne announces the most durable version of its high accuracy MM Series micro-machined silicon pressure transducers. The new hermetically sealed version of the MM Series features all welded Stainless Steel construction, 316 wetted parts and glass to metal seals (GMS) at the electrical outlets. This ensures that the unit is hermetically sealed from external environments and the media. Designed for use on automotive and aircraft test platforms and anywhere environmental concerns demand the most durable characteristics.
The sealed MMA500V Series has a micro machined silicon core that provides high accuracy, low drift and excellent long term stability in the harshest environments. Ranges from 100 psi to 5000 psi, accuracies from 0.08% to 0.03% and a variety of pressure and electrical connections make this MMA500V Series Transducer extremely versatile. The temperature compensation range can be as broad as -40 to 200*F (-40 to 104*C) and thermal errors as low as +/- 0.3% over the compensated range. Operating temperature range is -49 to 250*C (-45 to 121*C). The MMA500V Series design is further ruggedized with a secondary containment system in the event of diaphragm rupture. A 5-Point NIST traceable calibration certificate included.
The MM Series pressure transducers offer “Custom Designed” features with fast delivery. Our modular design allows you to construct a pressure sensor to meet your application requirements, with guaranteed fast delivery… typically from stock to 2 weeks. Our product configurator found online at omegadyne.com allows quick and easy configuration of a transducer that meets the exact needs of your project.
Designing with Thermistors
March 11, 2009 by Editor
Filed under Temperature
John R. Gyorki, Editorial Director
Temperature sensor applications usually fall into one of three general categories; monitoring, control, or circuit compensation, and four sensor types; thermocouples, thermistors, resistance-temperature detectors (RTD), and semiconductor temperature sensors. When selecting a sensor, some key characteristics to consider include temperature range, accuracy, response time, minimal temperature effect on the measured object, and the type of signal conditioning required. Other factors are long-term stability, mechanical ruggedness, and cost.
Unleaded NTC thermistor discs are frequently used in numerous automobile engine sensors to measure air and coolant temperature. The discs are located inside the tip of the housing, usually under a spring-load to maintain contact pressure.
The table compares thermistor characteristics with other types of temperature sensors and shows that thermistor devices are essentially passive variable resistors and require excitation current to produce an output signal. In other words, you cannot just connect a voltmeter across the leads of a thermistor, touch the sensor to a hot object, and expect to see a voltage.
Thermistors have a considerably higher sensitivity than most other sensors, but they are also much less linear. Although special high-temperature sensors, such as chromium oxide ceramic thermistors made by GE Sensing can operate up to 1000oC, conventional devices have a relatively narrow temperature range and are not an optimal choice when long-term accuracy is required. However, thermistors are usually less expensive than the other sensors and react faster to temperature changes.
All sensors require linearization, but each to a different degree. Also, to achieve high accuracy the circuit must be calibrated with the actual thermistor sensor connected. These two tasks can be accomplished with analog conditioners and calibration circuits, but they can be quite complex and require manual calibration. If a digital design is used instead, the sensor signal is digitized by an analog-to-digital converter (ADC) and the linearization and calibration are done in software with
minimum operator involvement.
Thermistor Fundamentals
Thermistors are solid-state, temperature-sensitive resistors that come in two types: negative temperature coefficient (NTC) and positive temperature coefficient (PTC). As the names imply, the resistance of an NTC thermistor is inversely proportional to temperature, whereas the resistance of a PTC thermistor is directly proportional. The sensors’ terminal resistance changes with the temperature change of the thermistor body, which can come from ambient heat, self-heating due to excitation current, or both.
PTC thermistors are used most often for circuit-overload protection, compared to NTC devices that are used primarily for temperature measurement and compensation. This article focuses on temperature measurement devices, so only NTC thermistors are discussed.
The resistance of an NTC thermistor decreases with an increase in its body temperature, however, the rate of resistance change is not linear. It is greatest at the lower temperature limit and gradually diminishes as the temperature increases.
NTC thermistors are a sintered mixture of metallic oxides, which include nickel, cobalt, manganese, and sometimes other oxides. The elements are formed as beads, chips, discs, rods, or thin-films. Bead thermistors are drops of semiconductor paste deposited on two platinum alloy wires, sintered at a high temperature. The wires are then cut to make individual thermistors. Chip and disc thermistors are fabricated as a thin sheet of material (wafer), and sintered at high temperature. The sides are silvered for attaching leads, and the wafers are cut into discs or chips. Rod thermistors are simply extruded.
Thermistor elements can be glass encapsulated, epoxy coated, or remain uncoated (bare). Bare thermistors respond faster, are smaller, and cost less, but they have no provisions for protection from the environment and mechanical impact. An epoxy coating can protect the device from the environment, but it slightly slows the response time and increases the cost. Glass encapsulation ensures a hermetic seal, high-voltage insulation, and resistance to corrosive atmospheres. Long-term stability of glass encapsulated parts is typically ten times better than the stability of epoxy coated parts.
Mounting features include unleaded discs that require spring-loaded contacts, silver or gold electrodes for wire bonding, and surface mounting provisions such as those for SMD chips. The leads can be axial or radial, bare or insulated, and straight or kinked. Axial lead and SMD parts are intended for automatic PCB insertion and pick-and-place equipment. Radial-lead devices and unleaded discs are well suited for temperature probe assemblies.
An unleaded NTC disc thermistor (a.) is commonly found in temperature probes. Adding radial leads to an uncoated disc thermistor (b.) lets it mount on a printed circuit board. Coating the disc thermistor with epoxy (c.) protects it from the environment. Epoxy-coated chip thermistors with flexible insulated leads are ideal for installations with limited space.
(Photos courtesy of GE Sensing & Inspection Technologies, Billerica, MA.)
One special type, thin-film thermistors, are deposited on a ceramic or flexible Kapton® base, only several tens of thousands of an inch thick. They have low dissipation values and fast reaction times due to their small mass. For example, TF series of thin-film NTC thermistors from Selco Products Company, are suitable for a -50oC to +90oC temperature range and have a dissipation value of 0.7 mW/oC with a thermal time constant of 2 s, both in still air. They are ideal for air and other gas temperature measurements as well as probe assemblies.
Accuracy ratings vary greatly between different devices, depending on the application. For example, general-purpose disc thermistors typically have tolerances that range from ± 20% to ± 2%, and interchangeable thermistors can have accuracies as high as ± 0.05oC in a narrow temperature range. They are available for probe replacement without system recalibration. For example, U.S. Sensor’s PR103J2 ultra-precision, interchangeable 10-kΩ thermistor is a highly accurate and stable sensor that matches the J-type NTC thermistor’s R-T curve with ± 0.05oC accuracy from 0oC to 50oC. Other resistance values from 2 kΩ to 50 kΩ are also available.
Several thermistor-related terms that are listed in catalogs and data sheets can help you select parts:
• The zero-power resistance, Ro, is a dc resistance specified at a particular temperature and an excitation current so small that the self-heating produced by power dissipation can be neglected. This special temperature is called the Standard Reference Temperature, and is typically 25oC.
• The resistance ratio characteristic is a ratio of zero-power resistance measurements made at two specific temperatures. It is typically the ratio of resistance at 25oC to the resistance at 125oC.
• The thermal time constant, τ, is the time in seconds required for a thermistor that dissipates zero power to change its body temperature 63.2% of the total temperature change in response to a step-function change in ambient temperature. This parameter characterizes the speed with which a thermistor can react to fast temperature changes and helps compare the response time of different devices.
• The dissipation constant, δ, is a ratio of the change in thermistor power dissipation to the change of thermistor body temperature. It is measured in mW/ oC and is specified at a certain temperature. Both τ and δ depend strongly on the measured object or media. For example, the dissipation constant of a GE type DC95 interchangeable chip thermistor is 8 mW/oC in stirred oil, but is only 1 mW/oC in still air. The thermal time constant is 1 second in stirred oil, but is ten times longer in still air.
• The maximum power rating is another characteristic related to power dissipation. It is the maximum power in mW at an ambient temperature of 25oC that a thermistor can dissipate for an extended period of time without degrading its characteristics. This rating must be derated based on the ambient temperature.
• The zero-power temperature coefficient of resistance (TCR), α, is the ratio of the rate of change of zero-power resistance at any temperature point, T, to the zero-power resistance at that point:
αT = 1/RT (dRT)/(dT)
Where:
αT = temperature coefficient of resistance at temperature T,
Ω / Ω / oC, or %/ oC
RT = resistance at temperature T, Ω
dRT = change of resistance, Ω
dT = change of temperature, oC
Another way to express the temperature coefficient is:
αT = – B/T2
Where:
B = material constant, oK
T = temperature, oK
Unfortunately, thermistor temperature coefficients are highly non-linear over their operating range, which means that the coefficient itself varies somewhat with temperature. A coefficient is at its highest value at its lowest temperature limit and gradually decreases as temperature increases. One value of a particular coefficient might work for a narrow temperature range, but most often, thermistor measurement circuits must be linearized to cover large temperature swings.
By simply adding one resistor in series with the thermistor, the output voltage vs. temperature curve can be linearized. When resistance vs. temperature linearization is desired, the resistor should be connected in parallel with the thermistor.
Circuits for linearizing thermistor outputs can be comprised of series, parallel, and series-parallel combinations of fixed resistors and additional thermistors. The simplest circuit is a parallel resistor, the value of which can be calculated from the following equation:
R = [RTM(RTL + RTH) – 2RTLRTH] / [RTL + RTH – 2RTM]
Where:
R = value of parallel resistor, Ω
RTL = thermistor resistance at the lowest temperature TL, ?
RTH = thermistor resistance at the highest temperature TH, Ω
RTM = thermistor resistance at the midpoint temperature TM, Ω
Midpoint temperature TM = (TL + TH) / 2, °C
Simple on/off temperature control circuits and applications with a narrow temperature range and relaxed accuracy requirements usually do not need linearization. A simple Wheatstone bridge circuit is usually quite adequate. Another example that does not require hardware linearization is a digital temperature circuit where the linearization is handled in software.
A simple on/off temperature control circuit can be designed using a thermistor in one leg of a Wheatstone bridge. Resistors R1, R2, and R3 must have a low temperature coefficient and be exactly matched to guarantee accuracy.
Operating Conditions
Certain operating conditions can significantly lower measurement accuracy or reliability and should be avoided. For instance, self-heating might become a hidden accuracy error. Thermistors generate their own heat when their excitation current is too high. The power it develops from the excitation current and its own resistance (P = I2R) can noticeably elevate the temperature of the thermistor body above the environment. Parts with a large dissipation constant, d, a low thermal resistance mounting, and other means of superior heat dissipation will have a lower temperature rise. But the primary way to avoid excessive self-heating is to keep the excitation current as low as possible.
Most measurement errors and premature failures often come from careless installation and operation. For example, although thermistors are considered to be rugged devices, take care not to crack a case, separate a bond, or exceed the upper temperature limit.
Lastly, aging is a phenomenon that is often overlooked and if not considered in the maintenance schedule, can lead to loss of calibration accuracy after extended periods of use. It manifests as an effective thermistor terminal resistance drift over time due to slowly changing resistances in the bulk material and in the contact areas between the leads and the thermistor material.
For more information:
Contact John Gyorki at the Engineering Exchange,
www.engineeringexchange.com
www.omega.com
www.gesensing.com
industrial.panasonic.com
www.murata.com
www.ussensor.com
www.vishay.com
www.selcoproducts.com
www.thermosensors.com
www.jumoplus.com
