Sensors are fundamental products in the electronic information equipment manufacturing industry and represent specialized components among the new electronic components prioritized for development. The sensor industry, widely recognized domestically and internationally as a promising high-tech sector, attracts global attention for its high technological content, strong economic returns, extensive market penetration, and broad market prospects. Driven by the booming electronic information industry market, China's sensor sector has established a solid industrial foundation. Significant progress has been made in technological innovation, independent R&D, commercialization of achievements, and competitive capabilities, contributing substantially to national economic development.
With the advent of the information age, sensors have become the primary means for humans to acquire information from both natural and industrial domains. In modern industrial production-particularly automated processes-various sensors monitor and control key parameters to maintain equipment within optimal operating conditions and ensure product quality. Sensors hold an even more prominent position in fundamental scientific research.
Today, sensors have permeated an extremely broad range of fields, including industrial production, space exploration, oceanography, environmental protection, resource surveys, medical diagnostics, bioengineering, and even cultural relic preservation. Evidently, the vital role of sensor technology in economic development and social progress is undeniable. Statistics indicate that the global smart sensor market's annual revenue is projected to grow at a rate of 10% per year. Currently, there are 65 million sensor devices with processors installed worldwide, a figure projected to reach 2.8 trillion by 2019.
Key Considerations for Sensor Selection
Sensor knowledge constitutes a substantial branch of electrical engineering requiring extensive experience to master. We will explore this further in future discussions. Today, we focus primarily on selection criteria.
1. Determine Type Based on Measurement Object and Output Requirements
To perform a specific measurement task, the first consideration is selecting the appropriate sensor principle. This decision requires analyzing multiple factors. For instance, flow meters include electromagnetic, vortex, and ultrasonic types. Selecting the appropriate flow meter requires targeting specific objectives. Additionally, consider the required output mode-such as 2-wire or 4-wire current signals (0-20mA, 4-20mA), 0-10V voltage signals, or communication protocols.
2. Selection Based on Sensitivity
Typically, within a sensor's linear range, higher sensitivity is preferred. Only with high sensitivity does the output signal correspond significantly to changes in the measured quantity, facilitating signal processing. However, note that high sensitivity also makes the sensor susceptible to external noise unrelated to the measured quantity. This noise can be amplified by the system, compromising measurement accuracy. Therefore, the sensor itself should possess a high signal-to-noise ratio to minimize interference from external sources.
Sensor sensitivity is directional. When measuring a unidirectional vector with high directional requirements, select a sensor with low sensitivity in other directions. For multidimensional vectors, minimize the sensor's cross-sensitivity.
3. Evaluating Frequency Response Characteristics
A sensor's frequency response determines the measurable frequency range, which must remain distortion-free within the permissible limits. In practice, sensor response always exhibits some delay, ideally minimized. Higher frequency response enables broader measurable signal frequencies. For dynamic measurements, response characteristics must align with signal properties (steady-state, transient, random, etc.) to prevent excessive errors.
4. Based on Sensor Stability
The ability of a sensor to maintain consistent performance over time is termed stability. Factors affecting long-term stability include not only the sensor's internal structure but primarily its operating environment. Therefore, to ensure good stability, sensors must possess strong environmental adaptability. Before selecting a sensor, investigate its intended operating environment. Choose an appropriate sensor based on specific conditions or implement measures to minimize environmental impact.
5. Range and Accuracy: The Most Challenging Pair to Balance
Accuracy is a critical performance metric for sensors, serving as a vital link in determining the overall measurement precision of a system. However, sensor accuracy is constrained by its range: generally, a larger range results in lower accuracy. Conversely, high-accuracy sensors often lack sufficient range, making high-accuracy, large-range sensors prohibitively expensive. Therefore, a balanced approach is required when selecting sensors.
When selecting sampling sensors, ensure the device meets the application's fundamental operating conditions (refer to manufacturer data sheets).
The six most critical operating conditions include:
1) Temperature range;
2) Specifications;
3) Protection rating;
4) Voltage range;
5) Discrete or analog output;
6) Parameter variation, specifically whether "adjustable parameters are beneficial."
When considering sensors with IO-Link configuration capability, six additional factors warrant attention:
1) Response speed;
2) Sensing range;
3) Repeatability;
4) Electrical connectivity;
5) Mounting type;
6) Visual display: Whether the application requires a visual indicator on the sensor.
In modern industrial production, particularly automated processes, various sensors monitor and control parameters throughout manufacturing. This ensures equipment operates normally or optimally while achieving the highest product quality. Thus, without a wide array of high-quality sensors, modern production would lack its foundation. The following details several most common sensor types in manufacturing, along with application tips and insights.
Most Common Sensor Types
Proximity Sensors
Proximity sensors detect the presence of objects in a nearby area without physical contact. These sensors are discrete output devices. Typically, magnetic proximity sensors detect whether an actuator has reached a specific position by sensing a magnet located within the actuator.
Purchasing an actuator from one company and a magnetic proximity sensor from another is generally not advisable. While sensor manufacturers may claim compatibility with X, Y, and Z actuators, variations in magnets and mounting positions can lead to sensing issues. For instance, the sensor may energize when the magnet is misaligned or fail to energize altogether. If the actuator manufacturer offers a matched proximity sensor, it should be the preferred choice.
Transistor-based proximity sensors have no moving parts and offer extended service life. Reed-based proximity sensors utilize mechanical contacts, resulting in shorter lifespans but lower costs compared to transistor types. Reed sensors are best suited for applications requiring AC power and high-temperature environments.
Position Sensors
Position sensors feature analog outputs that indicate actuator position based on the position of magnets on the actuator. From a control perspective, position sensors offer significant flexibility. Control engineers can define a series of setpoint values to match component variations.
Since these position sensors rely on magnets (like proximity sensors), it is advisable to source both sensors and actuators from the same manufacturer whenever possible. IO-Link functionality enables data acquisition from position sensors, simplifying control and facilitating parameterization.
Inductive Sensors
Inductive proximity sensors utilize Faraday's law of induction to detect object presence or simulate position output. The most critical factor when selecting inductive sensors is determining the type of metal the sensor detects, thereby establishing the sensing distance. Compared to ferrous metals, non-ferrous metals reduce the sensing range by over 50%. Sensor manufacturers' product manuals should provide the information needed for sample selection.
Pressure and Vacuum Sensors
Ensure pressure or vacuum sensors meet measurement ranges in both imperial (psi) and metric (bar) units. Specify the most suitable form factor for the allocated space. During equipment installation, consider whether sensors should include indicator lights or display screens for operator convenience. For rapid setting adjustments, consider pressure and vacuum sensors equipped with IO-Link.
Flow Sensors
Similar to pressure and vacuum sensors, flow sensors can be selected based on flow range, size, and variability of setpoints. Display options can be specified when ordering sensors. Flow sensors with relatively low flow rates can be chosen for specific zones or the entire equipment application.
Optical Sensors
The most common optical sensors are photoelectric scattering, reflective, and through-beam types. Laser sensors and fiber optic sensing devices also fall under optical sensor categories.
Most photoelectric sensors detect objects by reflecting light or interrupting a beam. Due to their low cost, versatility, and high reliability, these sensors are among the most widely used in manufacturing. Diffuse-reflective photoelectric sensors require no reflector. They offer excellent cost-performance for detecting the presence of nearby objects.
Through-beam photoelectric sensors offer the longest sensing range. These sensors install transmitter and receiver units at two separate points. Garage door safety sensors are examples of beam sensors. When the beam is interrupted, it indicates the presence of a target. The slot-type photoelectric sensor is an interesting through-beam variant; it integrates a transmitter and receiver into a single compact unit. Slot-type light sensors are used to detect the presence or absence of small components.
Reflective photoelectric sensors feature a sensor and reflector for mid-range presence detection. In terms of precision and cost, these sensors fall between diffuse and through-beam types.
Fiber optic sensors are used for presence and distance sensing. Parameters on these versatile sensors can be adjusted to detect various colors, backgrounds, and distance ranges.
Laser sensors can be used for long-range presence detection and offer the highest precision in short-range measurement applications.
Vision sensors are suitable for barcode reading, counting, shape verification, and similar tasks. They represent a cost-effective solution for vision applications where camera systems would be prohibitively expensive or complex. Vision sensors read barcodes, track individual components, and execute processes tailored to those components. Sensors can verify the number of features present on a part. Vision sensors can determine if a specified curve or other shape has been achieved. Since these sensors process light, testing them in conditions as close as possible to the actual operating environment-regarding ambient light and background reflectivity-is critical. In most applications, it is recommended to place vision sensors inside an enclosure to isolate them from external light sources. Seeking assistance from the vision sensor manufacturer during sensor testing is a good idea. Additionally, don't forget to ensure the selection of an appropriate fieldbus.
Signal converters transform the analog output signal from the sensor into a digital signal on the converter. Another option is conversion into IO-Link process data.
Other Sensors
1. Magnetic Switch: This is a specialized term for a cylinder sensor, primarily used to detect the position of a cylinder piston. Typically, cylinder suppliers provide these sensors tailored to customer usage scenarios. As the name implies, magnetic switches detect targets through electromagnetic induction, resulting in relatively low detection accuracy.
2. Proximity Switches: Designed based on electromagnetic induction principles, proximity switches can only detect metal targets, with slight variations in sensing distance depending on the metal type. Common detection ranges for proximity switches include: 1mm, 2mm, 4mm, 8mm, 12mm, etc. Proximity switches generally come in two types: flush-mount and non-flush-mount. A flush-mount switch means the sensing head does not detect metal targets around its circumference but only detects targets directly in front of it. In other words, the sensor's sensing head does not need to protrude beyond the metal mounting bracket. Non-flush-mounted types detect both front-facing and circumferential metal targets. Here, the sensor head must protrude from the metal mounting bracket, and no metal objects should be present within a certain circumferential range to prevent false detection. Proximity switches offer higher detection accuracy than magnetic switches. They are typically used in applications requiring relatively low positional precision, such as verifying product presence or fixture positioning.
3. Photoelectric Switches: Photoelectric detection offers advantages such as high precision, rapid response, and non-contact operation. It can measure multiple parameters, features simple sensor structures, and offers flexible configurations. Consequently, photoelectric sensors are widely applied in detection and control systems. Common types include: reflective photoelectric sensors, through-beam photoelectric sensors, and sensors utilizing reflective plates to reflect light beams. The latter two types detect by blocking light from the target object, while the former detects by sensing light reflected from the target. Consequently, the latter two typically offer longer detection ranges and higher precision. Due to their high detection accuracy, photoelectric sensors are commonly used for precise positioning of products or robotic workpieces, as well as in feedback systems for stepper and servo mechanisms.
4. Fiber Optic Sensors: Fiber optic sensors are another type of detection element utilizing photoelectric signal conversion. Compared to photoelectric switches, they can typically detect smaller objects, operate at greater distances, and offer higher precision. Consequently, fiber optic sensors are commonly employed in applications requiring greater accuracy and as positioning feedback devices in stepper and servo systems.
5. Photoelectric Sensors: Photoelectric sensors also utilize photoelectric signals. With a large detection area, they are often termed area sensors. Their primary application lies in interlocking and safety functions between equipment, particularly in human protection systems.
6. Thermocouples: Thermocouples are primarily used to detect ambient temperatures in their surroundings.
7. Laser Scanners: Laser scanners primarily function to precisely measure the dimensional contours of target objects.
8. Industrial Cameras: Also known as CCD (Charge-coupled Device) cameras in engineering, industrial cameras are mainly used to detect the shape and position of target objects. With advancements in CCD technology, high-resolution industrial cameras are now applicable in precision measurement fields.
9. Encoder: Based on operating principles, encoders are categorized into incremental and absolute types. Incremental encoders convert displacement into periodic electrical signals, which are then transformed into counting pulses. The displacement magnitude is represented by the number of pulses. Each position on an absolute encoder corresponds to a unique digital code. Therefore, its displayed value depends solely on the starting and ending positions of the measurement, not the intermediate process. Encoders are typically paired with stepper motors or servo motors to form closed-loop or semi-closed-loop control systems.
10. Micro Switch: A micro switch is a contact-type sensor primarily used for interconnecting equipment or detecting the status of safety guard doors on machinery.