Capacitive detection technology continues to be favored in traditional man-machine interface applications, such as laptop touch panel, MP3 player, touch screen display and short-range detector. In addition to using capacitive sensors to replace mechanical buttons, a little imagination and the basic principle of man-machine interface design will enable many other applications to use this technology. Figure 1 shows examples of application concepts that can be improved by using human contact detection.Figure 1. Device using capacitive sensor electrodes

For the device shown in Fig. 1, it is often advantageous to know the contact quality between the device and the skin before starting the device or measuring. These devices include medical probes that need to be close to the skin, biopotential electrode sensors, or housings for fixing catheters. In order to determine the contact conditions, several green capacitive sensor electrodes in the figure can be directly embedded into the plastic shell of the device during injection molding production. The host microcontroller reads some status registers on the capacitive sensor controller IC, which indicate how close the capacitive sensor is to the skin. Then, the basic detection algorithm running on the host microcontroller processes the status register information to determine whether the contact between each sensor electrode and the skin is appropriate.

In the traditional man-machine interface application of capacitive detection, people usually touch the sensor electrode by finger touch. The example in Fig. 1 uses a capacitive sensor in an unconventional way, and the user places a device containing a capacitive detection electrode on the human body. Developing such applications is simple, but in order to build a stable and reliable system, some key guidelines should be followed

Capacitance digital controller. To develop high-performance contact detection applications, we must first select a suitable capacitor digital controller (CDC). For the application shown in Fig. 1, the contact between the device surface and the skin is directly measured by a slight change in energy, which is distributed in the capacitive sensor electrode array and occurs when the device contacts the skin. The accuracy of this measurement depends on the sensitivity of the CDC analog front end and the number of sensor electrodes. The accuracy of capacitive sensor manufactured by traditional PCB process is usually in the range of 50 FF to 20 PF, so the high-precision measurement technology using 16 bit CDC is ideal.

When selecting CDC, we should first clarify some key characteristics, such as high-resolution analog front-end with 16 bit ADC, programmable sensor sensitivity setting, programmable sensor offset control, on-chip environment calibration, sufficient capacitive input channels supporting an ideal number of sensor electrodes, and integrated design without using external RC devices for sensor calibration. These features support reliable and flexible applications and bring the best user experience. For example, programmable sensitivity allows the interface designer to preset the best sensor sensitivity for a specific application rather than adopting a fixed solution that may lead to poor sensitivity. Programmable offset control is another important feature for interface designers, because the offset value of sensor boards in each production batch may be slightly different. The quick preview feature allows you to change the host firmware settings before putting a new sensor board into mass production. For applications where ambient temperature or humidity is expected to change, on-chip environmental calibration can achieve a more reliable solution. Note that the electrode sensor is constructed using standard PCB copper traces; The properties of the substrate will change with the change of temperature and humidity, so the baseline level of the sensor output will be changed. If CDC supports on-chip calibration, this baseline drift can be dynamically compensated during product use.

Small electrodes require high sensitivity. The objective of measurement is to determine the closeness of the equipment to the skin; The better the contact quality between the skin and the equipment, the more accurate the reading of the equipment. The accuracy of measurement depends on the number (the more electrodes, the higher resolution) and size of electrode sensors distributed in the contact area of the device. For the application shown in Figure 1, the surface area of the device is generally very small, and designers need to use small sensor electrodes when developing applications.

In order to reliably measure the small capacitance change related to the small sensor electrode (generally less than 50 PF), a high-sensitivity analog front-end controller is required. Remember that the type and thickness of plastic covering material will further affect the small signal emitted by the sensor through the plastic. The analog front-end measurement of the controller must have sufficient sensitivity to measure this small signal, and maintain a good signal margin between the measured signal and the threshold level detection setting under all operating conditions (such as different power supply voltage, temperature and humidity, and the thickness and type of coverage material). Low signal margin will increase the risk of false detection and sensor instability. In order to minimize risk, when using CDC with 16 bit ADC, a margin of at least 1000 LSB shall be maintained between the sensor baseline level (the sensor is not in contact with the skin) and the contact threshold level.

Ad7147 and ad7148 captouch programmable controllers are used for single electrode capacitance sensors. They have 16 bit resolution, can carry out nano farad level measurement, and can set 16 programmable threshold detection level values in the full-scale range. Both controllers support 3 mm & Tides under 1 mm plastic covering material with a dielectric constant of 3.0; 3 mm small sensor electrode while still maintaining the full-scale signal margin of 1000 ADC LSB. The full-scale signal margin is the difference between the sensor output without skin contact and with skin contact.

Maintain reliable performance. The capacitive sensor electrode is made of standard copper material or flexible material on PCB. The properties of this material change with temperature and humidity. This change will offset the baseline level (the electrical average of all sensor thresholds is based on the baseline level). A large baseline offset increases the risk that the contact threshold level is too low or too high (too low or too high depends on the direction of baseline offset), which will cause false contact error, or make the threshold level either too sensitive or not sensitive enough, resulting in the instability of contact state. In order to maintain the original signal contact threshold detection level margin (sensitivity) of the sensor, CDC needs to automatically track the amplitude of baseline offset error and readjust the threshold setting accordingly. The example in Figure 2 shows how the threshold levels of ad7147 and ad7148 are automatically tracked and adjusted for baseline offset changes caused by changes in environmental conditions.

Figure 2. Ad7147 / ad7148 on chip environment calibrationEliminate measurement errors. The modification of capacitive sensor electrode array in the device may cause space constraints and force the designer to place the CDC away from the capacitive sensor. This will increase the length of parallel sensor routing and make the wiring dense, which is not conducive to capacitive detection applications, because routing at different DC potentials will form the stray coupling path shown in Fig. 3a. The grounding layer of PCB cannot prevent this situation because the wiring and grounding layer are at different DC potentials, which will still form stray capacitance (Fig. 3b).Figure 3. The path of stray capacitance shows the results of the following parallel routing: parallel routing (a) without copper pouring layer, parallel routing (b) on grounding copper pouring layer, and parallel routing (c) on copper pouring layer with the same DC potential as the routing

To eliminate the stray capacitance error, one method is to surround the adjacent routing with a layer driven by the DC level (the DC level is the same as the DC level of the capacitive sensor electrode and routing). The ad7147 and ad7148 devices eliminate stray capacitance by providing a dedicated acschield output with this function, as shown in Figure 3C.

Consumer health care equipment such as spa and skin care products are entering ordinary families from professional institutions, and users are no longer technicians who are specially trained and familiar with products and their applications. Therefore, many of these products need a more intelligent user interface to enable untrained users to master the correct product use methods. Capacitive detection provides new choices for user interface designers, enabling them to explore various innovative methods to meet new user interface requirements. Capacitive digital technology provides contact information between capacitive sensor electrode and skin, which can be used to maintain the best product performance and safety.

Power supply standardIEC 60601-1:2005 and accompanying documents specify the safety and basic functional requirements of the power supply. The standard appears in the form of ANSI / AAMI document in the United States and in the form of "euronorm" in Europe and around the world, with some local differences. Since January 2021, the latest revision (a1:2012, a2:2020) has updated the standard, deleted outdated references, corrected errors and made it better consistent with IEC 62368-1. The current medical EMC standard IEC 60601-2 is the 4th Edition.Built in medical power standard

AC powered medical devices can have "built-in" power supplies or lower power external adapters. If it is an internal product, the product designer should consider high voltage and high energy, and usually choose to buy a "medically certified" power supply. However, the final product is the product certified for this application, so there are other considerations, such as labels, connectors, internal wiring of power supply and fuses.

Due to grounding, external wiring and interference from other noise sources, the built-in power supply conforming to EMC standards cannot be guaranteed to pass the test. A common practice is to add another EMI filter at the AC inlet, which may be included in the connector. However, it should be noted that the leakage current limit should not be exceeded. For the most stringent CF product rating, the leakage current limit can be as low as 10 a. The leakage of external filter may directly lead to the internal leakage of AC power supply, there is a risk of interaction between external filter and internal filter, and may even worsen EMI.

Secure power solutionsThe safe solution should select the target application and specify a power supply with 2 x MOPP certification and EMC compatible performance. For example, power supplies from recom racm230-g, racm550-g and the recently released racm-1200v series.Their peak or rated power is 230W, 550W and 1200W respectively. When fans are not allowed (usually in medical environments), these products can be floor cooled. All products have 2x MOPP / 250V AC rating to prevent leakage to ground (racm550 / 1200). The safety barrier in each recom power supply is shown in Figure 2.

Figure 2: safety fence racm230 / 550 / 1200 seriesFigure 3: a 2x MOPP DC / DC converter provides SIP / SOP isolationUsing a medically certified power supply (such as the power supply of recom) is a reliable way to ensure that the safety and EMC standards required by various medical application environments are met. It would be a lower overall cost approach if development, certification costs and time to market were taken into account.

Recom is a good choice when selecting AC / DC power supply for medical equipment. Eurotime electronics, as a distribution partner of recom, provides local inventory and technical support.Original title: eurotime classroom | AC / DC power supply in medical applicationsThe source of the article: WeChat official account: Euro time electronic RS] welcome to add attention! Please indicate the source of the article.

Application of DICOM standard in portable medical equipment1 IntroductionRadio frequency refers to the electromagnetic wave with a certain wavelength that can be used for radio communication. Radio frequency identification technology is a non-contact automatic identification technology rising in the 1990s. It uses the transmission characteristics of radio frequency signal and spatial coupling (inductive or electromagnetic coupling) or radar reflection to realize the automatic identification of the identified object. However, at present, there are still many bottlenecks in the development of RFID, and the low data reading rate is one of the main bottlenecks.

This paper will analyze the RFID system by introducing the basic composition and working principle of the RFID system. Combined with the problems encountered in the practical application of RFID system and the reasons for the low reading rate of the system due to the blind area in the reading range of readers, redundant data in different reading points, mutual interference of readers and other factors, this paper puts forward from the following aspects: reasonably optimizing the hardware configuration, perfecting the software design Play the role of middleware and integrate other technologies to improve the data reading rate of RFID system.

2. Basic composition of RFID systemRFID system at least consists of electronic tag (e-tag / transponder, also known as smart tag) and reader (reader / interleaver, also known as reader / writer).Electronic tag is the data carrier of RFID system. Electronic tag is composed of tag antenna and tag special chip. Electronic tags are divided into active tag, passive tag and semi passive tag according to different power supply modes; It can be divided into low-frequency electronic tag, high-frequency electronic tag, UHF electronic tag and microwave electronic tag according to different frequencies; It is divided into * label, linear label, paper label, glass tube label, circular label and special-purpose special-shaped label according to different packaging forms; According to their different working modes, they are divided into active tags and passive tags.

Reader is a device used to read or write electronic tag information. It can be designed into many kinds of products according to specific use environment and requirements. The reader communicates with the electronic tag wirelessly through the antenna, which can read or write the electronic tag identification code and memory data.A typical reader consists of a high frequency module (transmitter and receiver), a control unit, and a reader antenna. Of course, RFID system needs the support of computer and other hardware equipment and software in practical application. Figure 1 shows the composition of a typical RFID system.3 basic model of RFID system

The basic model of RFID system is shown in Figure 2. As the radio frequency carrier, the electronic tag and the reader realize the spatial (contactless) coupling of the radio frequency signal through the coupling element. In the coupling channel, the energy transmission and data exchange are realized according to the timing relationship.Discussion on reading rate of RFID systemThrough the introduction of RFID system, we believe that the main reasons for the low reading rate of RFID system are: there are blind areas in the reading range of readers, redundant data in different reading points, mutual interference of readers, etc. In view of the above problems, we discuss them from the following four aspects.

4.1 reasonably optimize hardware configurationIn terms of hardware, we must first clarify a problem. That's what you really need. Don't blindly think that "the price is expensive, the larger the reading range and the higher the frequency, the better". It is the so-called "tailor-made" that "what suits you" is the best. Based on this understanding, you can choose hardware devices that meet the actual needs.

At the same time, consider all RFID tags and readers as a complete "data network", so as to reasonably optimize the hardware configuration, so as to maximize the efficiency of the whole system. Taking the access control system as an example, in order to prevent the blind area in the reading range of the reader, resulting in missing reading, the number of readers or antennas can be increased to compensate for the defect of blind area in the reading range of the reader; In order to prevent mutual interference between readers, the method of relatively isolating readers or antennas in space can be adopted to avoid mutual interference. In addition, according to the actual needs, the data reading rate of RFID system can also be improved by properly adjusting the antenna layout and antenna transmission power.

4.2 improve software designAt present, the hardware facilities of the optimized RFID system can basically meet the needs of data reading rate. With the decline of reader price, end users can easily deploy a large number of readers in their application sites, which not only solves the problem of missing reading, but also obtains more useful information from these systems. But the new problem is: redundant data reading or cross data reading. A simple description of this problem is that "a tag that should not be read in a certain position is read by a reader that should not read this tag".The core of LV positioning logic is based on "picking out the required readout data from the spatial location and filtering out the unnecessary readout data". The result is that the correct and accurate tag position is extracted from the results obtained by all RFID readers. In short, LV positioning logic is a software algorithm based on eliminating "redundant" readout data according to the data set resident in the whole reader system. Colorwave algorithm gives a good solution to the conflict between multiple readers due to overlapping work ranges.

For electronic tag collision, in the high frequency band, the classical ALOHA protocol is generally used as the tag anti-collision algorithm. The tag of ALOHA protocol is used to avoid conflict by selecting the method of transmitting information to the reader after a random time; In UHF band, tree bifurcation algorithm is mainly used to avoid conflict. In addition, other optimization settings can be made for the software. For example, in the e-ticket system, the scanning time interval of the reader can be designed by software to adaptively adjust the scanning time. In case of large traffic, the scanning frequency of the reader can be accelerated through software control to prevent missing reading; In the case of less traffic, the scanning frequency can be relatively reduced, so as to avoid the emergence of redundant data.

4.3 play the role of MiddlewareRFID middleware is the nerve center in various RFID industrial applications. RFID middleware is a message oriented middleware (MOM). Information is transmitted from one program to another or more programs in the form of messages. RFID middleware plays an intermediary role between RFID tags and applications. From the application side, it uses a set of general application program interfaces (APIs) provided by the middleware, that is, it can connect to the reader and read tag data.Therefore, even if the database software or back-end application program storing RFID tag information is added or replaced by other software, or even the type of RFID reader is increased, the application end does not need to be modified. This not only effectively solves the problem of data reading rate, but also saves other problems such as the maintenance complexity of many to many connections. RFID middleware will have very good development prospects in service-oriented architecture (SOA) and business information security applications in the future.

4.4 integration of other technologiesIntegration with sensor technologyIn the next few years, an important application trend of RFID is the equipment that combines RFID with sensors (such as sensors for measuring temperature and pressure). At present, it has been implemented abroad. Because RFID has poor anti-interference and the effective distance is generally less than 10m, its application is limited. Combining WSN (Wireless Sensor Network) with RFID and using the effective radius of the former up to 100m to form wsid network will greatly make up for the shortcomings of RFID system itself.

Integration with WiMAX, 3G, GPS and other communication technologiesWiMax (global interoperability for microwave access) is simply defined as a wireless broadband data transmission system. The wireless service range of WiMAX can be as far as several kilometers under the condition of maintaining high data traffic in urban areas. Its performance far exceeds the existing wireless network technology. In directional communication connection, the service range can reach 50 km under the condition of maintaining certain data traffic. Due to its extremely high performance, WiMAX technology is considered to be the best backup scheme for DSL UMTS connection.

The integration of WiMAX, 3G, GPS and RFID is moving forward with the active participation of all parties. RFID tag has the characteristics of small volume, large capacity, long service life and reusable. It can support fast reading and writing, non visual identification, mobile identification, multi-target identification, positioning and long-term tracking management. The cost saving and efficiency improvement promote RFID technology to become an important entry point for various industries to realize informatization. They will build a wireless broadband network that can meet the needs of a variety of application environments and generate rich applications, expanding the application field of RFID technology.

Fusion with biometricsBiometric recognition technology is a solution to complete authentication by using automatic technology to measure their physical characteristics or personal behavior characteristics, and comparing these characteristics with the template data in the database. The biometric recognition system captures the samples of biometrics, and the unique features will be extracted and transformed into digital symbols, which are stored as personal feature templates. People interact with the identification system to authenticate their identity to determine whether they match or do not match. At present, the commonly used biometric recognition technologies include fingerprint, palmprint, face, voice, retina, signature recognition and so on.In short, the integration of RFID system with other technologies is imperative, and great achievements have been made. It solves the problem of low data reading rate of RFID system, which will inevitably make RFID technology widely used. Finally, it will go deep as bar code technology and slowly extend to all aspects of various industries, which plays a key role in improving operation efficiency and economic benefits of the industry, so as to promote a new leap in the global economy and have a far-reaching impact on human society.

5 Conclusion

On the whole, the development of RFID system will be better and better in the future. Although there are still some technical and application problems such as low reading rate, we believe that it is not difficult to overcome the current problems of RFID through a series of measures such as optimizing hardware configuration, improving software design, playing the role of middleware and integrating other technologies. Under the strong market orientation, RFID technology will cause a major change all over the world. It will become a new economic growth point in the future, and will eventually become the largest information technology support for the development direction of Chinese enterprises. It can be predicted that in the near future, as a global manufacturing base, China will be the world's largest RFID application market in the future. This will be a rare opportunity for domestic scientific research institutions and enterprises.