Reed sensor

Reed sensor - a device that changes the contact state depending on the presence or absence of the magnetic field.


Wikipedia states that the reed sensor invented in 1936 by Walter Ellwood, who worked at Bell Labs. This information has been carefully checked and found to be erroneous. The first of the patents filed by Ellwood - not that shown in the figure in Wikipedia - is dated March 29, 1938. There is a bug that inspires confidence that on Holmskoy Conference 2013 read a report on the reed relays, where reference is made to the said patent in 1938.

Holmskaya Conference - event held since 1953 in the form of a discussion of the progress of achievements in the field of electricity. In 1968 he was named a little-known physicist Ragnar Holm, 50 years of his life to the study of issues related to the theme. Since 1985 it is officially supported by IEEE Association, a legislator in most areas of technology.

So, in 2013, apparently, discussed advances in reed relays (see. below), and the authors (Stephen Day and Todd Christenson) carefully noticed that the first «reed switch» appeared "70 years ago." Simple subtraction obtain 43rd year. More like a July 14, 1942 - the date of publication of the patent number US2289830 A - rather than a hypothetical and it is not clear from which arose in 1936. Based on the available data on hand, we note that the date of birth of the sensors is permissible to consider 1938 the year of the patent application. In addition, it permitted the existence of the diaries and records, explaining what was happening at Bell Labs, but their real location is nothing to show.

The content of the patent

Shown in Wikipedia, taken from US Patent 2264746 and the author called electromagnetic switch. Publication took place December 2, 1942, after the release of the aforementioned US2289830 A. The picture seen sealed transparent glass flask that prevents contamination of the sensor contacts and their oxidation. Working considered part of the ferromagnetic strip, which interact with an external field (in the figure - Pos. 3 and 6).

The dielectric spacer is needed for reliable differentiation of groups of contacts. Conductors extending outside the bulb, as a rule, made of copper or brass. After contact with the sensor to the magnetic field iron plates are attracted to each other by changing the position of the contacts. It is shown that in the absence of external influence current enters the terminal 4, in the presence of - the fifth. This allows you to switch the circuit properly.

In fact, US Patent 2264746 filed in the relay. It can not switch the power circuit, for obvious reasons, but serves as an intermediate. Able to control other, more powerful devices. Regarding the patent US2289830 A, filed earlier, where we are talking about the sensor. Not difficult to guess that Ellwood wrote in a paper office later came up with a new device, and sent for review followed. Texts and published for each other: Ellwood further said if passed the first patent, there is no reason to reject the second. Which it was taken into account by the Commission.

From the screenshot it shows that the author proposed a number of ideas to bring to the contact action. Firstly, a strong magnetic field generated by the coil wound around the flask. The second version - backbone is used in the form of a coil, put on the flask. The third illustration suggests that the outer casing in the form of a fuse insert will allow the construction of any already existing solenoid. Finally, in the fourth embodiment serves to cover the copper contacts to interact field induction currents gold sputtering.

From this it is clear that the author has long experimented with the proposed device, or thinking about them. Based on this premise, we assume that the reed sensor is really conceived as early as 1936. Offers authors and other options, for example, platinum blotches on the contact surface. We go over the text of the patent:

  • The aim of the work is to create a cheap and durable switches to replace the existing devices while increasing reliability.
  • The new device is turned much smaller in size than the precursor, with a minimum of moving parts.
  • In the absence of air (helium, argon, etc.) May perform inside contacts from cheap iron, without fear of occurrence of rust.

Operating principle

The operating principle of the sensor reed viewed abstractly, for example, the device is given in Pat. When winding the coil around a sealed flask and passing current through the wire, a magnetic field whose lines are directed along a sensor axis (inside the flask). the field strength is enhanced in ferromagnets, multiplying into the tens of thousands of times. Orientation of the lines is the same. Consequently, at the end of the first iron contact occurs south pole, the second north. They will be attracted and will keep each other until there is no external field.

Residual magnetization is not enough to keep the system closed. Contacts will disperse to their former positions. Systems with gold plating may act due to induced currents, but the magnetic field strength assumed to be large. Acceptable called reed sensors on metal contacts more sensitive.

Advantages and disadvantages, the use of

Despite its apparent simplicity, reed sensors control significant currents to modest dimensions, in addition, is very durable and can withstand high mechanical stresses. The disadvantages include the complexity and high cost of manufacturing products. By the beginning of the XXI century it turned out that the further development of the technology is problematic due to advances in linear dimensions limit (5 mm in length). Since 1940, the size of the reed sensor decreased about 30 times.

5 mm are too large to use the product in cell phones, endoscopes, ear-phones and other mobile devices. Individual sellers are the merits of reed sensors include zero energy consumption. In a sense this is true, the device is completely passive.

millions of reed sensors are produced annually for automated test systems, engines, geological exploration equipment, medicine, appliances, plates. They serve to determine the orientation of the device in space, recording magnetic fields are capable of playing the role of a compass.

Modern reed sensors

Development of chips led to the creation of reed sensors and planar microstrip technology. The technological process goes by the scheme:

  1. The fixed contact is deposited on the silicon substrate.
  2. The movable contact has a cutout for reducing elastic properties, made of ferromagnetic material and fused to the substrate contact.
  3. The gap is so tiny, that operation is ensured minimum magnetic field strength.

manufacturing features

Is shown in the figure belongs to the planar structure MEMS - Micro-electro-mechanical systems (MicroElectroMechanical System). The disadvantages include sensitivity of the sensor to the substrate thickness, the parameter changes between the silicon plates, making unstable result. The elastic properties of the plate depends on the cube of its thickness, the slightest error leads to similar results. Finally, the temperature stress of the material during manufacture leads to an uneven change dimensions, which causes bending of the plate up or down, further introducing randomness in the resulting result.

Process recognized HARM - producing microcomponents with high density. As a result, the device becomes available stunning switching capacity - load hundreds of milliwatts. For example, the product is devoid of RedRock tendency to sticking contacts. Manages applied to a substrate area of ​​only 2.4 sq element. mm at a height of 0.95 mm construction. The problem is solved by applying plate thickness elements lithography methods, the bending direction parallel to the substrate varies. These assumptions allow to achieve high production repeatability.

The manufacturer claims that the HARM overcome the limitations listed above. In particular, it will be able to become full components of the mobile device. An additional advantage of the technology is the ability to fine-tune the threshold, which opens up a new direction in the use of reed sensors. HARM allows you to compare the device with Natural Born market leaders:

  1. Hall sensors.
  2. Anisotropic magnetoresistors.
  3. Planar switches.
  4. Giant magnetoresistors.

SMT technology allows us to hope that the device will be used. A planar assembly achieves a high density arrangement of microelements, automate the assembly process. And until some time reed sensors do not fit in SMT technology with a high degree of automation, but at the time of the second decade of the XXI century there were construction, lack of smoothing, eliminating him.

To date, it proved that photolithography can achieve greater accuracy in the production of reed sensors than any technology. The manufacturing process briefly:

  1. Special polymer (e.g., polymethyl methacrylate) through a mask is exposed to X-rays or ultraviolet light.
  2. External exposure changes the molecular polymer lattice that allows irradiated portions suitable rinse with solvent.
  3. Ferro-nickel alloy (permalloy-80) flush with the resulting sprayed form. polymer residues are removed.
  4. The desired layerwise applied design.

It is important current scaling device when miniaturization to obtain the desired properties. Valid considered low contact resistance at a high repeatability of parameters from one cycle to the next conveyor. It is necessary to create a relatively high force of attraction in the field of action: the miniaturization is accompanied by a decrease in shock attraction. Fortunately, especially HARM technology can solve the problem in an elegant manner. Increasing the contact area is achieved by increased thickness of the sprayed metal (see. above), movement occurs parallel to the substrate. A couple of hundred micrometers is irrelevant for placing components on the board (footprint is not changed).

Other technical solution is to create a radically new topology for HARM technology. Calculations show that the sensitivity of the sensor manages to raise a minimum of three times. At the same time elastic enough to withstand accidental knocks and vibrations, since the weight of the bridge with the contact is extremely small. Application of a flexible contact leads to the "anvil" thick, there are formed magnetic domains form strong attracting field. The resulting structure is described fairly simple mathematical formulas that allow to predict outcome. In particular, describe contact resistance mentioned above.

Technology of production of reed sensor has certain unexplored reserves for the introduction of products of the modern equipment.

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