Apa MEMS magneto-induktif?

What are magneto-inductive MEMS?

Steve Taranovich -January 29, 2017

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I recently missed a meeting at the University of Arizona in Tucson because their parking lot was full and, not being familiar with the area, I drove around looking for an empty parking space until it was too late to keep my commitment and I had to call to re-schedule the meeting. Driving around looking for a place to park your car is a ‘hit-and-miss’ endeavor that adds lots of unnecessary CO₂ to our environment (Europeans are especially conscious of this). How would you like to have a device in your car that can locate available parking spaces in the area and guide you right to it?

The above example is only one situation and need for an application using a magneto-inductive (MI) sensor in its design. There exists a myriad of other possibilities for this type of sensor, only limited by the designer’s imagination. Let’s first look at what the MI is and how it works, then we can discuss an example of one possible use for getting you a parking spot.

The magneto-inductive sensor¹

PNI Corp. has an MI sensor IC, the MS2100, consisting of two MI sensors making up a two-axis sensing solution plus a control ASIC. We will use this sensor as an example in the following tutorial. The following is how an MI sensor works and what sets the PNI solution apart from anything else out there that I have seen in performance.

There are many ways to employ an MI sensor but the usual circuit includes an L/R oscillator configuration with the MI in the feedback of a Schmitt trigger (Figure 1).

 

Figure 1 A typical MI sensing circuit design (Image courtesy of PNI)

The magnetic field parallel to the coil is shown as H_E. H is the total magnetic field that the MI sees and is a function of two elements: the magnetic field formed by the current, I, in the circuit and the external magnetic field. See Equation 1 in which k₀ is a constant depending upon the physical parameters of the particular sensor.

H = k₀I + H_E       Equation 1

For Figure 1, it is assumed that there is a “0” value at the Schmitt trigger input, A; i.e., 0 or some value less than the trigger value. The output then inverts to a logic “1” at a voltage we will call V_S. Now that V_S across the voltage up across the MI sensor until the point A voltage reaches the Schmitt trigger threshold voltage, V_H, and is seen as a logic “1” at Point A, thereby making the output a logic “0”. Now the voltage across the MI sensor is driven down and the result is a sustained oscillation (Figure 2).

 

Figure 2 The waveforms for the oscillator circuit. The current, I, follows the voltage waveform at point A. (Image courtesy of PNI)

Now here is the underlying principle to PNI’s magneto-inductive sensing technology. PNI designs its MI sensors with a solenoidal geometry coil which gets wrapped around a high-permeability magnetic core. The inductance of such a highly permeable material varies with the applied magnetic field. So the sensor’s inductance, µ, is a function of H, the magnetic field (Figure 3).

Figure 3 The graph shows the inductance, µ, of the highly permeable material vs. H, the magnetic field. (Image courtesy of PNI)

    In Figure 1, R_b is the bias resistance and it, along with the voltage V_S on the Schmitt trigger, are chosen so that the sensor’s magnetic field is in the non-linear region of the permeability curve shown in Figure 3.

Now let’s look at the voltage output when this circuit is driven with a positive or a negative bias without an applied external magnetic field (Figure 4).

 

Figure 4 This is the sensor circuit performance curve with no externally applied magnetic field. The period of the oscillation is the same when biased either positively or negatively. (Image courtesy of PNI)

Now we apply an external magnetic field, H_E which can also be the Earth’s magnetic field, and we see that both the positively and negatively biased curves shift in the same direction. It can be seen that when the circuit is positively biased, the shift causes the inductance to increase and when it is negatively biased, the inductance now decreases. This effect causes τ, the period between cycles, to increase for the positively biased circuit and decrease for the negatively biased (Figure 5).

 

Figure 5 The sensor circuit operation with an applied external magnetic field is shown here. (Image courtesy of PNI)

Now, if we measure the time to complete a fixed number of oscillations or periods which occur during the forward and reverse polarity directions, then take the difference between these two values, we can derive the strength of that external magnetic field.

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Apa MEMS magneto-induktif?
Steve Taranovich -January 29, 2017

dan, tidak menjadi akrab dengan daerah, aku pergi berkeliling mencari tempat parkir yang kosong sampai terlambat untuk menjaga komitmen saya dan saya harus menelepon untuk kembali menjadwalkan pertemuan. Berkeliling mencari tempat untuk memarkir mobil Anda adalah 'hit-and-miss' usaha yang menambahkan banyak CO2 yang tidak perlu untuk lingkungan kita (Eropa terutama sadar ini). Bagaimana Anda ingin memiliki perangkat di mobil Anda yang dapat menemukan ruang parkir yang tersedia di daerah dan memandu Anda hak untuk itu?

Contoh di atas hanya satu situasi dan kebutuhan untuk aplikasi yang menggunakan sensor magneto-induktif (MI) dalam desain. Ada ada segudang kemungkinan lain untuk jenis sensor, hanya dibatasi oleh imajinasi perancang. Mari kita pertama melihat apa MI dan bagaimana kerjanya, maka kita bisa mendiskusikan contoh salah satu penggunaan yang mungkin untuk mendapatkan Anda tempat parkir.

Magneto-induktif sensor1

PNI Corp memiliki MI sensor IC, MS2100, yang terdiri dari dua sensor MI membuat sebuah solusi penginderaan dua sumbu ditambah ASIC kontrol. Kami akan menggunakan sensor ini sebagai contoh dalam tutorial berikut. Berikut ini adalah bagaimana sebuah sensor MI bekerja dan apa yang membuat solusi PNI terlepas dari hal lain di luar sana yang saya lihat dalam kinerja.

Ada banyak cara untuk mempekerjakan sensor MI tetapi sirkuit biasa termasuk L / R osilator konfigurasi dengan MI di umpan balik dari pemicu Schmitt (Gambar 1)ekarang mari kita lihat output tegangan saat rangkaian ini didorong dengan positif atau bias negatif tanpa medan magnet luar diterapkan (Gambar 4).

 

Gambar 4 ini adalah sensor kurva kinerja sirkuit tanpa medan magnet diterapkan secara eksternal. Periode osilasi yang sama ketika bias positif atau negatif. (Gambar milik PNI)

Sekarang kita menerapkan medan magnet eksternal, HE yang juga dapat menjadi medan magnet bumi, dan kita melihat bahwa baik positif dan negatif kurva bias bergeser ke arah yang sama. Hal ini dapat dilihat bahwa ketika rangkaian positif bias, pergeseran menyebabkan induktansi untuk meningkatkan dan ketika negatif bias, induktansi sekarang menurun. Efek ini menyebabkan τ, periode antara siklus, untuk meningkatkan untuk rangkaian positif bias dan menurunkan untuk bias negatif (Gambar 5).

 

Gambar 5 Sensor sirkuit operasi dengan medan magnet luar diterapkan ditampilkan di sini. (Gambar milik PNI)

Sekarang, jika kita mengukur waktu untuk menyelesaikan sejumlah tetap osilasi atau periode yang terjadi selama maju dan mundur arah polaritas, kemudian mengambil perbedaan antara dua nilai ini, kita dapat memperoleh kekuatan yang medan magnet luar.