Recently, we disassembled the encoder system of a motor AV30 made by Nidec Avtron, which is specially designed for heavy loads, has an IP65 grade housing, and includes a set of robust bearings. It can also operate over a wider temperature range. The specific units owned can output more than 8,000 counting bits per revolution and provide information in binary form about the location of the so-called SSI connections.
If you’re not familiar with industrial encoders, the SSI protocol may be something new. SSI (Serial Synchronous Interface) is a widely used serial interface for connecting absolute position sensors and controllers. SSI uses the controller to send out a clock pulse train that initializes the threshold output of the sensor. A typical SSI connection has six wires: two for clock signals, two for data, and two for power and ground. The controller provides clock pulses, so the encoder does not need to generate a clock. Each time the encoder receives a clock pulse, it provides a new data bit on the output pin of the shift register. The data stream reflects binary 12 or 14 bits representing a particular axis position and then repeats.
The product includes two parts: the electronic device and the housing of the encoder. The cable terminates in a connector at the rear of the housing.
When the shell was removed, I was surprised: the inside of the shell was essentially empty. What you see is not the inside of the encoder, but another smaller metal casing. As a result, the housing can provide a seal on the element and secure a robust bearing to the encoder shaft. The inner housing is attached to the encoder shaft by screws. Because of the crimping method, the actual absolute encoder electronics must be exposed.
Absolute encoders like this motor are often referred to as Wiegand wire sensors because they use Wiegand wire to sense rotation. Weigand wire is a specially prepared Veka alloy wire (such as fe-Co vanadium). The special material makes it have a hard magnetic shell and soft magnetic core. The shell has a high magnetization ability. If the magnet is placed close to the wire, the shell shields the internal softcore from the magnetic field until the field becomes high enough that the entire wire – including the shell and the core – quickly switches magnetization polarity. The switch takes place within a few microseconds. This shift is known as the Wiegand effect.
An important feature of the Wiegand effect is that the energy produced by each reversal of magnetic polarization is constant and completely independent of the rate of change of the external magnetic field, even if this happens slowly. A typical application of the Wiegand effect is to place a Wiegand wire in the middle of a coil, and two magnets connected to the axis of rotation can generate a pulse based on magnetization.
At the center of the PCB is a Melexis chip and possibly an ARM MCU. A pair of rotating magnets in the encoder causes the Wiegand wire sensor to output two pulses per turn. But you might wonder how to get 4,096 different codes from the encoder when the Wiegand wire only produces two pulses per revolution. The answer lies on the other side of the PCB. In the middle of the board is Melexis’s special Hall sensor IC. Traditional planar Hall techniques are only sensitive to the flux density applied vertically to the IC surface. However, due to the use of a special material on the CMOS chip, the Melexis chip is sensitive to the flux density applied parallel to the IC surface. This allows the chip to decode absolute rotation or angular positions from 0 to 360 degrees using the directional component of flux density.
The only other identifiable chip we found on the encoder board was a flash memory chip, possibly used to track the starting position of the encoder. There is another chip on the board, which we suspect is an ARM processor, but the chip is not marked.
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