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Building a hollow shaft encoder by combining Hall elements and an ADC!

Episode 2. We will introduce hollow shaft encoder solutions by combining AKM's Hall elements and an ADC.

#02 Basic Knowledge of ADC

This is a story that young A, who works in a fictional motor company deepens the knowledge of ADCs with a senior colleague K, and his boss, M, manager.

Senior K

I know exactly what you mean. I sell motors.

Young A

The device that detects the rotation angle is generally called the encoder.

Senior K

Ok. Let's make an encoder with angular resolution of 12 bits or more with Hall elements and an ADC now!

Young A

Hall elements are often used for magnetic pole detection with motors, but can you make a 12-bit encoder?

Senior K

It is possible. If you do electric interpolation with an ADC.

Young A

Electric interpolation to interpolate..?

Senior K

Yes. It can be considered that the resolution is electrically improved with the number of ADC bits. Of course it depends on sensor distortion etc.

Young A

Well, You can do it for Hall elements.

Senior K

Also, in this configuration, the advantage is, it's easy to make a hollow shaft rotary encoder.

Young A

Hollow rotary?

Senior K

An encoder whose center of rotation axis is empty. It is also called a hollow shaft encoder.

Young A

A hollow shaft type is a recent trend in motors, isn't it?

Senior K

If it is an encoder of the type that detects at the shaft end, it will definitely become thick. If it is made hollow, you don't have to worry about the thickness and wiring can also be put inside.

Young A

I see. So, it is a 12-bit hollow shaft encoder.

Senior K

Well, I prepared two pole magnets that pierced the center to a donuts-shape. Magnetization is radial. Also I used two of AKM's HZ-116C and one of the AK924X series. This is the AK9240. Let's proceed with the 12-bit AK9240NK this time.

Young A

We have a few parts. Reduction of the mounting area is good.

Senior K

First, place the two Hall elements at 0°and 90°from the rotation axis. Next, attach a donuts-shaped magnet to the rotating body and place the substrate just below the magnet. It looks like this. (Figure 1)

Senior K

The magnetic flux density detected by each Hall element is a component of cos θ and sin θ according to the rotation of the magnet. Since the Hall element outputs a voltage proportional to the magnetic flux density, the Hall element outputs signals of cos θ and sin θ. It is a signal of 90 degree phase difference, so-called A phase and B phase signals.

Young A

Well, for one rotation, it is only necessary to think that one wave of A phase and B phase signals are output, right?

Senior K

Yes. One rotation can be represented by one wave. Let's input the output of this Hall element to an ADC, the AK9240NK!

Young A

Well, can we make A/D conversion immediately?

Senior K

You mean?

Young A

The output of the Hall element is quite small, isn't it? Well, I think it was about 100 mVpp.

Senior K

It also depends on the strength, thickness and space arrangement of the magnets, but that's about it.

Young A

The input range of the ADC is about 5V. It is necessary to insert an amplifier to the input stage to gain the Hall element outputs.

Senior K

That's exactly the advantage of the AK924X series! An instrumentation amplifier is installed in front of the ADC!

Young A

Wow! Even though it is so small?!

Senior K

The AK9240NK has a 2-channel 12-bit ADC in a 3 mm square package, an instrumentation amplifier, a 2-step variable low-pass filter, and an 8-level PGA, (programmable gain amplifier), integrated in the input stage of the ADC. (Figure 2)

Young A

Gorgeous!

Senior K

Look at this datasheet too. It guarantees specification for S/N=72dB at a setting value as high as 23 dB gain. Generally, with a preamplifier and built-in ADC product, there are many cases where S/N characteristics are not specified in such a high gain settings.

Young A

Does that mean that good noise characteristics can be obtained if it is connected to the Hall element without thinking? This is easy!

Senior K

Right. It really helps that you don't have to take time for analog design when there is no time and the deadline is very close.

Young A

By the way, I have heard that the Hall element has large temperature dependence. Is that okay?

Senior K

In principle, if the temperature dependence of the magnetic sensitivity is uniform with two Hall elements, since the signals of Phase A and Phase B are divided when calculating the angle θ, the parameters dependent on temperature are canceled out.

Young A

Hmm... But is it true that the temperature dependence of two Hall elements are uniform?

Senior K

Well, indeed. Does temperature dependence come together?

Expert M Manager

There is ingenuity there.

Young A

Oh, M manager, your appearance almost seemed timed as planned.

Expert M Manager

The HZ-116C is designed so that the temperature characteristics of two neighboring Hall elements in the reel are nearly uniform due to manufacturing innovation. Therefore, the temperature drift becomes smaller than that of a general Hall element.

Young A

I see.

Senior K

So it's better to use the AK924X in combination with the HZ-116Cs.