Electronic – Removing digital noise on quadrature encoder signals

digital-communicationsdigital-logicencodernoisequadrature

I am using a quadrature encoder with 3 lines to track the rotary position of a shaft attached to a BLDC motor(through a planetary gearbox).

When I rotate the shaft, the A, B and Z lines always have an erratic digital noise(observed using a logic analyzer), which always messes up the position readings. The picture of the digital signal is as follows
The order of the signals is A, B and the last one being Z(index)

The noise is always seen on all 3 lines together and compared to the actual encoder pulses, it is of a very short duration.

What could be the source of this noise? How can i remove the noise from the signals? Preferably through a minimal hardware solution.

My position data is processed by a controller who's code I cannot change, hence the option of compensating for the noise in software does not exist for me.

A closer view at the noise

EDIT: I am using a CUI AMT11-3S encoder, which is a capacitive encoder Link
& Datasheet

Best Answer

Looking at signals from a logic analyzer (especially where noise is a factor) can be confusing. It's possible that the noise you see is not at the same level as your quadrature signals, but because the analyzer either sees a signal over the logic threshhold or not, they look to be the same amplitude as your desired signal.

I'd suggest firstly that you check you have at least 10-20 uF on the supply voltage to the Quadrature detector. They can be sensitive to voltage fluctuations if you are using them near their bottom limit of supply voltage. Most are designed for 5-12 or 12-24 V supplies, but some are specified as 5 V only (and usually quite a different design). If you are using 5 or 12 V you are right on their lower limit for each variant.

Make sure the A, B, Z signals have a Schmitt trigger receiver such as the 74LS14, and check the datasheet for your Quadrature detector to see if it expects a pullup resistor at the receiver end of the cable (most do)....conversely, if you do have a pullup resistor make sure it's not too low a value.

As an example here is the datasheet for an Omron Rotary Encoder: http://www.mouser.com/ds/2/307/e6b2-c_ds_csm491-25665.pdf

Notice that the I/O structure varies considerably for the variants; pulldown, pullup and line drivers.

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Netduino Interrupts Affected By PWM – Diagnosis of Issue

Use software debouncing: In your interrupt routine deactivate interrupts (so no other interrupts after the current one are fired) and then set a flag, that the interrupt was called.

In your Main routine check for the flag, act on it and then enable interrupts again. Maybe add a small delay before enabling interrupts again.

This seems to be another solution to software debouncing:

 private static void Button1_OnInterrupt(uint data1, uint data2, DateTime time) {
        // glitch filter. events within 50 milliseconds of the first event are discarded
        if (Button1LastPushed.AddMilliseconds(50) > time)
            return;
        // Debug.Print("Button 1 Interrupt");
        Button1LastPushed = time;
        // actions on interrupt here
}

Of course you need to adapt it to your PWM.

PS: LEDs have a small backward current, this could be the source of your problem.

Can’t seem to determine quadrature encoder resolution

Well, 400 counts per revolution generally refers to counts in terms of the four possible state combinations of the two outputs. If you just count pulses on one channel, you will get 100 pulses per revolution on a 400 count per revolution encoder. You need a quadrature counter to actually get all 400 counts. 100 pulses times a gear ratio of 30 is 3000 pulses per rev, which is in the ballpark of what you are seeing.

Best Answer

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Netduino Interrupts Affected By PWM – Diagnosis of Issue

Can’t seem to determine quadrature encoder resolution

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