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PWM Voltage Issue


(Sjohns064) #1

Hello,

I’m having a problem with a servo which only intermittently responds. The servo is powered by a separate 6V UBEC (not connected to the servo rail). I’ve measured the PWM signal voltage, and it seems to be very low (3.1-3.4V), so I think this low voltage is the issue.

I’ve tried to add an IC buffer (74HC14N Hex Schmitt IC) to the PWM output on the Navio2 servo rail, in order to lift the voltage of the PWM signal to 5V (presumably what it should be?); however when the buffer circuit is connected to the PWM output pin, no PWM signal is sent by the Navio2. Could this be because the Navio2 is detecting an impedance mismatch between the buffer input and the PWM output, and then shutting down?

N.B. I’ve connected the power for the IC to the Navio2 rail, and the voltage across the connected IC circuit is showing 5V, so the powering of the IC seems to be ok.

Thanks for any help.
Regards,
Simon


(Tatiana Andreeva) #3

Hi @sjohns064,

May I ask you to share a hardware setup photo?


(Sjohns064) #4

Hi Tatiana,

Here’s an image showing the IC buffer setup:

The servo is a Savox SC-0252MG, and the UBEC powering it is a Turnigy 5A (7.5A peak) UBEC.

Without the IC buffer, the servo doesn’t move initially when you send it a signal, but if you manually move it to the desired position (when the PWM signal is being sent), then on all subsequent runs it works perfectly (no matter what the starting position of the servo shaft). The servo does have a large mechanical load on it, but it has more than enough torque to move, and it does move fine after this initialisation procedure.

It’s a confusing problem, but I have a friend who’s an electronics engineer and he suspects that the low PWM voltage is the issue. We have tried with other servos, and the PWM voltage (from the same signal pin on the Navio2) is higher, and they work fine without mechanical initialisation. Perhaps the larger mechanical load is somehow dropping the PWM voltage, hence why we are trying to use a buffer to lift the voltage to 5V. We have tested this buffer setup with a PWM input from a signal generator and it works fine, but it doesn’t work when using the signal from the Navio2 pins. Thanks very much for your help.


(Marc Lebret) #5

PWM signal voltage new “norm” is around 3V. Some “Old” servo need 5 volt signal. You can find servo amplifier in you need to use this servo.

Marc


(Sjohns064) #6

Hi Marc; thanks for the response. I wasn’t sure about the correct voltage, but I think your correct. I did a bit more research and I think the servo should be using TTL logic levels. i.e. 2-5V is interpreted as high.

Do you know if the Navio2 nominal signal output voltage is 3V? If it is, then presumably the issue is not related to voltage.


(Tatiana Andreeva) #7

Hi @sjohns064,

Yes, it’s 3V.

Navio2 can’t detect an impedance mismatch, so you can use any logic with it.

I don’t recommend you to use 74HC14 Hex inverter as it’s based on Schmitt flip-flop. The signal on the flip-flop output is inverted. We propose using something like 74LVC2G17.

May I ask you to provide us with the schematic for hardware setup photo you shared earlier?

How exactly did you measure the signal? Did you use an oscilloscope? If yes, could you please post here the screenshot of PWM output signal from Navio2 connected to your scheme?


(Sjohns064) #8

Thanks very much Tatiana for your suggestions. I’m going to try to use the 74HC14 to double invert the signal; i.e. get a non-inverted output just like the 74LVC2G17 provides.

I didn’t get any signal from the buffer board when I tried it with the inverted signal setup, because the Navio2 seemed to shutdown and not send any signal. I checked with a oscilloscope the signal on the Navio2 output pin, and there was no signal.
The output from the buffer board was just a constant 5V. I think this is because the buffer output is pulled up with the resistors when there’s no signal, when it should ideally be pulled down. I’ll let you know how I get on. Thanks again.


(Sjohns064) #10

Hi Tatiana,
Do you know there are any other Non-inverting schmitt trigger IC’s that come in any larger form factors, as I’m struggling to solder the 74LVC2G17 as it’s so small? Thanks again.


(Tatiana Andreeva) #11

Hi @sjohns064,

You can use any non-inverting buffers with Schmitt trigger input. I’m afraid I hardly can recommend you something more specific.