Hi, I have a motor driver which takes a 05V input speed control voltage. I'm driving this with an STM32 ARM GPIO line through an opto isolator.
I have a nice clean rect. wave input to the FOD817B and an RC filter on the output. It seems to work more or less as expected but it's not all that linear with respect to the PWM ratio.
Below is the cct and a graph of the input and output. The input is measures with a DVM which averages the 4kHz pulse train and gives a nice linear increase. The output clips a bit below 5V as expected due to VCE0 of about 0.14V. The output ripple is a fairly linear ramp up and down except for a little squiggle at the changeover. The RC is a bit too short.
lin(x) on the graph is a linear regression between 2000 and 18000 rpm. The "rpm" being the theoretically expected speed from which the PWM ratio is derived at 10V for 24000rpm range.
Since the opto is switching a digital signal I was expecting the output to be a lot more linear than this. The charging and discharge paths are not the same and I think this is pumping up the cap at higher mark/space ratios.
The motor driver is a Chinese VFD for which there is no spec for the input: I have no idea what I'm driving but guess it would fairly high input impedance.
Can anyone suggest how this could be improved without major additional complexity?
TIA.
I have a nice clean rect. wave input to the FOD817B and an RC filter on the output. It seems to work more or less as expected but it's not all that linear with respect to the PWM ratio.
Below is the cct and a graph of the input and output. The input is measures with a DVM which averages the 4kHz pulse train and gives a nice linear increase. The output clips a bit below 5V as expected due to VCE0 of about 0.14V. The output ripple is a fairly linear ramp up and down except for a little squiggle at the changeover. The RC is a bit too short.
lin(x) on the graph is a linear regression between 2000 and 18000 rpm. The "rpm" being the theoretically expected speed from which the PWM ratio is derived at 10V for 24000rpm range.
Since the opto is switching a digital signal I was expecting the output to be a lot more linear than this. The charging and discharge paths are not the same and I think this is pumping up the cap at higher mark/space ratios.
The motor driver is a Chinese VFD for which there is no spec for the input: I have no idea what I'm driving but guess it would fairly high input impedance.
Can anyone suggest how this could be improved without major additional complexity?
TIA.
Attachments

10.4 KB Views: 17

41.6 KB Views: 17
Last edited: