Yes and no. It is possible to control the RPM speed of the spindle using the PWM pins of the 32bits GRBL board. But the RPM rate will be defines in your Gcode and will be transmit to the spindle via the PWM pins of the 32bits GRBL board. The RPM will be the one you have chose in your CAM software.
With the TFT you can also define some customs buttons that would send a Gcode to set the speed on a specific value.
You can also change the the speed with the TFT using the "override" screen (upper right on the info screen - the button is not displayed but it should work)
mstrens will confirm, but I suppose you have to connect PIN "VOLTAGE OUTPUT - GND" (from the converter) to GND pin (but I can see it on the picture you posted) or maybe V0 pin (of the VDF) and "VOLTAGE OUTPUT - A0" (from the converter) to the V+10 pin (of the VDF). Then connect the converter like this :
Use a multimeter to calibrate the converter setting mini and max speed values via ARDUINO.IDE in serial monitoring mode. Considering the firmware version of the STM-32 you use, you will have to "un-invert" the PWM signal. Purchase the converter first. We will explain then
In order to test the PWM converter, you can for sure use the shema HTheatre provided.
Afterwards, when you will connect it the VFD, it is better not to use the 24V and Gnd from the cnc power supply.
The PWM converter seems to be optically isolated.
So just connect PWM + and PWM- to GRBL board.
It seems that you VFD can provide 24V. So connect it to power supply of the module.
The same for the Gnd of the VFD to the power supply (gnd) of the module.
Then, I presume (but I am not sure - look at the detailed doc of the vfd) you have to connect the voltage output A0 from module to V0 from VFD.
I expect that there is no need to connect voltage output (Gnd) because it would already be connected (internally) to power supply (gnd) inside the module.