Voltage variation and speed

Hi,

I'm using an ESC based upon VESC4 together with a 14-pole BLDC motor. I did the settings using the VESC tool and everything works fine at the nominal voltage of 24 VDC. The speed variation is made by sending PWM, with cycle rate as the speed variation criteria. For testing, I use a lab power supply but my final use will be made with a 19-29 VDC battery pack. When lowering or increasing the voltage between 19 and 29 VDC, the speed seems to change proportionally while keeping a steady PWM cycle rate. I guess I did not find the right setting, I'd like the speed to remain the same for a given PWM cycle rate, even if the input voltage changes. Does anybody know how to do that ?

Thank you very much !

Benjamin