Spindle Gear changing

Can someone please explain how to set up gear changes. My lathe spindle motor is connected to a gearbox that has two gear ratios and the gears are changed by a pneumatic actuator. The gearbox is then connected to the spindle via belts. The spindle has a encoder connected to it via a 1:1 geared belt drive. The feedback part of the system works fine.

How can I connect this to HAL so that the machine automatically selects the correct gear based on the requested spindle speed. I think that the gear change should take place when the spindle speed is very low, but not standing still as the gears wont change smoothly.

My spindle has an encoder on and at the moment, regardless of what gear I am in, it will always run at the selected RPM. I tested this, I selected the low gear ratio manually and ran S1000 M3 and it ran 1000RPM. I then did a M5, set the gear to high ratio manually, set S1000 M3 and the spindle ran at 1000RPM.

Unfortunately there is no clutch on the gearbox so I wont be able to change gears while running.

I basically just need a way to change gears automatically when a spindle speed command is given. When a S command is given, the spindle should first stop if it is running, then change to the appropriate gear based on the S command given, then start and run at the commanded speed. If possible, I would like the spindle to stop, then run at a very low rpm , then change gears if needed, then run at the given rpm.

Is there any way to implement this ?