Does anyone know how to make the input voltage vary from 0 to 1.8 instead of -0.9 to 0.9?

That's very easy: set VIN1 sin() function to:

sin(0.9 0.9 1000k)

first parameter is the

offset

, second one is the

amplitude

.

When I do this:

The input signal is correct. If you want see the comparator output changing state you must set the reference voltage VIN2 to an intermediate voltage, like 0.9V