AstroDude42 does it really have any belt? I thought it is a harmonic drive mount.
Aloha Olli,
Unlike your RST-135 which does not have any belts, the ZWO mount uses a belt to achieve the extra 3:1 (I think that is the ratio) gearing ratio, and to smooth out its strain wave gearing.
Keep in mind that ZWO uses Chinese strain wave gears, and not the one directly from the Harmonic Drive [tm] company (USA, Europe and Japan) that RainbowAstro uses. You can tell by looking at the first derivative of the AM5 periodic error curve, which is not a smooth sine wave.
The patent for strain wave gears has expired, so there are now other companies that manufacture them -- they just cannot call it Harmonic Drive [tm], since it is an active trademark in most of the world. So, ZWO can call it strain wave gear, but not Harmonic Drive[tm].
Additionally, the RST-135 uses real servo motors (analog behavior). ZWO uses stepper motors (finite discrete positions) -- if you have read my posting on measuring the stepper motor in the 5V EAF, you will see that all stepper motors have a nonlinearity (and that was what I was trying to quantify for the EAF, to see if the nonlinearity is large enough to be of concern), and that is what is causing the envelope of the periodic error of the ZWO drive to drift up and down, since not every step is equal, and as a consequence non-linear rotational angle.
Even though it tries to copy the RainbowAstro mounts, the AM5 is a cost reduced design, compared to the mount that you have. In the engineering world, there is no such thing as a free lunch.
Chen
