Hi,
Looks like the AC servos and drives are more expensive than steppers, but perhaps just because they go up in size quickly, while steppers with more than 1 N*M torque seem rare.
Yes, good quality servos but still not 'top shelf' are close to double the price of a stepper and drive.
All steppers lose torque the faster they go. The higher the winding inductance the worse the torque degradation. Most run of the mill
cheap 23/24 size steppers with 6mH inductance have as little as 5% of their holding torque at 1000 rpm. If you choose wisely and get steppers
of 1-1.5mH they may retain 35%-50% of their holding torque at 1000 rpm.
34 Size steppers have a lot more torque, 1200 oz.in is common but have very much higher inductance also and therefore will probably
not be much good beyond 500rpm.
All steppers, irrespective of size have no overload capacity, the first time you know that you are approaching the torque limit (at the given operating
speed) is a missed step or stall. Servos on the other hand just 'dig in', and produce the extra torque, commonly three to four times rated torque
with a heat limited duration.
The torque/speed diagram you posted is normal for servos. My Delta servos have torque/speed curves exactly like that.
I suggest you look at Delta, a Taiwanese brand manufactured in China or DMM, a Canadian brand manufactured in China, both good quality
and well supported that won't break the bank. There are even cheaper no-name Chinese brands but of questionable quality, support and
documentation, none-the-less seem good value.
Well chosen (low inductance) steppers with the best highest voltage drivers and highest voltage power supply deliver great results when used within
their envelope, but the operative words here are 'within their envelope'. If you attempt to stray out of that envelope they stall, no ifs or buts, they stall.
Having said that they have a great deal of torque in a small package, better than servos pound for pound. Where servos show their strength is
their overload capacity and speed, but you buy those advantages at a hefty price.
Craig