r691175002 wrote:Thanks for bringing them up, they are actually really neat. The marketing is pretty much bang on: they hit an amazing price/performance ratio that actually beats most Chinese systems in the Nema 23+ size range. I have been planning a build that uses leadshine easy-servos but will probably switch to the clearpath motors instead.
Unfortunately I doubt many hobbyist builds can make use of their increased performance. Unless your build is rigid enough to hold <0.005" under full load I wouldn't recommend a servo.
There are a huge number of differences between stepper motors and servo motors, but the most important ones are difficult to explain. Here is a quick overview:
[list][*]Surprisingly, stepper/servo motors are electrically identical and completely interchangable - there are only so many ways you can wind a brushless motor. Stepper motors often have higher pole counts, but the real difference is purely the presence of an encoder+control system.
No, servos and steppers are not electrically identical. They are two (bipolar), four(unipolar), or five phase motors. The rotors and stators are constructed differently as well, the serrated rotor and stators that direct the magnetic field to do the stepping. Feedback is what makes a servo a servo. That can one or more of a multitude systems. encoders, resolvers, hall effect, potentiometers, etc.
r691175002 wrote:[*]Servo motors often achieve a higher maximum speed but lower torque.
Servos have more running torque and speed, especially at high speeds. The torque rating you see listed for a stepper is not the same kind of torque for a servo, it is the step torque, the amount necessary to move the motor one step while the coils are energized at the rated parameters of the motors. Steppers are great for move and hold type situations.
r691175002 wrote:[*]Servo motors can achieve enormously higher resolution. Realistically you cannot expect accurate motion below half-step resolution out of a stepper - there is just too much sticktion. Servo motors can get to within a few counts even at 10,000+ PPR.
Yes, servos, especially modern ones, have insanely high encoder resolutions. The mitsubishi ones I am using on my telescope one come standard with 131072 line encoders and these are two generations old. I believe the current gen, the J4, has 2 million line encoders.
r691175002 wrote:[*]The biggest difference is rigidity. Magnets are really springy and if you put a large lever on a stepper you will see just how much play exists at the step level. High frequency control systems on a servo can be exceptionally rigid (similar to direct metal on metal contact).
r691175002 wrote:[*]This difference is also reflected in motion accuracy/reasonance. When you "step" a stepper motor you are only rotating the magnetic field - the shaft itself will lag and can over/undershoot. A servo can move exactly as commanded, especially in control systems that integrate lookahead.
When you advance a stepper you are really not rotating the field like you would with a three phase stepper. They are not wound like that, the cold cause a shift in the field which causes the teeth to index to the next spot, it is similar, but not the same. When you start micro stepping things change a bit and you are running them closer to a brushless motor and thats why you start loosing accuracy in between full steps.
r691175002 wrote:The deeper you dig the more advantages you find for servos over steppers, but the truth is that in most machines the motors contribute a tiny fraction of the total error. Unless you are right on the bleeding edge, money is better invested in the frame, spindle, linear motion, etc..
Servos are awesome, I am not sure about the chinese ones. Tecnik I dont know much about. In the past they have been very customer unfriendly.
I prefer to buy used Mitsubishi and Yaskawa motors and drives off ebay. The newer drives have incredibly good auto tuning and harmonic elimination that makes them real easy to use. Tuning is the bane of servo motors. Good drives have it built in, cheap ones you have to set manually and it can take quite a bit of time on a new machine, there are so many variables to get smooth servo operation. I had a heck of a time getting my laser cutter tuned, I chose not to use mitsubishi servos on that.
The teknic servos do look neat and the price is not terrible, the resolution is pretty awful though.