Closed loop Step Servo Drive Driver 2HSS86H + 4.5N.M Motor + Encoder

[JAZZCNC]

Servo motors are closed loop by definition. Whether you've got a closed loop driver on a stepper motor, or a more conventional motor, doesn't change the fact that the driver is continuously monitoring the following error. Following error and position loss are effectively the same thing - the closed loop control corrects for this error with both systems. With either system you should be able to select what happens when this error exceeds a certain threshold - i.e. loss of position.

So then by your definition just sticking a Servo motor and drive on a machine using Mach3 makes it closed loop.!!. . . . NOPE. . .Why not.? . . . .Because Closing the loop is the function of the Motion Controller not the Servo drive or Motor.

Again the Servo can only watch the following error report back to the motion controller and fault if goes past it's parameters. Positional correction is down to the Controller and if like Mach3 it doesn't have the abiilty to close the loop then position is not corrected.
The Closed loop Stepper system has built in controller to do the correction so it doesn't matter if the Controller sending the pulses doesn't close the loop position is still corrected AND just like a TRUE CLOSED LOOP servo system has the abilty to fault if outside set parameters.

Servo's have there place and like in Silyavski case with a very heavy gantry then I agree they are probably better suited.. . . .BUT . . My original case that for general DIY machines having or needing the luxury of CLOSED LOOP capabilty's still applies and they are much easier and cheaper than servo's plus can be just has accurate if correct size for the machine.

Now are they better than standard Stepper system then YES but only because they Close the Loop at the drive and in the case of the smaller 2Nm which use 3 phase motors with 1.2deg step angle makes for a very smooth running motor. They are no more powerful, faster or anything else and if you don't want the Luxury of Closed loop then with Very good drives then agree they are an expensive Luxury that standard steppers can match other than closing the loop and correcting positional loss.

For DIY slaved axis Stepper systems then IMO they are the DOGs Danglies offering the best all round solution with Minimal hassle of setup and high reliabilty against racking or positional loss.!!. . . . It just comes with a price tag.!!

Would I use them.? . . . . Yes but only on high end machine with Slaved Axis.!

[Jonathan]

I think you're mixing up two different control loops. Any servo motor drive with position control uses feedback to maintain the position error below a specified limit. The closed loop stepper system is just another type of servo system, hence both 'standard' servo motors and 'closed loop stepper' systems correct for position error. How well they can correct is dependent on the motor ratings, but with either system if you exceed the motor torque rating the result is the same - loss of position (i.e. position error outside limit for some time).

Within the motor driver, the position command (step&dir) is converted to a speed reference using a PID (+possible other bits) controller, then into a torque reference using a PID (+other bits) controller, which is the input to the vector current control algorithm that uses a further two PI controllers to set the current within the motor. The speed measured from the encoder, or back-emf sensing, is fed into the PI controller and used to correct the position error. So you actually have several control loops inside one another. Sometimes people implement the position control loop inside the motion controller, and instead send speed&direction commands to the motor drive, but doing that is a separate can of worms. It is these PID controllers that are tuned when a servo motor is set up - the difference for the stepper system is they only come with a limited set of motors, which combined with the different characteristics of stepper motors means they get away without the user tuning the system for 'most applications'.

Again, this control system is essentially the same (except for the 'other bits') for all the types of motors commonly used for servos, including steppers, and can be implemented without an encoder so long as the motor speed is sufficient.

just sticking a Servo motor and drive on a machine using Mach3 makes it closed loop.!!. . . . NOPE. . .Why not.? . . . .Because Closing the loop is the function of the Motion Controller not the Servo drive or Motor.

The servo motor+drive, or closed loop stepper+drive are themselves closed loop, but the system as a whole arguably isn't as Mach3 doesn't know what the following error is. When the fault pin from the driver is connected to an input to Mach3, then Mach3 knows when the error exceeded the set threshold (i.e. the motor was operated outside its ratings for too long) and can act accordingly. You can set it up this way for servos, stepper based servos and stepper drivers with stall-detection.
Whether you close the position control loop within the motion controller (e.g. using LinuxCNC) or the driver is a separate debate to the one we're having now about the virtues of steppers and servo motors. I don't think that's worth discussing here, given that due to bandwidth limitations it's not viable using a parallel port which is probably the system most people reading this will use.

I'm not saying there are no advantages to using the closed-loop stepper system, I'm just confident that they're not tangible for the vast majority of machines made on this forum and in the case where they are, a standard servo motor system is likely offer the same performance at a lower cost.

[JAZZCNC]

The servo motor+drive, or closed loop stepper+drive are themselves closed loop, but the system as a whole arguably isn't as Mach3 doesn't know what the following error is. When the fault pin from the driver is connected to an input to Mach3, then Mach3 knows when the error exceeded the set threshold (i.e. the motor was operated outside its ratings for too long) and can act accordingly. You can set it up this way for servos, stepper based servos and stepper drivers with stall-detection.

Yes I know all this and fully agree, I also admit I may have miss read what you meant but my initial point was that Servo's offer nothing more other than linear torque than these Closed loop Stepper systems do unless you close the Loop properly at the controller which like you say is another can of worms we both don't want to open.

Whether or not you can find servo's cheaper I think is a close call and I'd say not unless cheap chinese offerings and certainly not respected brand names Like Leadshine unless second hand. Then you also have the other can of worms of Pulse generation and what's needed to connect too and get the rated speed out of those servos, which makes those potentialy cheaper servo's suddenly not so cheap.!!

It's here where servo's don't match these drives for ease of use and setup which to your average DIY user with limitied knowledge and time is quite important. . . . They essentially just plug n play and offer just the same accuracy.!! . . . Which was my whole initial point.