FAQ Servos


 



Can a SmoothStepper run a Servo Motor?

The SmoothStepper can drive a servo motor if the motor driver has a digital interface (step & direction, quadrature, or CW/CCW).  The SmoothStepper does not have a PWM or analog output, so it cannot drive that style of motor driver.

For your axes (X, Y, Z, A, B and C or O.B. {Out of Band}), the SmoothSteppers can control stepper motor drivers and digital servo motor drivers. 

The SmoothStepper outputs 5V signals (with currents limited to 24 or 32 mA, dependent on the selected output pin). Your Servo will need to accept a 5V signal as Step/Dir, CW/CCW or Quadrature formats.

 



Do I have to tune my Servo motors?

YES, tuning is of critical importance!  Tuning will optimize your servo's PID loop parameters to work with the loads of your axes, along with your acceleration and velocity parameters, to get fast and smooth motion with minimum overshoot and minimum lag.  

If you don't tune, you will likely have jerky motion that is not smooth and/or sound noisy.

Make sure that the acceleration and velocity parameters you set in Mach's Motor Tuning window are the same or slightly lower than the acceleration and velocity parameters you set in your Servo's configuration before tuning.  If Mach has higher values for these parameters, you may get rough and jerky movement.  (Mach's acceleration and velocity parameters can be significantly lower than the Servo's parameters, you just won't achieve your hardware's maximum potential.)

 



What is the difference between a Servo and a Stepper?

There are several main differences:

 

Torque vs Speed curves

Servos have a very distinct advantage over steppers at speed, however steppers enjoy a BIG advantage in torque over servos at low
speed.

Stepper motors have a tremendous amount of torque (holding power) at a standstill and lower speeds.  For precision mill work, where you are producing a lot of cutting torque and need lots of positional holding power, steppers do a great job! In this area, servos will need to be oversized in order to provide a comparable amount of torque as steppers at these speeds and loads. 

As you go to higher speeds, servo motors take the lead in torque and speed.

 

Open vs Closed Loop Control

In Open Loop Control mode, every time the SmoothStepper sends a step pulse, the motor driver will advance the motor, without providing positional feedback to the motor driver.  The motor driver will assume that the motor successfully made that move.  If the axis binds up, there was material blocking movement or cutting too aggressively into material the motor will stall and the motor driver will be unaware.  Once a machine is set up properly, this is typically not a problem, however, it is a possibility.

In Closed Loop Control mode, the motor driver will record the incoming step pulses from the SmoothStepper and know what the motor's commanded position should be at all times.  The motor's current position is measured constantly and fed back into the motor driver.  The current position and the commanded are fed into a PID loop, and the motor driver will adjust the motor's actual position to correct for any missed or lost steps, while not lagging too far behind the commanded position or overshooting the commanded position. The SmoothStepper can read the positional data in via encoder inputs, and will display the positional data, but you do not need to send the positional data back to the SmoothStepper in closed loop mode.

Most Stepper motors are driven with an Open Loop control, although some stepper motors now come with closed loop controls.  If you are considering going with closed loop steppers, it is usually worth spending a little more and go for servos with closed loop control. 

Servo motors are closed loop systems.   The downside is that the servo motor may lag slightly behind its commanded position, especially at high speeds. Also, servos  may hunt for its position while at a standstill (jitter back and forth) and can run very rough in an improperly tuned systems.  However, if your system is tuned properly, having closed loop servos is more of an advantage than a disadvantage.

 

Cost

Servos will cost more than the roughly comparable steppers.

 



How do I connect my Servo's Fault or Error output to the SmoothStepper and Mach?

The Fault or Error output from a Servo driver can be connected to unused SmoothStepper inputs and assigned to unused limit switch signals.  If one of the drives fault, the resulting "limit switch" event will stop Mach and motion.   However, you should not rely just on software to stop movement.  You should have a hardware circuit to shutdown motion (the other servo motors) and spindles, lasers or other potentially dangerous outputs.  Please review our safety information page. 

Most servo motor drivers (controllers) have a method to connect all of the Fault/Error lines together, so if one faults, they will all shut down.

 



What brands of Servos are there?

We have a forum thread dedicated to which servos are used by SmoothStepper customers.

Here is an alphabetical list of servo motor brands that have been successfully used with the SmoothStepper, let us know if we missed one that you use:

Adtech (Europe)

Allen Bradley

Clearpath by Teknic

Delta

Dynamic Motor Motion (DMM)

Gecko Drive

Leadshine Note that "Easy Servos" are closed loop steppers, not true servo motors. 

Mitsubishi

Yaskawa 

 

 

 

 

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