Servo vs Stepper

  • I'm a new one for this community.
    This also not directly related with 3D printing.
    I searched about this and I couldn't find good answer.

    One of my friends told me CNC machining centers (Milling) mostly use servo motors and CNC laser cutter and plasma cutters use stepper motors mostly.

    Position controlling is more accurate in servo motors than steppers.

    I think position controlling is more important in laser and plasma cutters than CNC machining centers, but laser and plasma cutters use stepper motors.

    Why do laser and plasma cutters use steppers without using servo motors?

    This question has more area than 3D printing and CNC routing.And also, This question asked for more reason for why use steppers in laser cutters,plasma cutters and CNC router.SO, this is not a duplicate of this one.

    I think it has to do with the mass in the control loop. Servos don't have a control loop as such, so are simpler to work with, but are more limited in their ability to move a large mass precisely.

    @SeanHoulihane, Actually servos do have a control loop (often called a PID loop because of the parameters that drive it). the loop is the defining nature of what makes it a servo. Stepper motors on the other hand run open loop because they have no feedback. In addition, I would say servos are the better choice for controlling a large mass. Hmm... maybe I'll add this to my answer.

    You don't want "position control" if you are moving your tool along a path. The tool (3dprinting hot end, Laser, or whatever) shall move exactly on the path. When you need to drive to a position the "position control" is important. When driving along a part the control loop means deviations from the path.

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    @0scar Some time its hard to accept one answer because several answers can be accepted. Anyway I vote for most helpful one.

  • Servos do have several advantages; but, they are more expensive and more difficult to control.

    Generally, a servo motor is a DC motor but with an encoder to provide position feedback. A circuit (can be a computer) then compares the actual position (from the encoder) against the commanded position and uses the error to determine how much power to put to the motor (usually by PWM).

    Some of the advantages of servos:

    1. The encoders on the motor often have thousands of counts per revolution so they are accurate.

    2. They are a great choice for controlling a large mass. When beginning a motion, the control loop can detect that more power is required when the encoder does not respond as fast as expected thus putting more power to the motor. This will them automatically reduce as the motor reaches speed and no longer needs the acceleration torque. Also, the servo loop can also apply reverse torque when trying to slow down the large mass to limit overshot.

    Some of the disadvantages of servos:

    1. The DC Motors used for servos reach peak power at thousands of RPM. That means to use them on a printer you will need to gear them down. This adds to the expense.

    2. You need electronics to PWM the power to the DC Motor and to close the servo loop (usually at least 1 KHz). This can require a lot of the CPU. Probably would be more than a Melzi could do since it is already maxed out.

    3. The servo loop tuning can cause the motor to buzz when it is holding position on an unloaded axis. This could cause print issues.

    I know you have likely seen cheap servos out there often called "hobby servos". These are often used in RC. These use a creative trick that allows them to use a cheap potentiometer to create an inexpensive control loop. The limit to this "trick" is that it CAN NOT rotate a full 360°; thus, it CAN NOT run a continuous axis. Yes, I know there are hobby servos out there that are called continuous rotation servos; but, they do that by disconnecting the potentiometer. In that case they are no longer servos. This is just a way to use the same control interface to control a standard DC motor and the motors are not accurate.

    Stepper motors on the other hand:

    1. Are really cheap;

    2. Don't require complicated drive circuits or control loops;

    3. Love to hold position without a load.

    Their downside is that their rotational accuracy is limited by the physical poles of the motor. This can be improved using micro-stepping; but, there are limits. Also, it is difficult (often impractical) to determine if the motor missed a step. That can usually be handled by just making sure that the load on the motor is always well below the step torque. This often involves managing the motor acceleration.

    In summary, servos are great for some applications; but, for low cost situations like 3D printing, steppers are hard to beat. It is likely servos needed for milling CNCs because the cutting head is much more massive than an extruder or laser and the servo control loop is needed to provide accurate motion for the higher mass.

    Thanks, I think I fixed it. When I changed the answer, I wrote based on my memory of the question (which is often not as good as I think it is). That was exacerbated further by Stack Exchange hidding the question when you are editing and answer.

    Thanks for your answer. According to your explanation of servo and steppers, I can get one reason for why steppers are used for a laser and plasma cutters. CNC millings have heavy load/weight to handle. So, Accelerating and decelerating is more important than laser cutters. therefore, CNC milling need a servo. Otherwise, laser cutters have low weight load to move. So, position controlling can be reached using steppers.No need more expensive and complicated system using servo.

    CNC Machines use AC Servo Motor.

    Servomotors can use steppers internally. The "servo" part refers to closed-loop position control, and any motor can be used to move the mass around. It's just that usually it's cheaper to put a large BLDC or brushed motor instead of a stepper once you have position control.

    @NSiri - Yes, that is what I think too.

    @JashJacob I have heard two different ways for a servo being called an "AC Servo". One type is a DC Servo whose controller uses AC line power input. Rectified AC line allows for a much higher drive voltage. The voltage a DC motor runs at limits its max RPM due do back-emf; so, the higher voltage means higher RPM and HP. Another motor I have heard called an "AC Servo" is an AC induction motor run in a servo control loop. This requires a more complicated drive; but for high HP, AC induction motors are WAY cheaper than brushless DC motors.

    @BryanBoettcher You are correct, stepper motors and servos motors are both brushless DC motors. Like a servo, a stepper is more defined by its control scheme than the motor it uses. A stepper uses the poles within the motor to step the motor around its axis. The more steps, the more positions you can achieve. Similarly, servos benefit from more poles as that makes them run smoother. Thus both schemes typically use the same motors. Servo control; however, will allow you to push the envelope of the motor further than stepper control can; but, at the cost of more expensive control.

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