Servo vs Stepper

Question

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?

P.S. 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.

Answer 1

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.

Answer 2

The main reason why stepper motors are used is their low price. Small stepper motors (NEMA 17 and smaller) are powerful enough for 3D printers because the mass is so small. These motors generally require a maximum of 2 amps of current. 2 amps is small enough that control chips with all the circuitry and drive electronics can be manufactured as an integrated unit. The most common chips in 3D printers are the Allegro line of chips. Since they are so common, the price per chip is extremely low.

Servo motors could have a similar fully integrated chip, but the issue is servos need to be tuned for their specific application. A servo needs to be tuned separately for each axis of each model of a 3D printer (or other CNC device). This makes servos much more difficult to use for hobbyists in particular. If a servo is not tuned properly, it could oscillate dramatically or respond to inputs too slowly. The interface for tuning would probably be a serial interface, which would increase the cost and complexity dramatically.

Additionally, servo motors require an encoder of some type. Often it is an optical encoder, but there are also magnetic encoders and capacitive encoders. The cost of the encoder is essentially constant no matter how powerful the motor is, so when you have a small motor, the cost of the encoder is significant. High resolution magnetic encoders are $3 each in bulk. That would add a lot of cost to the motor, not including the other more expensive drive electronics. Optical encoders can be cheaper; I'm not sure of the specific pricing on those.

Ignoring cost, servo motors with high resolution encoders would definitely be better for 3D printers. I think we will likely see some higher end consumer printers with servos come onto the market soon. Servos would eliminate missed steps, potentially increase acceleration and maximum speed (depending on the size), and would reduce vibrations if properly tuned.

The answer given by markshancock brings up good points too, but I wanted to elaborate on the cost issue.

Answer 3

Servo is best when you primarily provide an exact position to go to, sounds like a human arm isn't it? However, number of positions is sometimes overwhelming.

Think about a painting, is it easier to draw point by point or just throwing the pencil relatively with instincts? That is why you need less computing when using stepper motors because they just step locally without worrying about the overall painting. Having said that, it is more costly for a stepper motor to travel far distances because it has to think about its every step.

Answer 4

The basic difference between a traditional stepper and a servo-based system is the type of motor and how it is controlled. Steppers typically use 50 to 100 pole brushless motors while typical servo motors have only 4 to 12 poles.

Answer 5

Servo for spindle to control rotation speed of cutting bit. Stepper to control movement and tool position. All CNC machines.