Some of our Googled results for 

"Servo verses Stepper"

from the MIT website:

Open Loop step-motor or Closed-loop servo motor : 

1. Is load constant or variable ?
Office printer vs. machine tool - Stepper can not take a overload,  a servo can.
Close-loop servo can recover position after an overload cycle (a stepper can not)

2. Can the application tolerate position loss?
Servos are usually operating close loop, while steppers usually are open-loop.

3. How much load-inertia can you tolerate?
For stepper motors, do not go higher than a 10:1 ratio of load inertia vs. motor
inertia. For servo motors, that could go as high as 50:1. For higher dynamic
performance (fast acceleration/de acceleration) keep the ratio 1:1

from the Woodweb knowledge base:

Stepper systems are often “open loop” which means that the controller only tells the motors how many steps and how fast to move, but does not have any way of knowing where they actually are. This can lead to errors, should a situation arise where the motors are unable to comply with the commanded move. This can be very obvious, where the motion stops and it sounds like you stripped a gear, or subtle, where the motor only misses a “few” steps. The result is the same - the controller thinks you are at X25.5, Y15.5 and in reality you might be at X25.3, Y15.4 . This can lead to a cumulative error, which may in turn lead to crashes, not to mention out of spec parts.

from the Danaher Motion website:

In many respects, stepper and brushless permanent magnet (PM) motors are the same. This is because they share the benefit of maintenance free operation due to the elimination of brushes and commutator. A primary differentiation however is that stepper motors operate open loop, whereas brushless motor operation is closed loop. This means that the brushless servo motor provides real-time feedback to the drive amplifier to optimize voltage and current, whereas the stepper motor does not.

Stepper Motors
Stepper motor operation is done by the commutation of current through discrete stator windings resulting in the synchronous rotation (stepping) of the rotor. This is done with the assumption that the rotor shaft maintains a speed proportional with the frequency of commutation pulses. No real-time feedback is provided to assure the motor maintains pace with the desired motion sequence. If an external load on the system exceeds the motor's torque producing capability, the motor will stall. If the motor had been running at speed when this occurred, it will not be able to restart even if the load goes away, until the frequency of switching is slowed to the point where the motor has enough torque to pull into synchronism. To minimize this condition, the torque rating of the motor chosen is often two or more times the required torque. This adds to the size, weight and inertia of the motor, minimizing it's dynamic capability. To assure that a stepper motor does not overheat, voltage applied to the windings is limited so that the maximum continuous current of the motor is not exceeded. This is done regardless of whether the motor is 'running' or not, and independent of the required load. This current is defined by the relationship: I = V/R, where: I = DC amps, V = DC volts, and R = resistance of the motor winding. Because of the limitation of applied voltage, the motor electrical time constant L/R plays a significant role in determining the maximum operating speed of the motor. This is because as the switching frequency increases, the rise time of current will ultimately limit the average current per step, which relates directly to the torque developed. Stepping motors, as a result, are often limited to about 2000 RPM. The value of stepping motors for simple, low power applications is their ability to provide positioning and velocity control at a very low cost.

Brushless Motors
Brushless motor operation is also done by the commutation of current through multiple windings to the extent that the rotor flux synchronizes with the stator flux. Compared to steppers however, this commutation is done based on rotor position feedback such that synchronism is always maintained. This is referred to as self-synchronism. The motor maintains the ability to deliver torque regardless of the velocity error. In addition, because of the gain factors in the drive amplifier, current (and thus torque) can be increased when needed by applying higher voltage to the motor. The ability to deliver current based on demand is a key feature of servos. If the motor is at rest and there is no load, no current is consumed, and the motor can cool. Additionally, peak currents two to five times or more the continuous rating can be delivered for acceleration. An additional benefit of servos is the ability to operate at very high commutation frequencies and thus high speeds. Unlike the stepper drive, the servo drive has a current loop that controls the current. High bus voltage compared to what is needed to force current into the windings is available, and mitigate the electrical time constant of the winding. Speeds of 10,000 RPM or more are common, and are most often limited by mechanical factors.

from the motion system design website:

Designers who choose sides between steppers and servos limit only themselves. Don t be shorted. Learn where each motor type works best, and bring the full power of technology to your next project.
Servomotors are the clear choice for today s motion applications. Then again, so are stepper motors. What' s more, stepper motors are also less expensive than they were a few years ago and easier to select, install, and operate. But for that matter, so are servomotors.

Servomotors have two distinct advantages over steppers: They can generate high torque over a wide speed range, and they do it in a small package. They've also dropped in price over the past few years more so than steppers largely because of high-volume manufacturing.

Tuning problems, once the bane of servo users, are for the most part history. Some servo systems, in fact, tune themselves automatically and adapt to any mechanical system without a decrease in performance.

Although servomotors are designed to run at high speeds, they can run at extremely low speeds under precision control, even down to 0 rpm.   Where precision is not an issue, however, stepper motors are usually a more economical solution for low-speed applications.  Generally speaking, low speed is anything less than 1,000 rpm. Above 1,000 rpm, stepmotor torque begins to fall off, the result of energy losses and magnetic circuit time constants.  In contrast, servomotors with comparable torque do not begin to fall off until around 2,500 to 3,000 rpm, or more.

from the NEAT website:

Stepping and servo motors each have specific strengths and weaknesses which must be weighed relative to your application requirements. The stepping motor requires no tuning and can be used open loop (with no encoder). This makes them easier and less expensive to use than servo motors. A servomotor should be considered for applications requiring high speed, high accuracy, and/or particularly smooth, quiet motion.

Brushless servo motors house the coils in the stator and the magnet in the center. (Conversely, the coils are found in the center of a brush type servo motor, with the magnets surrounding them.) Both types of servo motors, when operated as part of a closed loop system, continually try to move to the commanded position. If they are not at that position for any reason (even if they were there briefly), they will see the difference in position as an error and try to correct. Due to this nature of operation, a servo motor does not “stall” like a stepping motor does, and typically moves more smoothly. At higher speeds, the servo motor is more efficient than the stepper motor. Due to its high number of poles to commutate, and greater inductance, the stepper’s torque falls off rapidly with speed. The servo’s speed-torque curve, on the other hand, is quite flat. Since power is proportional to speed x torque, the servo’s power increases with speed, while the stepper’s power levels off. The maximum continuous mechanical shaft power of the servo is nearly triple that of its stepper counterpart.

from the euclid research website:

Stepping motors can be used in simple open-loop control systems; these are generally adequate for systems that operate at low accelerations with static loads, but closed loop control may be essential for high accelerations, particularly if they involve variable loads. If a stepper in an open-loop control system is over-torqued, all knowledge of rotor position is lost and the system must be reinitialized; servomotors are not subject to this problem.

Micro-stepping is often used to position the shaft of a stepper motor between the full step positions. As illustrated in the left graph below, the shaft of a motor doesn't always follow the ideal micro-step position dictated by the indexer. In this case, 256:1 microstepping was applied to a 1.8° motor over one electrical cycle (4 full steps). The ideal shaft position (indexer pulse), the actual shaft position and the driver currents were measured simultaneously at every micro-step command for 1024 steps. Micro-stepping accuracy is determined by the construction of the motor and the accuracy of the driver. In the right graph below, notice how the driver is unable to generate clean sinusoidal currents near zero. Micro-stepping is rarely more accurate than 1/10th of a step. In the presence of a frictional load, micro-stepping can result in a stick and slide motion.

from a google website:

Cost: Once the main factor in choosing between servos and steppers. At a time when both motor types were quite expensive, cost favored stepper systems because of their simplicity. Today, lower costs seem to favor servomotors more, widening the applications that can take advantage of their capabilities.

Load inertia: As a rule of thumb, step motors usually don't exceed a 10:1 ratio of load to motor inertia. On the other hand, direct-drive servosystems with high resolution feedback and no compliance, can run as high as 50+:1 with quicker response times relative to previous technology. For a typical servosystem with a drive train that requires high acceleration or deceleration, it is best to keep the ratio within a range of 1:1 to 5:1 for quick response. To achieve a good system bandwidth with higher inertia mismatches — compliance must be minimized or even eliminated, feedback resolution maximized, and current, velocity, and position-loop update rates made as fast as possible.

Torque: Consider a constant or variable load. Servo systems can recover from an overloaded condition, but stepper systems cannot. Steppers can give you a lot of torque in a small package, under 1,000 rpm. Servomotors, on the other hand, handle torque requirements well above 1,000 rpm (as well as below). Regarding torque, designers should select the motor that provides the higher value from speed-torque curves. For the same price, most designers prefer to use servomotors.

Complexity: One change that improves reliability and maintenance in servos has been a reduction in the number of wires necessary between the power and feedback devices. Manufacturers also have taken much of the guesswork out of tuning and determining when a system needs maintenance. Automated or calculated tuning techniques and built-in diagnostic programs help simplify these tasks. Most servo drives can use traditional "step" and "direction" (stepper) inputs, usually in "position" mode to eliminate the potential for loss or addition of steps.

Resolution: Servomotor resolution is theoretically infinite, but in closed-loop operation, system positioning depends primarily on the resolution of the feedback device, be it a sine encoder, resolver, or digital-type encoder.

With steppers, there's also a difference between theoretical and actual resolution. For example, a two-phase, full stepping, 1.8° step-angle motor may have 200 possible positions in one revolution (360°/1.8°), but whether or not it's achieved depends on the application. Same is true of half-stepping and microstepping motors; a 1.8° microstepper, though specified as having ten microsteps per each full step, cannot necessarily find any position within 0.18°. Several microsteps may be commanded before torque builds up enough to overcome friction and load inertia. In a real-world situation, the motor could easily jump one or more microsteps beyond the number commanded and stabilize there. When positioning resolution must exceed 200 steps/rev, steppers may be used with a feedback encoder. In closed-loop mode, it's possible to go as high as 1,000 steps/rev. Five-phase motors and, with caution, microstepping motors, can improve on this as well.

Repeatability: Servomotors are extremely repeatable because they run closed loop. But steppers can be just as repeatable, especially when running in one direction. However, when friction load increases (as during direction reversal) the situation changes. Similar to how a gearbox must take up backlash, the stepper must also catch up to system command. During the first move in a new direction, motor accuracy is affected because the stepper is overcoming friction (the affects of the load). Once that happens, the system regains its specified repeatability.