Servo drives vs. stepper motors: differences, applications, and how to choose

Servo drives and stepper motors differ primarily in their method of motion control. A standard stepper motor performs positioning based on sequential control pulses, often without feedback regarding the shaft’s actual position. A servo drive, however, utilizes an encoder to continuously correct the difference between the commanded position and the actual position.

This does not mean, however, that a servo is always the better choice. Stepper motors perform well in applications requiring precise positioning, low-to-medium speeds, and moderate dynamics. Servo drives gain the advantage in scenarios involving rapid speed changes, heavy loads, and applications where the drive must respond in real-time to positional errors. Therefore, the choice should be based on the specific characteristics of the axis in question rather than the drive technology alone.

Servo drives vs. stepper motors: how do their operating principles differ?

A standard stepper motor moves by a specific angle upon receiving a control pulse. The controller counts the pulses to determine the commanded position; however, in a typical open-loop system, it does not verify whether the shaft has actually completed the full programmed movement.

To gain a deeper understanding of how a stepper motor works, it is worth paying particular attention to the relationship between pulse frequency, speed, and torque. This relationship largely determines the operational limits of the stepper motor. A servo drive, on the other hand, utilizes a closed feedback loop:

1. the controller commands a position, speed, or torque,
2. the motor executes the movement,
3. an encoder measures the actual position,
4. the regulator compares the actual value with the commanded value,
5. the controller corrects the error.

This enables the servo to respond to load changes during operation. The system’s operation is best illustrated by the working principle of a servomechanism, in which the motor, feedback mechanism, and regulator form a unified motion control system.

It should be noted, however, that modern stepper motors can also operate with an encoder. Consequently, integrated closed-loop stepper drives partially narrow the traditional gap between stepper motors and servos.

Stepper motor vs. servo – comparison of key parameters

Parameter	Stepper motor	Servo drive
Typical control	Pulse-based, often open-loop	Closed-loop with encoder
Low-speed torque	High	High
High- speed torque	Drops significantly	Maintained over a wider range
Dynamics	Good for moderate requirements	Very high
Position control	Based on pulses or encoder (closed-loop)	Continuous encoder feedback
Risk of undetected position loss	Możliwe w układzie otwartym	Sterownik wykrywa błąd pozycji
Very low-speed operation	Very good	Very good
Torque control	Limited	Precise
System complexity	Usually lower	Higher
Cost	Usually lower	Usually higher
Typical application	Dispensing, positioning, linear modules	Robotics, CNC, dynamic machine axes

The biggest mistake in such a comparison is looking solely at nominal torque. A drive’s suitability is determined by the torque available at the required operating speed, not the maximum value available when the shaft is stationary.

Stepper motor power – how to determine it?

Stepper motor power is not a parameter to be analyzed independently of torque and speed. Mechanical power at the shaft is derived from these two quantities.

It can be calculated using the formula:

P = M × ω

where:
● P – mechanical power [W],
● M – torque [Nm],
● ω – angular velocity [rad/s].

If the speed is known in rpm:

P = M × 2π × n / 60

For example, a motor generating a torque of 1 Nm at a speed of 600 rpm develops:

P ≈ 62.8 W

However, the phrase “at a speed of 600 rpm” is crucial. If the motor’s torque drops to 0.4 Nm at that speed, the power output will be only about 25 W.

Therefore, when selecting a stepper motor, one should use the torque-speed curve rather than relying solely on the holding torque value.

Stepper motor rotational speed – what does it depend on?

Stepper motor speed depends directly on the frequency of the pulses sent by the controller.

For a simplified system:

n = 60 × f / N

where:
● n – speed in rpm,
● f – pulse frequency [Hz],
● N – number of pulses required for one full rotation.

If a standard motor performs 200 full steps per revolution and the controller generates 2,000 pulses per second:

n = 60 × 2,000 / 200 = 600 rpm

Mathematically, speed can thus be increased by raising the pulse frequency. In practice, however, a limitation arises: stepper motor torque decreases as speed increases.

This is primarily influenced by:

● winding inductance,
● driver supply voltage,
● phase current,
● rotor and load inertia,
● required acceleration,
● control method.

It is precisely in this area that a servo drive often gains the advantage.

Stepper motor torque vs. servo drive torque

A stepper motor generates torque very effectively at low speeds and when holding a position. Its characteristics change however, it changes significantly as rotational speed increases.

Stepper motor holding torque

Holding torque defines the maximum torque a powered, stationary motor can withstand without losing its commanded position.

It therefore applies to the condition:

n = 0 rpm

This is not the torque available across the motor’s entire operating range.

If a stepper motor has a holding torque of 3 Nm, the available torque at higher speeds may be significantly lower. A torque-speed curve is necessary for proper selection.

With JVL integrated stepper motors available from AMG, options range from compact drives to NEMA 34 designs delivering torque in the range of over ten Nm; however, even here, the specific operating point must be verified against the given model’s performance curve.

How does a servo motor behave?

A servo drive is designed to maintain a commanded torque over a much wider speed range. Additionally, it often provides peak torque for short periods of acceleration and deceleration.

This is crucial for an axis that must:

● start quickly,
● accelerate the load,
● brake precisely,
● change direction instantly.

In such a cycle, the zero-speed torque alone reveals little about the drive’s actual capabilities.

Is a stepper motor less accurate than a servo motor?

Not necessarily. Accuracy depends on the entire mechanical system and the control method.

A stepper motor moves in discrete steps. A standard 1.8° motor has:

200 full steps per revolution.

Microstepping can significantly increase control resolution, and integrated stepper drives can achieve very high internal resolution.

However, this does not mean that every microstep corresponds to identical mechanical accuracy. Factors influencing this include:

● motor tolerances,
● load,
● system elasticity,
● gear backlash,
● coupling stiffness,
● friction,
● resonances.

A servo drive offers another advantage: it measures the actual position and can correct errors.

Therefore, in applications where guaranteed control of the actual position under varying loads is paramount, a servo drive may be the more suitable solution.

When is a stepper motor sufficient?

A stepper motor works very well when the application:

● operates at low to medium speeds,
● performs repetitive movements,
● involves a predictable load,
● does not require extreme acceleration,
● requires high motion precision at low speeds,
● needs to remain relatively simple in design.

Typical applications include:

● dispensing systems,
● packaging machines,
● linear guides and axes,
● adjustment mechanisms,
● industrial printers,
● positioning tables,
● small manipulators,
● laboratory equipment.

In more demanding systems, it is worth using a stepper motor with an encoder. This allows the controller to receive feedback on actual movement and detect deviations from the target position.

When is a servo drive the better solution?

Servo systems are particularly advantageous when an axis must operate dynamically and under varying loads.

They are a good choice when the following are required:

● high speeds,
● very rapid acceleration,
● frequent changes in direction,
● torque control,
● a wide speed regulation range,
● real-time position error correction,
● operation with rapidly changing loads.

Typical applications include:

● industrial robots,
● CNC machines,
● high-speed pick-and-place systems,
● high-performance packaging machines,
● gantry axes,
● electronically synchronized applications.

In such systems, the ability to continuously compare the commanded position with the actual position gives the servo drive a significant advantage.

And when should an integrated stepper motor be considered?

Between the classic stepper motor and the traditional servo drive lies a third category of solutions: integrated stepper motors operating with feedback.

In JVL ServoStep drives, the motor works in conjunction with an encoder and integrated control electronics. This allows for the retention of stepper motor advantages—such as high torque at low speeds—while simultaneously enabling actual position monitoring.

Such a solution can be beneficial when:

● a classic stepper motor is too simple,
● a full servo system is unnecessary,
● a compact design is important,
● wiring needs to be minimized,
● closed-loop position control is required.

This is precisely why the boundary between a stepper motor and a servo drive is not as clearly defined today as it was in traditional drive systems.

How to choose: servo drive or stepper motor?

It is best to start with the actual application parameters.

Choose a stepper motor when:

● peak torque is required at low speeds,
● movement is predictable,
● the load does not change abruptly,
● dynamic requirements are moderate,
● simplicity and cost-effectiveness are paramount.

Choose a servo drive when:

● high speeds are required,
● the axis undergoes rapid acceleration and deceleration,
● the load changes during movement,
● precise torque control is needed,
● position errors must be detected and corrected in real time.

Consider a stepper motor with an encoder, when:

● you need feedback,
● most movements occur at lower speeds,
● you want to reduce installation complexity,
● the application does not require the dynamic performance typical of a full servo drive.

The final selection should consider not only the motor’s nominal torque but also the torque-speed curve, inertia, duty cycle, accuracy, and communication requirements.

Stepper or servo? First, define the operating point

The most important question is not “which technology is better?” but rather “what torque and dynamic performance does the specific axis require at a given speed?”

For a proper comparison, it is worth preparing the following data:

● required continuous and peak torque,
● maximum speed,
● acceleration,
● load inertia,
● required accuracy,
● duty cycle,
● range of load variations,
● required position control method.

If the axis operates primarily at lower speeds and performs repetitive movements, a stepper motor can deliver excellent performance with a simpler design. In cases requiring high dynamic performance and variable loads, a servo drive usually holds the advantage. Stepper solutions with feedback further expand the range of applications where a stepper motor can be used instead of a conventional servo.

FAQ

What is the difference between a servo drive and a stepper motor?

A conventional stepper motor positions the shaft based on pulses without feedback regarding its actual position. A servo drive uses an encoder and a closed-loop control system, allowing it to detect and correct position errors in real time.

When should you choose a stepper motor, and when a servo?

A stepper motor is suitable for applications involving moderate dynamics, lower speeds, and predictable loads. A servo motor is a better choice for high speeds, rapid changes in direction, variable loads, and applications requiring precise position and torque control.

What is the torque of a typical stepper motor?

Torque depends primarily on the motor’s size and design. Small stepper motors may offer values ​​below 1 Nm, larger NEMA 23 units a few Nm, and NEMA 34 motors can reach values ​​in the range of ten to twenty Nm. However, when selecting a motor, the torque available at the required operating speed is more important than the holding torque alone.

Is a stepper motor as accurate as a servo motor?

A stepper motor can provide very high positioning resolution, especially when using microstepping. However, a servo motor has the advantage of continuous real-time position monitoring and error correction. The accuracy of the entire axis is also determined by the gearing, backlash, rigidity, and the machine’s mechanical components.