Stepper Motor vs Servos
A servo motor would usually be the motor of choice for engineers seeking advanced motion control. They are tried, tested and trusted over many years and can be found in a multitude of devices, from small home electronics right up to cars and planes. However, a servo motor may not always be the most appropriate motor for each application; they can be costly and problematic, especially when it comes to extremely precise positioning, an issue in sensitive applications. Still, servos are often selected because they are more well-known than stepper motors and are suitable for a great many uses. Cheaper stepper motors can encourage the use of servos as well –it’s a false economy to use a cheap ‘good enough’ stepper motor. The quality of the stepper driver is the limiting factor in how well a stepper motor operates. The less smooth the stepping, the less accurate the results will be. Use a high quality stepper motor and the results will follow suit.
What Types of Stepper Motor are Available?
There are three types of stepper motor:
- Advanced closed loop–used where systems require feedback. Position, velocity and applied torque are constantly monitored and maintained with great accuracy, without needing to include external sensors in the machine build. The most versatile of the stepper motors and particularly effective for rapid indexing and applications with complex motion profiles;
- Advanced open loop–a simpler and lower cost motor, used in simple processes, particularly in systems where feedback is not critical –for example, fluid or gel dosing systems, or machines with a constant simply indexing profile;
- Traditional stepper motor–used where straightforward and accurate positioning operation is required.
Stepper motors are reliable, easy to use, low cost and highly adaptable. They are used in a wide-range of applications and industries in systems that require precise control. This includes medical equipment, food processing, factory automation applications and solar panel manufacturing. They can often be used where servo motors are traditionally used and may be a more suitable choice of motor due to their accuracy. This is especially true of the closed loop stepper which can also be utilised in applications that require smooth operation, short settling times and precision across many positions.
Main Benefits of Stepper Motors
- Precise control and outstanding stopping accuracy. With a closed loop stepper, measurement errors do not accumulate between steps so a stepper motor can set positions with a high degree of accuracy.
- Stepper motors provide stable rotation and high torque at low speeds.
- Open loop control transmission of commands means they have excellent followability and no command-response lag, thanks to the commands being synchronised to a pulse.
How to Select a Stepper Motor
There are several factors to consider when selecting the best stepper motor for the application: inertia ratio; that the required torque of the application (including safety factor) is within the permissible torque value of the motor selected; the PN or harmonic gears need to be confirmed if highly accurate positioning is required; the gear ratio for the right size steps to match required indexing distance; and operational speed required.