10 tips for servos and steppers a simple guide What are the basic application differences between servos and steppers? Where would you choose one over the other? This short 10 point guide, offers a simple non technical explanation of the differences between these positioning technologies and attempts to de-mystify the subject, putting straight many misconceptions. By Nigel Dawson Edrives Product Manager nigel_dawson@festo.com
1. Torque comparison of steppers and servos Many people believe stepper motors do not have the torque capacity of servo motors. This is not necessarily true. The nominal torque of a stepper motor is very similar to a servo for a comparable frame size. Of course a servo has an additional time dependant peak toque rating and their speed curve is considerably more flexible and higher performance than a stepper. If a stepper solution is correctly sized, it offers a real low cost alternative to a servo. 2. Good applications for steppers Servo motors, despite many claims, are not always the best performance choice for applications. The structure of a stepper motor means that positions are discrete steps in its revolution cycle. If an application requires very good stand still stability, for instance positioning a vision system, a stepper will always be better than a servo. Servo motors resonate backwards and forwards at standstill, giving them their positioning characteristics; stepper motors stay very stable at standstill. Positioning of vision systems is an ideal application for stepper motors 2
3. When is a servo not a servo? The term servo, often refers to a system with closed loop feedback, hence a servo motor is really only a servo when coupled to the drive to close the loop. This also means that any motor type can be built into a servo system to offer a cost optimised servo system. Such low cost servos utilise stepper motors to give all the functionality of a servo at 50% of the cost with speed being the only drawback. 4. What to watch on a stepper application Certain characteristics of a stepper motor necessitate careful selection on applications. The torque curve of a stepper is very short with high RPMs leading to a drop off in torque quite quickly. Very low speeds on steppers also offer low torque; therefore vertical applications where the motor holds a load stationary are often not suitable for steppers. Applications that require very smooth steady speed are also difficult with steppers due to the resonance created by the discrete steps; this however can often be improved by micro stepping or closing the loop. An example of a servo drive, designed specifically for encoder enabled stepper motors 3
5. What is micro stepping? How can a motor with only 200 steps offer 200,000 positions in one revolution? The stepping driver that gives the motor the correct power and phases for it s task is the product that contains the micro stepping function. By varying the current between 2 poles in the motor we can affect the rotor position between the two poles. If the driver gives 50% of the current to each pole, the rotor will be exactly half way between the poles, therefore offering half steps. Many drivers can offer 1/1000 steps, therefore increasing the resolution considerably. Be careful, micro stepping has a negative effect on the torque curve for stepper motors, reducing the torque by as much as 30%. 6. Torque the torque One major advantage of servo motors is their ability to control their torque in the application. Steppers motors operate constantly at full torque, therefore making it impossible to utilise them in applications where force or torque control is required. If a servo motor is connected to a ballscrew drive, the application can offer precise force control for pushing a product or press fitting products together in assembly. However, closed loop stepper systems do have the ability to offer torque control. 4
7. Size IS everything Servo motors and stepper motors are sized in EXACTLY the same way. Care must be taken to ensure that the torque requirements for the stepper motor during acceleration (the most power hungry phase of the movement) lies within the nominal torque curve of motor. Servo Motors have more flexibility as the acceleration torque generally can lie in the peak torque curve of the servo motor and only the Root Mean Square torque of the overall cycle needs to lie within the nominal torque curve. Stepper motor torque curves showing single profile for three alternative voltages for the same motor Servo motor torque curves showing the peak and nominal (sustainable) torques 5
8. Sizes for all applications Remember, the majority of stepper and servo motor manufacturers offer the same frame size motor in differing lengths. This feature allows different torques and inertia values to be obtained by the same size motor, allowing a greater degree of freedom in selection. It also allows greater performance without increasing the motor size and therefore the cost significantly, resulting is a solution that is more cost effective, lighter and with smaller installation space. By keeping the frame size smaller and increasing the length, any costs of additional gearboxes are reduced significantly too. 9. Don t use a sledgehammer to crack a nut. Many servo motors are attached to mechanical actuators such as belt drives to convert the rotary motion to linear motion. Often stepper motors are ignored for this application on the presumption that they are too slow. However, belt drives generally have long travel distances per revolution, with many medium sized belt drives moving 100mm per rev. Servo motors on belt drives are often oversized on speed For a relatively fast movement of 2 metres/sec., the motor would have to rotate at only 1200 rpm, well within the scope of a good quality stepper motor. A servo motor with 12000 rpm performance, would offer a top speed of 20 metres/second, far faster than any belt drive can travel. Don t over specify the application when you do not have to! Different length motors with the same frame size offer significant advantages 6
10. Power equals performance On both stepper systems and servo systems, the more current you have, the more torque is generated by the motor. If a 100mm frame size servo motor is the only size that will fit the application space, then using a 3 phase 400V AC drive will offer significantly more torque than a 240V AC variant. Similarly with a stepper drive you have the choice of varying the DC voltage to the drive, many systems work on 24V DC, 48V DC or 72V DC, obviously a 72V power supply will offer an increase in performance on 24V. One advantage of a stepper drive is the amount of torque that can be generated from a simple 48V DC power supply, meaning the drive is running at a safe supply voltage, adding the benefit of safe and easy maintenance. Examples of 24V and 48V DC power supplies for use with stepper drives 7
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