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Selecting the proper motor What size motor should I use? What type-stepper or DC-brushed or brushless-permanent magnet-rare earth-AC servo.or..? Overwhelmed by choices with no clear definition of the trade-offs? Read on. Steppers vs. DC Motors A great deal has been written regarding the benefits of stepper motors vs. DC motors. In many cost-sensitive applications, a stepper is automatically chosen because it is assumed to be the lowest cost solution. Sometimes it is. A stepper motor can be operated "open loop", meaning that under normal conditions, it can be assumed that the motor will make exactly one step with each cycle of the stepping sequence to the motor coils. Normally, it does. However, if the load is too high or the acceleration is too high or the bearings begin to rust or lose lubricant or the velocity is too near the resonant frequency of the motor, or the moon is in Hades, then the motor may stop, turn erratically or run very happily in the opposite direction. The results of such uncontrolled behavior can range from amusing, through expensive to deadly. If they are so unreliable, then why are they so popular? The answer is that they are perceived to have lower cost and be easier to use than servo motors. This perception was formed and propagated when electronics were expensive and before modern electronics made it practical to use a low cost permanent magnet DC motor as a servo motor. It is true that stepper motors may be reliably operated open-loop within certain operational conditions, but if the conditions change, the reliability disappears. To operate a closed loop control, there must be some form of measurement device (encoder) to provide the position data. When this perception was being formed, encoders were expensive. Perhaps the biggest problem encountered with steppers is the resonant frequency. The first step taken in most systems to reduce the effects of resonance in stepper motors is one that nullifies the cost advantage of open loop control. This step is to add a viscous damper to the motor shaft to damp oscillations. Although viscous dampers are very effective in preventing loss of control and excessive oscillation, they cost more than many incremental encoders, which would allow closed-loop control, bringing many more advantages to the system than simple damping.
The biggest advantage that steppers have is not one of cost. Rather, the advantage lies in the fact that steppers can apply more torque in small motions than DC motors. When a stepper motor has been aligned to its stable position, no amount of current applied through the motor windings will cause it to move away from this position. A DC motor has no stable position and will attempt to run continuously at higher and higher speeds as the current is increased. Open-loop position control is not possible with a DC motor. Positioning is possible only through the use of a position feedback device and suitable electronics to control the polarity of the current through the motor. When the task to be performed requires reliable motion of an exact number of steps with each command cycle, a well-designed, damped, stepper motor may be the best choice, but it may not cost less than an equivalent closed loop DC motor system. There is another advantage of open loop systems, but it has no place in even moderately high precision systems. If there is a significant amount of "slop", "loose play", "backlash" or however it may be defined in the system, it is extremely difficult to overcome the effects of it with a closed loop system. An open loop stepper drive will stop at some non-repeatable position within the loose play. A closed loop system will want to oscillate between the bounds of the slop. Many vendors claim to have systems for overcoming backlash electronically, by always approaching the target position from the same direction, but the value of such approaches is suspect. DIVA's position is that the backlash should be designed out of the system. Methods for reducing backlash are readily available and when adopted will lengthen the life and increase the accuracy of the system.
To summarize, here is how stepper motors compare with DC motors.
So, if you need to move something very quickly, with high dynamic forces, forget the stepper. If absolute minimum cost outweighs all other considerations, perhaps the stepper is a good choice. Most systems fall somewhere between these two extremes.
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