The Merits of Adding Optical Encoders to Stepping Motor

Step motors are widely used in automation due to their high resolution, precision positioning, minimal control electronics, and low cost. As an open loop system, traditional step motors are driven without the need for sensors to feed information back to a controller; however, the open loop configuration of step motors has challenges. Today’s complex projects, such as quadcopters, often call for enhanced position control. Stepper motors have been developed to deliver excellent precision, allowing very predictable performance when the step angle and gearbox ratio are both known. As a result, stepper motors have naturally become the motor of choice for many new electronics. But even with the great visibility into position and velocity that a stepper motor offers, some engineers have taken things a step further. Not content with the stepper's native open loop control, they’ve added stepper motor encoders to their motor systems. rnThe Motor Encoder: When Closed Loop Control is Essential Anstepper motor encoder is a rotary device that can be mounted onto an electric motor. It provides closed loop feedback signals, tracking the motor shaft’s position, speed, or both. At first, this functionality might seem redundant. Isn’t precision control essentially what the stepper does on its own? The answer, it seems, is yes and no. Recent research has shown that the apparently “unnecessary” guidance provided by the encoder can have a powerful impact on how well a stepper motor works. Motion controller brand Galil conducted a series of side by side tests between open and closed loop stepper motor systems. Their team discovered that the addition of an encoder could produce appreciable gains in major performance metrics. rnCompared to the open loop system, the closed loop system offered:rnSignificantly improved velocity smoothness;rnReduced overall current consumption;rnHigher torque at lower velocities than comparable three-phase brushless servo motors.rnGalil's researchers determined that “dramatic” performance gains for stepper motors were possible by integrating a positional feedback device along with a two-phase brushless amplifier. Their testing methodology and results have been made public on their website. These results are truly exciting news in the motion control world. Engineers at all levels can enhance stepper motors into even more powerful and economical solutions through the use of readily available encoder technologies. WHICH ENCODERS ARE BEST?rnAn established technology, optical encoders offer accurate and reliable performance along with a wide range of resolutions; however, they are susceptible to signal degradation and loss when exposed to dust, oil, or similar contaminants. They are best used in clean environments. Capacitive encoders utilize newer technology, offer similar benefits, and ultimately provide the same position and speed information as optical encoders. But they are immune to environmental contaminants.