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| 36 principles and are constructed using a single primary coil, either fixed to or built into the rotor. The primary coil is paired with two secondary windings fixed at a 90° angle to each other (Figure 2). When an alternating current (AC) energizes the primary winding, the secondary outputs will be out of phase due to their placement, giving different peak voltages proportional to the rotor's angle. The secondary readings are then demodulated using the primary signal as a reference. A high-resolution measurement of the shaft angle can be calculated from that information. With no physical contact between the primary and secondary windings, resolvers are very rugged and reliable. On the other hand, they can be relatively expensive, complex, and power-hungry. Encoders Several types of encoders can be used to provide positional feedback. They all have unique benefits and drawbacks. Optical Encoders Optical encoders consist of an LED and two photosensors that are situated at a 90° angle from each other. The LED and photosensors are separated by a glass or plastic disk that rotates with the rotor. The disk has alternating opaque and clear lines or slots that radiate from its center. The number of lines or slots determines the resolution of the encoder. As the disk rotates, the sensors see patterns of light and dark, which are converted into pulse streams by the photosensors. A circuit then translates those streams into two bitstreams that show position and direction. This type of encoder can provide only an incremental indicator of motion. To determine absolute position, a third photosensor can be used as a reference. As optical encoders rely on vision, their performance can be hampered by any contamination or blockages that obscure a slot. The disk can also be damaged by vibrations from the motor or warped by temperature extremes. Additionally, the LEDs can degrade over time. Finally, optical encoders consume more power than other feedback-sensing methods. Despite these drawbacks, optical sensing is one of the most popular options for positional sensing. Magnetic Encoders Magnetic encoders use a disk with several poles around the circumference. Sensors—such as Hall effect devices to detect voltage change or magnetoresistive devices to detect a change in magnetic field—are placed equidistantly from each other and output a sine wave as the disk rotates. The number of poles and sensors determines the encoder's initial resolution, and a circuit then determines the position of the rotor. This technique provides incremental readings. Engineers can achieve an absolute reading by assigning a digital code to each measuring position. Magnetic encoders are rugged and capable of withstanding the shock and vibration found in electrical motors (Figure 3). Their operation is unaffected by oil, dirt, moisture, or other contaminants. However, they are susceptible to temperature extremes and the magnetic interference caused by electric motors. They also tend to deliver lower resolution and accuracy than other encoders, though modern Figure 3: This image of a magnetic encoder shows the poles on the rotor and the sensors that perform the readings. (Source: "aicandy/stock.adobe.com"; generated with AI)