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Encoders vs Resolvers

An encoder is a sensing device that provides a feedback signal by converting mechanical motion into a digital electrical signal. Encoders form an integral part of most motion control systems, in which they are used to measure linear/angular position, distance, or speed. Their ability to monitor and control rotational or linear motion is invaluable to engineers who require precise and advanced feedback to fine-tune the performance of motion control systems in real-time.

Due to the variability of existing motion control systems, there are many types of encoders available for use in different applications. Choosing the right encoder for a given application depends on the components making up the motion control system in question. In terms of application, there are three major classifications of encoders, including:

  • Rotary Encoders: These encoders measure rotational motion, and they are typically coupled to a rotating shaft to detect and communicate any changes in its movement. They are mainly used for both speed and position control of rotary applications (i.e. in rotary motors). Rotary encoders feature an accuracy of above ± 10″ (arcseconds) and most of them are equipped with important safety functions. 
  • Linear Encoders: They measure motion along a straight line, with their sensor heads attaching to the moving component of machinery operating along guideways. These encoders are often used to manage errors like pitch error of a linear actuator and backlash error (which occurs when comparing linear feedback on the load to the actual feedback of a rotary motor). They also assist motion control systems to position loads at precise linear positions. High-precision linear encoders can capture linear motion measurements down to sub-micron levels. 
  • Angle Encoders: Similar to rotary encoders, angle encoders also measure rotational motion but of typically larger arc diameters. They are mostly used in applications that require precise measurements at the external diameter of a mechanical system like a huge curved surface or a circular turn table. Angle encoders with high precision can capture angular motion measurements down to sub-micron levels. Available angle encoders have an accuracy of ± .04″ (arcseconds) and up to 29 bits resolution. 

Difference Between Absolute and Incremental Encoders

Rotary and Linear encoders (discussed above) may produce either absolute or incremental signals. Thus, they are further divided into either absolute or incremental encoders. Incremental encoders only indicate that a position has changed by providing high and low signal waves that show movement from one position to the next. These encoders do not indicate a specific position. On the other hand, absolute encoders use a unique identifier for each position; so, an absolute encoder provides an indication that a given position has changed and the specific position of the encoder.

Working Principle of Encoders

Despite there being multiple types of encoders currently available on the market, they all execute the same basic function–translating linear or rotary mechanical motion into a digital electrical signal for the purpose of controlling or monitoring various motion parameters such as distance, speed, position, rate, count or direction. However, these encoders differ in the manner in which they’re constructed and how deliver the acquired feedback signal to a control device (i.e. a PLC or High-Speed Counter) within a motion control system.

Different encoders use different types of technologies to create a feedback signal. Some of these technologies include:

  • Optical Sensing Technology: This is the most common technology employed by encoders to create a feedback signal. In optical sensing technology, the encoder provides a feedback signal based on the interruption or blocking of a ray of light.

In such an encoder, a beam of light emitted by the LED light source is transmitted through the Code Disk consisting of opaque line patterns (more like spokes on a bicycle wheel). As the actuator or motor shaft coupled to the encoder rotor rotates, the LED light beam is interrupted by the Code Disk’s opaque lines before it’s picked up by the Photodetector Assembly. This creates a pulse or digital signal: Light = ON; Light = OFF. The resulting output signal is sent to the connected electronic control circuit, which will then send the appropriate command signal to produce the desired motion control function.

A Magnetic Encoder
  • Magnetic Technology: This encoder technology makes use of magnetic fields to produce feedback. For example, a rotary magnetic encoder will detect the rotational speed or angle of rotation of a rotor shaft as per the changes in the magnetic field.

The simplest magnetic encoder design is composed of a magnetic sensor and a permanent magnet. The permanent magnet is connected to a motor or actuator shaft while the magnetic sensor is mounted on a Printed Circuit Board (PCB) and integrated into the encoder structure. As the motor shaft rotates, the permanent magnet also rotates and the magnetic sensor detects the change in the electromagnetic field produced by the permanent magnet as it rotates and generates a digital signal (pulse train). Therefore, a magnetic encoder senses information on the rotational speed and position of the motor shaft as changes in the electromagnetic field then converts them into electrical pulses and outputs them as digital feedback signals to motion controllers.

Note: Magnetic encoders are not as environmentally sensitive as their optical counterparts. They can thus be used in industrial environments characterized by higher humidity, excessive vibration, and dust. But optical encoders provide the highest levels of resolution and accuracy compared to magnetic encoders whose resolution and positional accuracy are affected by inherent non-linearities within the magnetic field.

  • Mechanical Technology: This encoder technology makes use of sliding contacts and metal discs for measurement. Whenever the metal disc rotates, some of the sliding contacts meet with the metal surface while others fall into gaps. Each sliding contact is connected to a separate sensor which produces an electrical signal. 
  • Inductive Technology: Inductive encoders respond to the presence of an electrically conductive or a ferromagnetic metal at a given point. Such inductive sensors function via electromagnetic fields and coils.   
  • Resistive Technology: An encoder using this technology consists of conductive material with shaded areas and an insulating material with unshaded areas on its scale.  
  • Capacitive encoder technology: Capacitive encoders include a sinusoidal rotor. When this rotor rotates, a transmitter electrical signal is simulated. The encoder then monitors changes in capacitance on the board receiving the transmitter signal and translates them into a feedback signal 

What is a Resolver? 

A resolver is a special type of rotary transformer used to determine the angle of rotation and displacement speed of its rotor. Its structure is similar to that of a small synchronous electric motor comprising a machined metal (iron or steel) rotor and copper windings on the stator. Resolvers are similar in design to rotary position sensors with the ability to monitor the angular speed, position, and direction of a motor or actuator shaft. 

The basic design of a resolver consists of a rotor (reference coil) coupled to the actuator or motor shaft being monitored and a stator connected to an AC power source. The reference coil (rotor) may be coupled to the rotating shaft using brushes but most often it’s inductively coupled across an air gap. An inductively-coupled or brushless resolver design can be thought of as a rotating electrical transformer. The stator consists of an excitation (primary) winding and a pair of stationary stator coils mechanically displaced 90° from each other (positioned orthogonal to one another). The stationary stator coils are designated as COSINE (COS) and SINE (SIN) windings.

How Does a Resolver Operate?

Resolvers operate on the same physical principles as electrical transformers. Such transformers consist of stator and rotor windings made of copper. And the inductive coupling of those windings keeps changing depending on the angular position of the rotor. Likewise, the resolver generally includes a set of windings in the rotor and three types of copper windings (a primary winding and a pair of secondary windings) located on the stator.

The primary winding of the stator induces AC current to the primary winding of the reference coil (rotor) in form of a sinusoidal AC signal. This signal is also referred to as Input Sin Signal. Next, the secondary windings (COSINE and SINE windings) receive or pick up feedback from the rotor winding. Since the stator secondary windings are stationary and located at 90° relative to each other, they output sine and cosine signals. These output signals are then measured to provide an electrical signal relating the position, speed, or direction of the rotor coupled to the rotating shaft being monitored.

The position or angle of rotation of the resolvers’ rotor is simply the Arctan of the SIN winding output voltage divided by the COS winding output voltage. For each position of the rotor, the resolver provides a unique ratio of SINE and COSINE signals. Hence, it can determine the actual angle of rotation and rotational speed of its rotor to provide the absolute position of the coupled motor or actuator shaft. Resolvers transmit the absolute position of the rotating shaft as an analog output signal.

Note: The operation of a resolver can be varied depending on different operating parameters like input excitation frequency, the phase shift of the output voltage from the input voltage, input excitation voltage, input maximum current, null voltage, transformation ratio of the voltage output to the input voltage, and encoder accuracy. Also, the accuracy of the resolver can be improved by internally shorting one set of rotor windings.
Digital Converters in Resolver Systems

Digital Converters in Resolver Systems

Since a resolver is an analog sensing device, its output has to be converted to a digital output so as to extract information regarding the position of the coupled rotating shaft. For this reason, Analog-to-Digital (A/D) converters or Digital Signal Processors (DSPs) are essential devices for connecting resolvers to a digital controller (i.e. a PLC) or an industrial PC. Such electronic devices convert the analog output signal from the resolver to a more understandable format for the digital industrial motion control systems.

Types of Resolvers

Resolvers are classified into a variety of different types, some of these types include: 

  • Variable Reluctance (VR) Resolver: This type of resolver does not include any rotor windings but it has a primary winding and a pair of secondary windings (SINE and COSINE windings) on its stator. 
  • Classical Resolver: It includes three types of windings, with a primary winding on the rotor and two secondary stationary windings on the stator. 
  • Differential Resolvers: These resolvers merge two di-phase primary (main) windings into one of the multi-slot laminations, as with the receiver type, and two di-phase secondary windings in the other stack of sheets. The two secondary windings are used to obtain the electrical angle relation, while the remaining angle is mechanical, and secondary & primary electrical angles. 
  • Computing Resolver: This type of resolver generates SINE, TANGENT, and COSINE functions. These functions are then used to solve the geometric angle relationships for mechanical and electrical angles 
  • Transmitter Resolvers: They are characterized by a single excitation input to the primary winding of the rotor (rotor input) and Sine/Cosine outputs. 
  • Receiver Resolvers: The two windings of these resolvers are energized while the electrical angle is represented using the ratio of the sine wave output and the cosine wave output. The system revolves around the rotor windings to obtain a zero voltage. At this juncture, the mechanical angle of the rotor is equivalent to the applied electrical angle to the resolvers’ stator. 
  • Synchro Resolver: This is a highly precise resolver type used to perform various functions like receiving, and transmitting feedback signals. 

What’s the Difference between Resolvers and Encoders?

From the discussion above, we can infer that resolvers and encoders (rotary or angle encoders) perform the same function of providing feedback on the rotation speed and angle of rotation of the connected motor or actuator shaft.

Let’s now look at their differences in line with the following characteristics:

Input Signal 

Encoders are generally powered by simple DC voltage. However, resolvers require an AC reference sine wave for excitation. This input sine wave is created with a dedicated resolver power supply, which is usually powered with DC voltage.

Output Signal

Encoders produce a digital pulse train to indicate changes in motion over a short distance; counting the pulses indicates the distance, and speed over time while checking the pulse order in Channel A versus Channel B indicates the direction of movement (quadrature).

Conversely, resolvers produce a set of SINE/COSINE signal waves (analog voltage signals) to indicate the absolute position of a rotor shaft within a single revolution. The SINE/COSINE signals are essentially converted with a resolver interface board (i.e. Digital Signal Processor or A/D converter) to a digital signal to be sent to the connected digital controller or industrial PC.

Electronics Assembly 

Typical encoders have onboard electronics; this results in minimal interconnections but it limits their operating temperatures. On the other hand, resolver systems mount the interface board (i.e. resolver digital converter) and resolver power supply near the input device. This requires substantial inter-device wiring, but it allows the resolver to withstand higher operating temperatures than encoders.

Accuracy

The accuracy and resolution of encoders vary mainly depending on the disk model used in the encoder construction. Some encoders have a high resolution of 10,000 Pulses per Revolution (PPR) and even more. In fact, the best optical encoder models can produce a resolution value of 27 or 29 bits.

In contrast, the accuracy of a given resolver mainly depends on the number of stator poles (on SINE/COSINE secondary windings). The higher the number of poles on the resolver’s stator, the more accurate its output signal is likely to be. But this resolver accuracy is usually less than that of an equivalent encoder.

In essence, encoder accuracy is normally within the range of 20 arcseconds whereas resolver accuracy is in the 3 arcminutes range. Thus, encoders are more accurate compared to equivalent resolvers.

Reliability

Resolvers are much more reliable compared to encoders because they don’t include onboard electronics or other sensitive elements in their basic structure. Also, they can function optimally in extremely harsh industrial environments because they’re resistant to very high temperatures, mechanical vibrations, contaminants, air pollutants (dust), and ionizing radiation.

On the contrary, encoders are sensitive to high operating temperatures above 248°F (120 °C). Mechanical vibration and shocks also have a significant influence on the encoder output. Besides, despite optical encoders being the most accurate feedback devices, their performance is highly dependent on the operating temperature levels and the presence/absence of contamination. Moreover, magnetic encoders which feature greater environmental strength have a major drawback–inherent non-linearities within the magnetic field that affect their results. Thus, resolvers can substitute encoders in extreme environments where the former is bound to fail.

Integration

The feedback signals provided by encoders are often quadrature digital signals. For this reason, you’ll not need additional equipment to connect an encoder to a digital controller; as the encoder can transmit signals in form of Lines per Revolution (LPR), Pulses per Revolution (PPR), Counts per Revolution (CPR), etc.

On the other hand, resolvers output analog feedback signals. Thus, to integrate a resolver into a digital industrial control system you’ll require a Digital Signal Processor (DSP) or a Resolver Digital Converter (RDC). The RDC should be tuned to the drive system, as the drive’s maximum working speed is limited by the conversion rate of the RDC.

Note: You can connect a resolver directly to a control device, if the device can accept SINE and COSINE input signals as an alternative to pulse input. The control device should also include suitable software for processing the SINE/COSINE signals.

Applications

Encoders are used for speed and position control of DC motors, AC induction motors, as well as DC & AC servo motors (with the addition of motor commutation function). They are critical components in almost all types of automated electro-mechanical systems. They are especially common in industrial applications involving large machinery performing high-precision prototyping, repeatable tasks, or any other delicate work. Some of the most common applications of encoders in automation systems include CNC machining, electronics manufacturing, unmanned mechanical systems, and robotic arms, MRI and CT scanning machinery, speed and braking systems, etc.

In contrast, resolvers are commonly used for motor commutation and speed control of AC & DC servo motors, and Permanent Magnet (PM) motors. Also, due to their high reliability, resolvers are widely used in the most demanding and extreme applications where strength and stability are required. Some of these applications include steel plants and paper mills for speed & position feedback, control systems of military vehicles, oil and gas production, jet engine fuel systems, metallurgy, space industries, etc.

This entry was posted on September 22nd, 2022 and is filed under Hardware Comparison, Uncategorized. Both comments and pings are currently closed.

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