Foreword
Rotary transformers are mechanical or analog sensors used to determine the absolute position and speed of a motor. These sensors are commonly used in industrial, automotive, and aerospace industries—especially in harsh environments where the motor or sensor may be contaminated. Common contaminants include oil, dirt, food particles, and even extreme temperatures that would prevent other rotary sensing technologies from being implemented. Specific end products present unique design challenges and requirements, some of which are common to most industrial applications. Two of these challenges are: 1) absolute accuracy of angular position and velocity data; and 2) minimizing or eliminating electromagnetic interference (EMI).
Industrial applications of rotary transformer sensors
Some absolute rotation sensing technologies (such as optical encoders) have been repeatedly chosen for industrial applications. However , when dealing with harsh environments or for cost considerations, resolvers are the ideal choice. Servo motors are commonly used in industrial applications, connected to resolvers and other types of position sensors. Applications that typically use servo motors and servo drives in conjunction with resolvers for angular velocity and position measurement include:
CNC and injection molding machines
·lift
robotic arm
Electric vehicles (electric bicycles, electric scooters, electric wheelchairs, etc.)
Railway transportation
Agricultural and construction equipment
Buses and heavy trucks
Golf carts and low-speed electric vehicles
Main requirements for rotary transformer sensing systems
Precise and timely rotary transformer angle output
Before finding ways to mitigate the effects of electromagnetic interference on industrial systems using resolvers, it's crucial to understand why precise position control is essential. A resolver provides an analog output that is theoretically equivalent to infinite resolution. Analog-to-digital conversion techniques limit resolution by dividing the output into blocks or steps. Finitely dividing a continuous angle will lead to quantitative errors. For example, you might use a 12-bit resolver to provide an angular output. One rotation of the resolver axis is divided into 4096 steps (2^12 corresponds to one 12-bit resolution). Since one degree equals 60 minutes, one rotation (360 degrees) equals 21600 arcminutes (60x360). Therefore, the interval between each step is 5.27 arcminutes (21600/4096). It's impossible for the system to provide better information than 5.27 arcminutes.
Two key factors in determining the correct angular position are system accuracy and system settling time. The latter primarily refers to how long it takes for the angular output to display the precise position. Each component of the system needs to be evaluated to identify limiting factors. In a system, the typical error accuracy is the sum of the resolver error and the resolver analog-to-digital converter (RDC) error. Most commonly, a resolver error occurs between 3 and 10 arcminutes. Adding the resolver analog-to-digital converter error at 5.27 arcminutes, we can derive the accurate error range to be 8.27–15.27 arcminutes . Therefore, selecting the correct RDC is crucial. The following factors affect system accuracy and settling time in typical resolver applications:
Mechanical factors
• Sensor structure (zero-point voltage, transformation ratio, etc.)
Sensor specifications vary with temperature
• Unbalanced coils: The output voltages of the sine and cosine coils may be unbalanced, leading to errors.
• Rotary transformer sensor misalignment: The rotary transformer may be installed incorrectly, causing static errors in the system.
• Number of poles in a rotary transformer sensor: Since each additional pair of poles allows for the detection of an additional 360 degrees, increasing the number of poles reduces angular error.
Electrical factors
• Rotary transformer analog-to-digital conversion structure
• Time delay from signal input to angular output of the rotary transformer, resulting in rapid angular change settling time.
Imbalance of Analog Front-End (AFE) components
The system has the ability to handle environmental factors (e.g. , external magnetic fields or common-mode noise).
Stabilization time
When the motor position or output signal of the rotary transformer changes rapidly, the settling time is a fast performance indicator of the RDC control system [2]. Figure 1 shows an example of the settling time of an RDC feedback control system with a step input change (black line). The blue signal shows the response to the circuit in normal mode, and the red signal shows the response during acceleration mode (rapid angle change). In order to track the rotation angle under rapidly changing conditions, acceleration mode helps the control loop to easily track a rapid rotation angle [4].
Figure 1: Settlement time of RDC step response
EMC/EMI affects rotary transformer systems
Electromagnetic compatibility (EMC) refers to how an electronic system can operate in an electromagnetic environment without causing problems (immunity). Similarly, the pulses emitted by a system must not interfere with any products within its range. In industrial equipment applications, variable speed drives and control circuits are major sources of interference. The rapid switching of power components, such as insulated-gate bipolar transistors (IGBTs) and microcontrollers, is a primary source of high-frequency emissions or interference. IGBT switching times can be as long as 100 ns.
Electrical equipment should be unaffected by high-frequency phenomena, for example:
1. Electrostatic Discharge (ESD)
2. Fast Transient (also known as EFT)
3. Radiated electromagnetic field
4. Conducted radio frequency interference
5. Surge pulse
The constraints are determined by industry standards, such as the IEC 61800-3 standard which specifies the electromagnetic compatibility requirements for variable speed drives that include AC/DC motors and control circuits. In such an environment, any design should adhere to established basic electrical design principles to mitigate the impact of noise [3].
1. Electronic PCB Schematic and Layout Design:
a . Perform power supply and simulated grounding separately.
b . Use analog filters to eliminate common-mode noise on the sensor signal.
c . High-frequency, low-impedance filters for high-frequency interference (e.g., ferrite beads)
d . Minimize the loop area so that ground can provide the lowest possible impedance for the signal return path.
2. Mechanical Design:
a . Use armored cables and connectors (e.g., DB-9 armored connectors).
b . Wiring: Minimize cable length between driver and sensor components.
c . Use armored twisted-pair power and control cables to avoid interference.
d . Use double armor to reduce radiation interference
Electromagnetic Interference Immunity Requirements for Variable Speed Drives
TI engineers tested the IEC 61800-3 standard to obtain environmental specifications (Table 1). The design uses armored connectors and armored cables (length > 30m). The standard definitions are shown in Table 2.
Table 1: EMC Specifications for Variable Speed Drives as Specified by IEC 61800-3
Table 2: IEC61800-3 Pass Performance Standards
Where do the EMI results come from?
Any high dl/dt or dV/dt can be a significant potential source of electromagnetic interference (EMI). The EDGE rate of an electronic signal can introduce harmonics and intermodulation distortion. For example, an EDGE rate of 10 ns on one side and 1 ns on the other results in a 10 MHz square wave. This demonstrates how increased harmonic content can accompany a square wave with a faster EdgeRate. Equation 1 can be used as a general formula for calculating the harmonic frequency range at a given EdgeRate:
(1)
According to this formula, the harmonic frequency corresponding to a 10ns EdgeRate is approximately 31.8MHz . Figure 3 shows that the last significant harmonic frequency is 30MHz. Meanwhile, the harmonic frequency corresponding to a 1ns EdgeRate is 318MHz (Figure 2). If the frequency range is extended beyond 300MHz, the harmonics are still noticeable, but they decrease rapidly at the relevant frequencies.
Figure 2: Spectrum of a 10MHz square wave
Figure 3 : Spectrum of a 31.8MHz square wave
These methods can help reduce the overall impact of noise on the accuracy of rotary transformer systems:
1. Using differential signals helps reduce electrical noise in cables.
2. Noise from armored cables can penetrate underground before affecting sensor circuitry and causing errors.
3. The analog front-end (AFE) used in the RDC structure can filter out common-mode noise.
4. Strive to obtain a near-perfect grounding method with the lowest possible impedance.
5. Minimize the loop that acts as an EMI antenna.
Shielding and filtering
All conductive components, such as cables, ground, and metal sheaths, can propagate radiation. The transfer impedance of the cable must be below 100 mΩ/m in the frequency range up to 100 MHz. The highest shielding effectiveness can be achieved using metal conduits or corrugated aluminum shielding. The longer the cable path, the lower the required transfer impedance. Common-mode inductors can be used in signal cables to suppress common-mode interference at a specific power level. An ideal common-mode inductor will not always transmit differential-mode signals. Faraday cage technology is another commonly used method for controlling radiated interference.
Figure 4: Example of choke suppressing common-mode noise
in conclusion
The unique challenges of high precision and noise in industrial motor position sensing applications can be addressed through comprehensive design considerations and careful selection of electronic components. When designing a resolver, designers should consider system settling time specifications, chip performance related to EMI/EMC, and how these factors affect overall system accuracy.
References
1. Verma , Ankur; Chellamuthu , Anand . “Design considerations for resolver-to-digital converters for electric vehicles , ” TI Analog Applications Journal, 1Q2016
2. Irfan Ahmed , “Implementation of PID and In-line Controllers Using TMS320 Series Digital Signal Processors (DSPs) (SPRA083),” TI Application Report, 1997
3. Martin Staebler , “Designing an Electromagnetic Compatibility (EMC) Compliant Interface for Motor Position Encoders – Part 1 , ” TI Motor Drives & Control Blog, August 31, 2015
4. R. Mancini , “TI Op-Amplifier Fundamentals and Design Guide,” ISBN : 978-0-7506-7701-1 . Elsevier 2003
