What is a resolver? See how it is used in industrial applications
Servo motors are commonly used in industrial applications, connected to resolvers and other types of position sensors. Since the 1960s, resolvers have been used in servo systems as an angle signal generation and detection element. Resolvers are mainly used in angular position servo control systems. Since two-phase resolvers are easier to improve accuracy than synchro, resolvers are more widely used.
Some absolute rotation sensing technologies (such as optical encoders) have been selected for industrial applications many times. However, resolvers are the most ideal choice for harsh environments or low cost factors.Industrial Applications of Resolver Sensors: Applications that commonly use servo motors and servo drives in conjunction with resolvers to achieve angular velocity and position measurement include: CNC and injection molding machines, elevators, robotic arms, electric vehicles (electric bicycles, electric scooters, electric wheelchairs, etc.), rail transportation, agricultural and construction equipment, buses and heavy trucks, golf carts and low-speed electric vehicles.
Main resolver sensing system requirements Accurate and timely resolver angle output. Before finding ways to use resolvers to mitigate the effects of electromagnetic interference on industrial systems, it is important to understand why accurate position control is essential. Resolvers provide an analog output that theoretically has infinite resolution. Analog to digital conversion techniques limit resolution by the degree to which the output is divided into blocks or steps. Finite division of a continuous angle will result in quantitative errors. For example, a 12-bit resolution converter is used to provide the angular output. One revolution of the converter shaft is divided into 4096 steps (2^12 corresponds to a 12-bit resolution). Since one degree equals 60 minutes, one revolution (360 degrees) equals 21600 arc minutes (60x360). The interval between each step is 5.27 arc minutes (21600/4096). It is impossible for the system to provide better information than 5.27 arc minutes. The two key factors in determining the correct angular position are system accuracy and system settling time. The latter refers to how long it takes for the angular output to show the exact position. Each component of the system needs to be evaluated to determine the limiting factor. In a system, the typical error accuracy is the sum of the resolver error and the resolver analog to digital converter (RDC’s) error. Most commonly, a resolver error occurs at 3-10 arc minutes. Add to that the resolver analog-to-digital conversion error at 5.27 arc minutes, and we can conclude that the exact error range is 8.27-15.27 arc minutes. Therefore, it is important to choose the right RDC. What factors affect the system accuracy and settling time of resolver applications?1. Electrical factors ·Resolver analog-to-digital conversion structure ·Time delay from resolver signal input to angle output, fast response to angle change settling time ·Unbalance of analog front-end (AFE) components ·System's ability to handle environmental factors (e.g., external magnetic field or common-mode noise)
2. Mechanical factors ·Sensor structure (zero voltage, transformation ratio, etc.) ·Changes in sensor specifications with temperature ·Coil imbalance: Sine and cosine coil output voltages may be unbalanced, resulting in errors ·Resolver sensor misalignment: The resolver may be installed incorrectly, resulting in system static errors ·Number of poles of resolver sensor: Since each additional pair of poles detects 360 degrees more, the increased number of poles will reduce the angle error 3. EMC/EMI affects resolver system Electromagnetic compatibility (EMC) refers to how electronic systems operate in an electromagnetic environment without causing problems (immunity). Similarly, the system's transmitted pulses must not interfere with any product in the range. In industrial equipment applications, variable speed drives and control circuits are the main sources of interference. The fast switching of power components, such as insulated gate bipolar transistors (IGBTs) and microcontrollers, is a major source of high-frequency emissions or interference. IGBT switching time can be as long as 100nS.Electrical equipment should be immune to high-frequency phenomena such as: 1. Electrostatic discharge (ESD) 2. Fast transients (also known as EFT) 3. Radiated electromagnetic fields 4. Conducted radio frequency interference 5. Surge pulses IV. Settling time When the motor position or output signal of the resolver changes rapidly, the settling time is a fast performance indicator of the RDC control system. 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 normal mode response of the circuit, and the red signal shows the response during the acceleration mode (rapid angle change). In order to track the rotation angle under fast changing conditions, the acceleration mode helps the control loop easily track a fast rotation angle.
Alignment method of electrical angle phase between resolver and servo motor: 1. Use a DC power supply to pass DC less than rated current to UV winding of motor, U in, V out; 2. Then use oscilloscope to observe signal lead output of SIN coil of resolver; 3. According to the convenience of operation, adjust the relative position of resolver rotor and motor shaft, or the relative position of resolver stator and motor housing; 4. While adjusting, observe the envelope of resolver SIN signal, and adjust until the amplitude of signal envelope is completely zero, and lock resolver; 5. Twist the motor shaft back and forth, and after letting go, if the amplitude of signal envelope can accurately reproduce the zero point every time the motor shaft returns to the equilibrium position, the alignment is effective. Remove the DC power supply and perform alignment verification:
1. Use an oscilloscope to observe the SIN signal of the resolver and the UV line back EMF waveform of the motor;
2. Rotate the motor shaft to verify that the zero crossing point of the resolver's SIN signal envelope coincides with the zero crossing point of the motor's UV line back EMF waveform from low to high.
This verification method can also be used as an alignment method.
At this time, the zero crossing point of the SIN signal envelope is aligned with the -30 degree point of the motor electrical angle phase. If you want to align directly with the 0 degree point of the motor electrical angle, you can consider:
1. Connect three resistors of equal resistance into a star shape, and then connect the three resistors connected in the star shape to the UVW three-phase winding leads of the motor respectively;
2. Use an oscilloscope to observe the midpoint of the motor U phase input and the star resistor, and you can approximate the U reverse potential waveform of the motor;
3. According to the convenience of operation, adjust the relative position of the encoder shaft and the motor shaft, or the relative position of the encoder housing and the motor housing;
4. While adjusting, observe the zero crossing point of the SIN signal envelope of the resolver and the zero crossing point of the motor U reverse potential waveform from low to high, and finally make these two zero crossing points coincide, lock the relative position relationship between the encoder and the motor, and complete the alignment.It should be pointed out that in the above operation, it is necessary to effectively distinguish the positive half cycle and negative half cycle in the SIN envelope signal of the resolver. Since the SIN signal is the modulation result of the excitation signal with the sinθ value of the angle θ between the rotor and the stator, the modulated excitation signal is in phase with the original excitation signal in the SIN signal envelope corresponding to the positive half-cycle of sinθ, and the modulated excitation signal is in anti-phase with the original excitation signal in the SIN signal envelope corresponding to the negative half-cycle of sinθ. Based on this, the positive and negative half-cycles in the SIN envelope signal waveform output by the resolver can be distinguished. When aligning, it is necessary to take the zero crossing point of the SIN envelope signal corresponding to the transition point from the negative half-cycle to the positive half-cycle of sinθ. If it is taken inversely or not accurately judged, the electrical angle after alignment may be misplaced by 180 degrees, which may cause the speed outer loop to enter positive feedback. If the servo drive can provide absolute position information related to the electrical angle of the motor, you can consider: 1. Use a DC power supply to pass a DC current less than the rated current through the UV winding of the motor, U in, V out, and orient the motor shaft to a balanced position; 2. Use the servo drive to read and display the absolute position information related to the electrical angle of the motor obtained from the resolver signal; 3. According to the convenience of operation, adjust the relative position of the resolver shaft and the motor shaft, or the relative position of the resolver housing and the motor housing; 4. After the above adjustment, make the displayed absolute position value sufficiently close to the absolute position point corresponding to the motor's -30 degree electrical angle calculated according to the number of motor pole pairs, and lock the relative position relationship between the encoder and the motor; 5. Twist the motor shaft back and forth, and after letting go, if the above-mentioned converted absolute position points can be accurately reproduced each time the motor shaft freely returns to the balanced position, the alignment is effective. After that, after removing the DC power supply, you can get the alignment verification effect that is basically the same as before: 1. Use an oscilloscope to observe the SIN signal of the resolver and the UV line back EMF waveform of the motor; 2. Rotate the motor shaft to verify that the zero crossing point of the SIN signal envelope of the resolver coincides with the zero crossing point of the UV line back EMF waveform of the motor from low to high. If you use the EEPROM and other non-volatile memory inside the driver, you can also store the phase measured after the resolver is randomly installed on the motor shaft. The specific method is as follows: 1. Install the resolver randomly on the motor, that is, consolidate the resolver shaft and the motor shaft, as well as the resolver housing and the motor housing; 2. Use a DC power supply to pass a DC current less than the rated current through the UV winding of the motor, U in, V out, and orient the motor shaft to a balanced position; 3. Use a servo drive to read the absolute position value related to the electrical angle analyzed by the resolver, and store it in the EEPROM and other non-volatile memory inside the driver that records the initial installation phase of the motor electrical angle; 4. The alignment process is completed. At this point, the motor shaft is oriented in the direction of -30 degrees of the electrical angle phase, so the position detection value stored in the non-volatile memory such as the EEPROM inside the driver corresponds to the -30 degree phase of the motor electrical angle.Thereafter, the driver will subtract the absolute position value related to the electrical angle analyzed by the resolver at any time from this stored value, and perform necessary conversions based on the number of motor pole pairs, and then add -30 degrees to get the motor electrical angle phase at that moment.
