The rotary transformer is simply referred to as "rotary transformer". It is divided into various series such as magnetic resistance type, brushed rotor winding type, brushless rotor winding type, dual-channel type, etc. In simple terms, a rotary transformer is a type of analog output angle sensor. It needs to be combined with rotary transformer decoding to obtain the required measured angle.The reason why permanent magnet servo motors use rotary transformers and other angle sensors is to perform vector control, so that the electromagnetic field generated by the stator winding always remains orthogonal to the rotor permanent magnetic field, thereby achieving the best output effect. To achieve this characteristic, it is necessary to accurately obtain the current rotor position status of the motor. To achieve this goal, there are two very important parameter indicators that need to be controlled: 1. The deviation angle between the angle zero position of the rotary transformer and the zero position of the motor; 2. The angle deviation after the installation of the rotary transformer, that is, the "combined electrical error" of the rotary transformer.
For example: Suppose the deviation between the zero position of the rotary transformer and the zero position of the motor is +30′, the electrical error of the rotary transformer is ±25′, and the error of the rotary transformer decoding board is ±10′. Then within the range of 0 to 360 degrees, the deviation of the rotary transformer's expression of the actual position of the motor rotor is: -5′ to +65′.
The current main measurement methods for zero deviation include three types:
Static measurement of zero position deviation is the most widely used method in domestic applications. It only requires a DC power supply and a resolver calculation device to calibrate the zero position. The common practice is as follows: First, apply a low-voltage DC current to the motor windings. Connect U phase positively and V phase or VW phase negatively. At this point, the motor rotor will be pulled to a fixed position. For example, in the UVW connection method, the theoretical electrical angle of the rotor is 0°. Reading the resolver calculation angle value at this time is the resolver's zero position deviation relative to the motor.
The equipment is simple in design and low in cost. Only a low-voltage constant current source is needed to supply power to the motor, and a resolver decoding device is required to display the resolver angle value for zeroing.
The operating condition is static and safe, allowing for continuous adjustment. The operation is simple.
Disadvantages:
Static measurement assumes that the three-phase windings of the motor are balanced and remain in a fixed position after being energized. In reality, due to factors such as tooth-slot torque, friction, and unbalanced two-phase currents, the motor rotor will deviate from the zero position, and this deviation is difficult to measure and perceive. This results in the actual zero position deviation test accuracy of static measurement being not very high, with the maximum deviation ranging from 0.5° to 2°.
The zero position static test deviation mainly consists of two parts:
1. Random error: Due to the existence of friction torque and rotor inertia, when power is supplied from different rotor positions, the final position where the rotor stops has a certain randomness. This random error can be reduced by increasing the direct current supply current to the motor, but when zeroing the motor, the cooling device often does not work, so it is necessary to consider the current value that the motor can withstand in this state to avoid causing the motor to burn out.
2. Fixed deviation: If the currents flowing through the motor's UV and UW phases are unbalanced, it will cause a fixed deviation in the position where the motor rotor stops. This deviation is not only due to the imbalance of the three-phase windings of the motor, but also is mainly affected by the inconsistent DC resistances of the power cables of U and W phases and the inconsistent contact resistances of the test clamps. Because the DC resistance of the motor windings is very small, even a deviation of several milliohms in the contact resistance will cause a significant deviation in the current flowing through the UV and UW phases. Currently, some zeroing systems have paid attention to this issue, for example, in the 2019 version of the Resolver Analyzer Pro resolver zeroing analyzer, an current monitoring function has been added, which can effectively avoid measurement errors caused by current imbalance during static testing.
Dynamic measurement is based on the principle that the armature position state of the motor can be accurately reflected by the waveform of the reverse electromotive force. The zero position of the motor rotor and the angle deviation from the rotation variation are measured under the condition of the tested motor rotating and generating electricity. There are two common methods: the inertia method and the trailing method.
1. Inertia method: Drive the tested motor to a certain speed, then remove the driving voltage, and use the inertia of the motor to complete the measurement.
2.Trailing method: Use another motor to drive the tested motor to be in the power generation state, and measure the reverse electromotive force voltage.
The position measurement accuracy of the motor rotor is very high. Therefore, the measurement accuracy of the zero position deviation is one order of magnitude higher than that of the static measurement and can reach sub-level precision.
Dynamic measurement requires the motor to be in the power generation state.
Self-learning is a function that enables the controller to automatically determine the zero position of the resolver. This function requires support from both the controller's hardware and software. There are three main approaches: the first is static measurement, where a direct current is applied to the motor windings to determine the position state of the motor rotor; the second is dynamic measurement, based on the principle of reverse electromotive force; and the third is injecting high-frequency voltage or current into the motor windings to measure the state of the motor rotor.
Since the first method requires the motor to be in a free-axis state and the second method requires the motor to operate with inertia, it is often only possible to complete zeroing without connecting to the load. The third method can be used for motor position sensorless control and can complete zero position measurement in any assembly state.
Self-learning is usually used in conjunction with software compensation, allowing the zeroing process to be completed without adjusting the resolver stator.
Zero position measurement can be accomplished without the need for an external zeroing device.
Specific controllers are required. Methods one and two require specific operating conditions, and they can only be implemented after the motor and the load are separated.