[Introduction]In motor control applications in industrial and consumer fields, designers often face torque loss, insufficient margin, excessive power consumption and heat generation, noise and vibration that are difficult to eliminate, etc. caused by traditional motor control methods. question. To this end, Renesas has introduced a resolver-based motor control solution, which can effectively deal with the above problems and adapt to various complex working environments to achieve precise motor control.
First, let’s take a look at what is a resolver? Resolver is an electromagnetic sensor used to measure the angular displacement and angular velocity of rotating objects. As shown in the figure below, the resolver consists of a stator and a rotor. Usually, the rotor is fixed on the motor shaft and rotates synchronously.
The working principle of resolver is basically similar to that of ordinary transformer. The stator winding of the resolver acts as the primary side of the transformer and receives the excitation voltage. The rotor winding is used as the secondary side of the transformer, and the induced voltage is obtained through the effect of electric coupling, resonance and magnetic coupling. The relative position of the primary and secondary windings of the resolver changes with the angular displacement of the rotor, so that the voltage amplitude of the output winding has a sine and cosine function relationship with the rotor angle. Perform analog-digital conversion on the output signal and calculate the arc tangent value to obtain the current angular displacement of the rotor. The differential of the angular displacement to time is the rotational speed.
Relying on its special structure and principle, the resolver has become a precise angle, position, and speed detection device, which can well adapt to other encoders such as high temperature, severe cold, humidity, dust, high speed (above 8000r/m), and high vibration. In cases where it cannot work normally, it has higher positional accuracy and has a longer service life than other sensors. Due to the above characteristics of the resolver, it can completely replace the photoelectric encoder and is widely used in servo control, robot systems, machine tools, automobiles, electric power, metallurgy, textiles, printing, aerospace, ships, weapons, electronics, metallurgy, mining, oil fields , water conservancy, chemical industry, light industry, construction and other fields of angle, position detection system.
1 Renesas Resolver Motor Control Solution
Renesas has developed high-precision motor control solutions based on resolvers, independently developed a new RDC (Resolver-to-Digital Converters) IC, and combined with the RX series microcontrollers (MCU) to launch A high-precision resolver motor control solution. This solution can be widely used in motor control systems for industrial and consumer applications.
The figure above is a block diagram of the composition of the scheme. In this scheme, the RDC IC and RX MCU process the signal from the resolver into angle information, and the RX MCU can also control the motor operation at the same time. A dedicated driver for the RDC IC is provided on the RX MCU, and resolver processing can be easily performed using the API interface.
2 Main Features of Renesas RDC ICs
Renesas RDC ICs have the following key features:
First, the Renesas RDC IC adopts a new phase error detection method, which obtains position information by counting the phase difference on the RX-MCU. Phase difference data is generated from the excitation signal and rotation angle information in the resolver signal. Compared with the traditional phase tracking method, the phase difference detection method can realize real-time motor control with higher precision and higher speed.
The traditional phase tracking detection method using PI control servo loop contains frequency characteristics, so there will be a delay time when the motor changes speed.
When using 1KHz as the main frequency characteristic, a delay of 125μs occurs.
※The delay of 1KHz input is 45°→125μs=1ms*45°/360°
→ Servo tracking time delay for step input increases due to sharp changes in angle. Example: 1.5-8ms@180°step.
With Renesas’ phase difference detection method, there is no delay time because the frequency characteristic is not related.
→ For step input caused by severe angle change, it can respond quickly (10μs, 20° step input).
In the configuration of resolver sensor, this solution is compatible with voltage detection type and current detection type. The resolver must be a single-phase excitation two-phase output type resolver. Not only a common voltage detection type resolver, but also a current detection type resolver manufactured at low cost can be used.
Combined with a multi-pole resolver, high position accuracy can be achieved with a high resolution of 200,000 P/R mechanical angular position. And since the RX MCU can calculate the motor home position, there is no need for the Z-phase output of the motor home position.
By combining the RDC IC with the RX MCU, the MCU shares some functions of the angle calculation, thereby simplifying the circuit design of the RDC IC and reducing the device cost of the RDC IC. On the other hand, by adding a unique winding error correction function, the error of a common resolver can be reduced from 1.91° to about 0.48°, so that a lower-cost resolver can be selected.