Improving System Efficiency and Providing More Precise Control

Figure 1 shows the block diagram of a servo motor. A servo motor feeds back signals of current sensor and encoder to control torque, speed and position. The signal of current sensor is used for torque control.

The specifications of current sensors that contribute to the torque control performance include below:

1.  offset accuracy
2.  current sensitivity accuracy
3.  noise 

Compared to other coreless current sensor ICs, Currentier has better accuracy in each aspect.

Figure 2 shows the result of calculating how this specification difference appears as a torque error. The calculated system uses a coreless current sensor IC as the phase current control of the inverter and is assumed to be used in two-phase current detection.

         Orange : Competitor's coreless current sensor IC
         Blue      : Currentier CZ375, CZ372, CZ370 series

The horizontal axis is the electrical angle and the vertical axis is the amount of torque error (standardized to CZ37), assuming an ideal system with no errors except for the offset and current sensitivity error of the current sensor IC itself. 

As it can be seen from Figure 2, the error of the coreless current sensor IC is directly related to the torque error, and selecting a current sensor with high accuracy and a small error will lead to improve torque control performance.

In addition, there is a significant influence on torque error caused by the noise. Figure 3 shows how much the amount of torque can be influenced by the noise of CZ375, CZ372, CZ370 series and competitor's coreless current sensor IC. As in Figure 2, it is calculated in the system of two phase current detection of the inverter, assuming an ideal system with no errors except for the noise of the current sensor. As the results show, it can be controlled with a much smaller torque error than other products by using CZ375, CZ372, CZ370 series.

Current sensor is used for torque control as already explained in section 1. However, feedback circuits affect position and speed control as well because it operates at the highest frequency.

Particularly in the field of AC servo motors which high-end encoders (24bit) are used, the resolution of current sensor is mostly insufficient to match the resolution of encoder. In such cases, by using Currentier which detects the current with high-resolution, not only noise in torque control but also in speed and position control can be reduced. As a result, it achieves low motor vibration. CZ375, CZ372, CZ370 series is the coreless current IC that can provide significantly high-resolution and its specifications are nearly 5 times better than those of competitors. (Figure 6)  

In recent years, with increasing interest in the environment, improving the efficiency of industrial and residential equipment has become a major issue. Thus, an attention has been paid to the power-factor correction (PFC) circuit. In the past, control ICs for PFCs were widely used, but recently, circuit composed of discrete components such as interleaved PFCs (Figure 7) have been adopted in order to achieve even higher power factor and higher efficiency as well as downsizing. For example, in multi air conditioners, commercial air conditioners and room air conditioners, SEER (Seasonal Energy Efficient Ratio) is regarded as important. To improve SEER, new development of the above PFC circuit is underway.

The higher power factor and the higher efficiency can be made possible by a newly developed PFC circuit method and high accuracy current detection. On the other hand, for system downsizing, it is necessary to increase carrier frequency. (By increasing carrier frequency, L and C in the Figure 7 can be small (*2). Thus, to achieve high power factor, high efficiency and system downsizing all together, a current sensor with high accuracy and fast response is needed.

AKM's coreless current sensor "Currentier" features both high accuracy and fast response, which is ideal for PFCs in power supplies for air conditioners and other applications.

AKM's coreless current sensor IC Currentier contributes to "Improving system efficiency and providing more precise control" by

• Improving torque control performance
• Low vibration motors
• Improving system efficiency

We have prepared a tool for simple simulation of servo motor torque error. You can experience the low noise and high accuracy of AKM's current sensor. You can also compare it with the current detection method that you are currently using.