Technical Information

E-2 Servo Control Technology

E-2 Servo Control Technology

Toshiba Machine has developed the special LSIs (V-engine and C-engine) to improve the servo performance, thus realizing the current control period of about 3μs and shortest speed control period of 25μs. As a result, we could control the micro-vibration of the motor during stop to about 1/10, compared with the previous one in the linear motor control. Additionally, by controlling D-axis current "Id" affecting magnetic flux density of the motor and by feeding current "Id" according to shortage of voltage as output from the C-engine in the "Id" control in which torque characteristic is improved at high-speed motor revolution, "Id" could be reduced by 50% or more at mold clamping motion of the injection molding machine, compared with the general method. Thus heat generated in the motor could be minimized.

[Platform]
E. Control and mechatronics technologies.
[Applications]
Machine tool, electric injection molding machine, industrial robot.

[Technical points]

  • Development of special LSIs (V-engine and C-engine) to realize the current control period of about 3μs and shortest speed control period of 25μs.
  • Reduction of "Id" by 50% or more at mold clamping motion of the injection molding machine.
  • Detection of voltage shortage by the C-engine to realize energy-saving with effective "Id" control.

1. Introduction

In the industrial equipment field, the servo forms an essential constituent. Toshiba Machine is now engaged in the development and manufacture of general-purpose servo systems for direct sales and servo systems for driving Toshiba Machine built machines and equipment. The servo system is a very important element affecting the machine performance it drives and we have to continue developing high performance of the servo system. Now, we describe the tasks we are tackling concerning realization of high performance of our servos.

2. Enhancing servo performance

2.1 High-Speed Specification (Improvement of basic performance)

Fig. 1 Structure of servo system

Fig. 1 Structure of servo system

The quality of a servo is determined by the fact that the machine can be operated to its full capacity when the servo is combined with the machine. In this paragraph, we focus on the performance of the servo itself.
The performance of the servo itself is largely affected by the time delay in the control. This time delay refers to the time lag covering from feedback detection and calculation for control up to output of the manipulated variable. If the time delay is long, phase delay in the high frequency zone increases and the zone where stable control can be performed will be limited. Therefore, to improve the servo performance, it is necessary to shorten the time delay in the control as much as possible. For servo adjustment, we aim at setting the gain value as high as possible in the stable zone.
As the time delay is reduced, phase delay in the high-frequency zone becomes smaller, thus the higher gain value can be specified.
To reduce the time delay, the digital control of servo, which has been processed by the software previously, is now executed by the LSI, thus high-speed digital control of servo can be realized.
A general-purpose single control axis servo system as used in hydraulic pump control or transfer equipment is shown in Fig. 1 (a). Position control is executed by the CPU implemented in single control axis amplifier, The speed and current control are executed by our specially developed LSI, called the "V-engine". As a result, speed control can be executed in the shortest cycle of 25μs.
As shown in Fig. 1 (b), industrial machines such as machine tools, injection molding machines etc. are controlled in coordination with multiple axes. Position and speed control of the multiple axes are realized by the software using the CPU in the main controller. It enables synchronized control of multiple servo motors; various compensation functions and smooth motion without causing an impact.

Fig. 2 Current control LSI, C-engine

Fig. 2 Current control LSI, C-engine

The host controller and amplifier are connected via high-speed communication of 160Mbps. Current control is realized by LSI, called "C-engine", equipped on the amplifier, as shown in Fig. 2. The C-engine executes current control of 3-phase synchronous motor in the control cycle of about 3us, which results in realization of stable current control with less time delay. Thus, small vibration in the servo system, which is caused by the current error, has been reduced. Compared with the previous amplifier in which current control was done by software; vibration at stop of the linear motor has been reduced to one-tenth (1/10). For high-speed injection molding machine, high-speed position control is necessary to stabilize molding, and position control is realized in the period of 62.5μs. Thus, fluctuating range is controlled to 0.05mm even at injection speed of 1300mm/s to achieve stable and continuous molding operations without short-shot.

2.2 Energy conservation (example)

Fig. 3 Results of measuring 'Id' cutback at mold clamping

Fig. 3 Results of measuring 'Id' cutback at mold clamping

First of all, injection molding machines have a high requirement for high-speed injection axis and mold clamp axis, in addition to high-speed revolution and high torque increasing, in addition to high-speed revolution and high torque. If you select motors satisfying the requirements, you will need upgraded motors with higher output power. To avoid this, the motor characteristic is compensated by the current control, thus realizing high-speed revolution and high torque.
As the specific motor characteristic, as counter electromotive voltage of the motor approaches the power-supply voltage upon high-speed revolution, the current cannot flow and there is a zone where torque is restricted.
Then, when current "Id" affecting magnetic flux density is fed to the zone though it is unrelated to torque generation, the counter electromotive voltage can be reduced to develop the torque characteristic to the high-speed zone. This is called the "Id control."
Because the counter electromotive voltage is proportional to the motor speed, "Id" is generally fed according to the motor speed. The method is easier way; however it will also pass additional current at high-speed revolution if the load is light and the motor will generates the heat.
As our control systems allow the C-engine to detect the shortage of voltage, required "Id" can be fed when necessary and contributes to save energy and minimize the heat generation in the motor. Fig. 3 exemplifies the "Id" control applied for mold clamping of the injection molding machine. Compared with the general system in which "Id" is controlled simply in accordance with speed, the energy--saving system has slashed D-axis current "Id" by more than 50%.

3. Conclusion

We are determined to be involved in the development of software algorithm and LSI by incorporating and utilizing diversified technologies to realize low-priced, high performance servo which will be surely contribute to enhancement of the machine performance in the coming years.