The Development of Ultra-fast Current Loop Technology (Part 1 of 2)

By Enzo Stampacchia, Inovance Technology Italy

Servo drives have evolved over the decades and many now feature ultra-fast current loop technology as a result of a series of technological advancements.

The evolution of servo drive technology has led to the development of ultra-fast current loop technology, which provides better control of servo motors by delivering near-instantaneous feedback, improving the bandwidth of the current control loop and increasing speed and positioning capabilities. In order to get to this point there have a been a series of important developments along the way, not least improvements to power transistors that enabled drives designers to create products adhering to the rule of 10, an empirical law relating to the three control loops of a servo drive.

This article is the first part of two blogs that will examine the evolution of servo drives leading up to the development of ultra-fast current loop technology, such as that found in a range of Inovance servo drives. In part two, I will explore in more detail what ultra-fast current loop technology provides in terms of enhancements to servo drive performance and capabilities.

The evolution of servo drives

Ultra-fast current loop represents the next generation of servo drives technology and is part of a long history of innovation, dating back to when servo drives for industrial applications were developed more than half a century ago. When computer controlled machines (NC) first came into use, the only power semiconductors capable of managing the current required to drive servomotors were thyristors. At that time, the current loop power stage was limited by the use of thyristor technology to 25-30 Hz.

^{Source : The Power Thyristor and its Applications by David Finney}

A well-known empirical law to run the three loops of a servo drive (position, speed and current) is the rule of 10. So, the speed loop has to run 10 times faster than the position loop, and the current loop has to run 10 times faster than speed loop. If we consider the thyristor current loop frequency was limited to 25-30 Hz, the position loop would be limited to 0.3 Hz, which means a position loop gain Kp of 0.3 x 2 x π = 1.9. This is too slow and so, initially, drives designers looked for at least a position gain Kp of around 10 (1.6 Hz). 

Because of the limits placed on the current loop power stage by thyristors, the rule of 10 was abandoned and drive designers used every possible method to get a better response out of the inner speed and position loops. This meant difficult parameter settings, nonlinearities, and complicated axis control; all making creating a servo application and tuning it a difficult job. What followed was a long intermediate history of improvements to power semiconductor devices like power transistors and IGBTs. Motor control was achieved in the past using analog drives, then hybrid (a mix of analog and digital), then full digital control, as a result of substantial improvements in performance and falling costs of microcontrollers.

Developments in servo motor technology

During this time, servomotor technology changed a lot; developing from the first generation wire wound DC brushed motors, through to the permanent magnet DC brushed motor, and on to high performance permanent magnet AC brushless motors. The introduction of the Neodymium Iron permanent magnet represented a big leap forward in the development of servomotors. Last but not least: the transducers. In the past, servo drives were set up in speed control and the feedback transducer was a brushed analog tachogenerator. Closing the position loop was initially carried out by the NC Controller, then the CNC controller and, later, by the transducer in the form of a magnetic resolver or optical encoder.

With the introduction of brushless technology, a position transducer mounted on the motor could be utilized for control. This was followed by an optical encoder, which had initially been unsuited to running in high temperature environments. SIN-COS interpolation techniques made the optical encoder more robust and capable of producing very high position resolution feedback (many million counts per rev), which enabled it to maintain pace with the current loop controller.

The pursuit of the rule of 10

The philosophical approach, pursued by all servo companies and following control theory, was to reinstate the rule of 10, specifically for the two inner loops: the speed loop and current loop. Increasing the gain of the speed loop allows the servo to run faster, with higher motion dynamics, a reduction in trajectory following errors and more precision in point-to-point position applications. The result of industry R&D in this area is a big improvement in frequency bandwidth in the most recent generation of servos.

Inovance, having reached this target for high bandwidth speed loop, increased its efforts to follow the rule of 10 on the current loop. The result was our Ultra Fast Current Loop Project. The performance achieved was excellent, very linear, and totally predictable by the servo theory, with the absence of non-linearities, no dead times and no delay.

To find out more about the performance and potential offered by ultra-fast current loop technology, look out for the upcoming Part 2 of this blog: Ultra-Fast Current Loop technology delivers speed and precision