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Closed Loop without a feedback device?

Author : Mark Checkley is from KEB UK

09 September 2016

Feedback devices are the usual choice to help with motor control, here Mark Checkley from KEB UK explains why sensorless closed loop control is chosen by customers to add value and a competitive advantage to their machines.

More than simply rotating a motor, the variable speed drive has long been recognised as a critical piece of most modern machinery. It offers a gateway into the machine, capable of significantly increasing the performance of the machine as well as offering sophisticated monitoring of the process, e.g. highlighting the onset of mechanical problems that could be the source of unscheduled downtime. There are a variety of features offered by the manufacturers, which is worth looking at more closely to understand which would be most advantageous for them to use.

One function that offers increased performance and also helps to reduce cost is sensorless closed loop control. Traditionally running motors in closed loop, which gives the best speed and torque performance, requires an encoder or resolver for the feedback. This adds cost for this component but also with the cabling and sockets required. Additionally, encoders and resolvers can be affected by the heat transfer from the motor as well as being vulnerable to vibration.

Eliminating the need for a feedback device

Sensorless closed loop control builds a mathematical model of the motor, usually through an auto-tuning process within the inverter. Whilst in operation the stator current is continuously measured, the flux and torque is then calculated and compared to the mathematical model. The current can then be regulated to match the required speed and torque.

Versions of this type of control have been available for standard induction (asynchronous & squirrel cage) motors for a few years. However not all algorithms are equal. Knowing which one is better or worse enables OEM’s to maximise their competitive advantage.

The algorithms effectiveness can be measured by its ability to provide constant torque across the full speed range and by its speed response to step changes in load. 

As an example, the torque characteristics of KEB’s F5 range of inverters is detailed in the graph below, alongside a graph of the dynamic response behaviour of a load. What we see are high speed stability and an extremely fast response comparable to the very best traditional closed loop systems.

The reason for this performance is the hardware (fast microprocessors) combined with the highly tuned software resulting from many years of R&D. The result is a much higher performance than other sensorless closed loop strategies. The KEB technology works with a wide range of motors from many manufacturers.

KEB’s F5 range of inverters

Typical applications that have used this KEB technology are: extruder main drives, crusher drives, centrifuges, test systems, mixer units, meat cutters and mincers, heat pumps, hydraulic pumps, generators and machine tools.

Using with servo motors

The inability to use this control with permanent magnet synchronous motors (servo motors) has often proved challenging due to the low inductances of the motors. This makes it harder to accurately calculate the required flux and torque generating current values, resulting in engineers less able to reap the benefits of using a synchronous motor which is significantly smaller, vastly efficient and more dynamic, all of which could represent a significant competitive advantage but only if physical feedback devices and cables weren’t required.

KEB’s algorithm, however, is able to handle the lower inductances of these motors with ease, enabling users to reap all the benefits of this motor technology, while eliminating the cost of the physical feedback device and its associated cabling and interface.

Actual and potential applications include driven tools, textile machines, extruder drives, injection moulding and blow moulding machines, and high frequency pump drives in compressors, screws and vacuum pumps. The same algorithm is also suitable for use with linear motors, opening up a vast range of new applications and opportunities.

KEB offer Sensorless Vector Control on the majority of its drives, being standard on the large power units enabling OEM’s a real means of achieving a significant competitive advantage.

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