Inside the Advantages of Sensorless Closed Loop Control

By Mark Checkley, KEB Automation

While feedback devices may be the usual choice for control of motors, sensorless closed loop control can provide design engineers and machine builders with the ability to add value and a competitive advantage to their machines.

With ever lower cost machines providing increasingly stiff competition for western European OEMs, rather than competing purely on price, these companies are looking for ways of adding value to their machines by increasing their performance, flexibility, efficiency and availability.

Variable speed drives are an essential tool here. 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, for example, highlighting the onset of mechanical problems that could be the source of unplanned downtime. Manufacturers of variable speed drives offer a variety of features, which are worth looking at more closely to understand which of these features would be most advantageous to use.

One function that offers increased performance and also helps to reduce cost is Sensorless Closed Loop (SCL) 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 the component parts but also adds cost in terms of the cabling and sockets required. In addition, encoders and resolvers can be affected by the heat transfer from the motor, as well as being vulnerable to vibration under certain conditions.

How does Sensorless Closed Loop control eliminate the need for a feedback device?

First, a mathematical model of the motor is built, typically through an auto-tuning process within the inverter. Whilst in operation the stator current is continuously measured, the flux and torque 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 number of years. However, not all software algorithms are equal. Knowing what makes one better or worse enables OEMs to maximise their competitive advantage with this technology and helps them select the most suitable drive.

The effectiveness of an algorithm 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 F6 range of inverters is detailed in the graph below, along with a graph of the dynamic response behaviour of a load. What we see are stability and a fast response comparable to the traditional closed loop systems.

The reason for this performance is the combination of utilising hardware with the software resulting from many years of R&D. The result is much higher performance than other sensorless closed loop strategies. The technology also works with a wide range of motors from many manufacturers.

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

Use 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 being less able to reap the benefits of using a synchronous motor, which is smaller, more efficient and more dynamic – all of which could represent a competitive advantage, but only if physical feedback devices and cables weren’t required.

Actual and potential applications include driven tools, textile machines, extruder drives, hybrid drives, injection moulding/ blow moulding machines, diesel electric drives in conveyor systems, container or heavy vehicles, electric drives in yachts/boats, 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.

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