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Open-loop drive technology `comes close to closed-loop`
Published:  01 March, 2006

Open-loop drive technology `comes close to closed-loop`

The UK drives-maker Control Techniques has devised a new method for open-loop control of variable speed drives which, it claims, comes as close as possible to closed-loop control, without needing encoder feedback. The technique, called Rotor Flux Control (RFC), is now being incorporated as standard into the company`s flagship Unidrive SP range.

Emerson-owned CT says that RFC will be a particular advantage for applications requiring a combination of dynamic performance, stability, speed accuracy, and low noise levels, without using encoders. It adds that RFC has advantages over rival technologies, such as ABB`s Direct Torque Control (DTC).

"With RFC, we can provide a simulated speed feedback that gives excellent speed accuracy - certainly the best results without actually having the cost of direct feedback from an encoder," declares CT product manager, Richard Smith.

The new technique (outlined in the block diagram above) applies the principles of closed-loop control to an open-loop motor. It uses a simple, accurate mathematical model of the motor to calculate a simulated speed and position feedback, allowing the loop to be closed without using an encoder.

CT concedes that this approach is not unique, but claims that it is the first to offer a simple algorithm that allows the calculations to be performed synchronously with the drive`s speed and current loops. It describes the resulting speed accuracy as "excellent" and stable, even with troublesome light loads.

Accurate and dynamic control of standard AC induction motors without measuring speed is not easy. Considerable amounts of money have been spent trying to develop the perfect control algorithm to achieve this goal, yet closed-loop control still offers significant advantages that make it the only choice for some applications.

The crux of the problem is that the standard AC motor demonstrates some quite unattractive control characteristics, such as instability and non-linearity, for which it is difficult to compensate. Some drives suppliers have claimed to have developed the universal open-loop algorithm, but, in practice, says CT, these often require weeks of commissioning work to get anything really working.

The original method of varying the speed of an AC motor - known as V/f control - was to apply a variable frequency to the motor and assume that the motor would be operating at approximately the correct speed. However, the speed of a motor changes significantly as the load and temperature vary, so that good speed-holding at low speed becomes impossible. Under certain load conditions and at certain frequencies, the motor speed can oscillate around the required speed, even though the applied frequency is constant.

Despite its inherent limitations, for some applications the simple V/f method has advantages - the set-up is minimal, several motors can be connected to one drive and, for many applications, the lack of speed regulation is not a big problem.

Another approach has been trying to model some aspects of the motor, to estimate how the speed of the motor may be changing with load and temperature and to compensate accordingly. This approach is often known as open-loop vector control, although some drives manufacturers use a similar method marketed under a proprietary name.

Open-loop vector control solves some of the problems associated with V/f control and provides moderately good dynamic performance and fairly intuitive set-up. It is perfect for applications such as controlling conveyors or centrifuges, where relatively good speed-holding is needed, but dynamic performance is not, and where stability problems are rarely seen.

Another method that has received considerable attention is Direct Torque Control, devised in 1985 by Takahashi and Noguchi, and subsequently implemented by ABB in its drives. DTC offers good dynamic performance, similar to open-loop vector drives, making it suitable for most applications.

But, says CT, DTC has limitations. The switching method for the power transistors is derived from a look-up table in the drive`s memory, resulting in a variable switching frequency and significant audible noise from the motor, plus added motor losses. For applications that are sensitive to noise, such as lifts and building controls, this can present a problem. In addition, to prevent undue stress being placed on the motor insulation, the algorithm also requires careful internal limits to be set.

Control Techniques claims that RFC is different because its designers have approached the problem from the perspective of closed-loop control technology. It says that this gives it advantages over DTC. For example, the torque response - the time taken for a motor to generate 100% torque in response to a step demand - for a DTC-based system is typically up to 2ms. According to CT, RFC delivers full torque in less than 0.5ms, improving dynamic performance and extending system bandwidth.

Unlike DTC, the switching frequency of the RFC system is selectable, making the drives suitable for applications, such as lifts and HVAC controls, where low audible noise levels are important.

CT admits that RFC will not be ideal for every application. Systems that demand accurate speed and torque control down to zero speed will still need feedback devices, it says. But the technology will allow open-loop drives to be used in many more applications where, in the past, only a closed-loop drive would do.

Because RFC is based on closed-loop technology, the drive can switch seamlessly between true closed-loop control and the new RFC mode. This could be useful for applications with wide speed ranges, such as spindle motors, where an encoder is needed for low-speed positioning and performance, but is unable to transmit coherent data at higher speeds. Another possibility is to add a built-in level of encoder redundancy at no extra cost.

CT points out that its Unidrive SP family (shown above) now offers a choice of V/f, open-loop vector, RFC and true closed-loop control, allowing users to pick the best control method for any application.

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