Digital servo drive suits brushless ac servomotors with resolver feedback

Paul Boughton

The drive provides users with a cost-effective means of increasing the speed and smoothness of existing automation through the application of advanced digital control technology, without having to replace the servomotor. It is also likely to prove popular with OEMs and machine builders designing servo-based systems for use in challenging environments, since resolvers are generally considered more mechanically and electrically robust than encoders, especially at elevated temperatures.

Microflex servo drives are all-digital control products, using DSP (digital signal processor) technology to maximise performance and minimise cost. The new resolver input model uses a resolver-to-digital-converter (RDC) IC to convert the analogue motor feedback signal into digital data immediately prior to processing. The drive's DSP enables users to optimise system performance via software-configurable anti-resonance filters. Instead of employing 'soft' servo loop gains to eliminate noise and vibration by de-tuning the system - which is a compromise often faced when retrofitting a drive to an existing installation - users can set up the filters to optimise dynamic performance and minimise settling times.

Advanced space vector modulation (SVM) techniques are used to control the IGBT (insulated gate bipolar transistor) power devices in the drive's output stage, resulting in significantly reduced switching losses and harmonics. Compared to conventional drives with a pulse width modulated (PWM) output stage, Microflex units enable servomotors to run more smoothly and up to 15 per cent faster – equating to greater control flexibility, improved accuracy and increased machine throughput.

Capable of operating in torque or velocity control mode, the drive uses sinusoidal, resolver-based commutation to minimise torque ripple – making it excellent for precision motion control applications – and features a simulated encoder output for connection to an external motion controller. Like all Microflex drives, it is available with a choice of 3, 6 and 9A continuous power ratings to simplify application matching, and accommodates peak currents of 200 per cent to maximise dynamic performance.

The drive operates from any single- or three-phase 50/60Hz supply in the range 115 to 230V ac, enabling it to be used virtually anywhere in the world, and generates a nominal bus supply of 160 to 325V dc. It incorporates dc bus undervoltage and overvoltage monitoring, and is comprehensively protected against conditions such as overcurrent, overtemperature and motor short-circuits. A built-in regenerative braking IGBT, in association with an external dump resistor, facilitates dissipation of braking energy.

Microflex drives are configured using Baldor’s Windows software environment, Mint Workbench. The same tool is used across the company's extensive range of servo drives, intelligent drives and motion controllers, providing OEMs who build a variety of machines with a familiar configuration environment, supported by a choice of hardware with common features. Mint Workbench includes a sophisticated auto-tuning tool to help users install Microflex drives rapidly and efficiently, together with highly intuitive FFT (Fast Fourier Transform) analysis tools and automated frequency response tests to facilitate creation of custom anti-resonance filters.

Baldor offers a comprehensive range of complementary products for use with Microflex drives, including Nextmove motion controllers in PCB and packaged forms, and rotary and linear brushless ac servomotors.

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