Digital servos dispense with costly analogue parts

Posidrive MDS 5000 servo drives are compact, modular and fully digital in design.

When the economy falters, competitive pressures increase, and this can have serious consequences for the life of a business.

One option to emerge stronger from a crisis is to reduce product costs by innovative drive technology.

This article presents a new generation of servo drives in which the analogue system gives way to purely digital feedback.

By means of a changeover switch, a number of servomotors (also new) can be operated sequentially on a single drive.

The drives also provide an integrated IEEE1394 (FireWire) interface for economic real-time communication.

Servo drive technology is gaining ground because the requirements for movement dynamics and precision are growing more stringent.

At the same time, increasing machine flexibility and modularisation demands additional servo axes.

The drive costs can then make up a large proportion of the total costs of a machine, even more so when another trend is considered, that of absolute measuring systems.

Safe collision avoidance or elimination of a reference run after switching on are desirable or even essential requirements but they can drive costs sky high.

Can - but do not have to, as this article shows.

Where do the comparatively high costs of a conventional servo drive come from? A servo drive is characterised by fast-response control circuits, a generously sized output module, a wide interface variety and a high-quality analogue system to record set point and actual values.

Naturally, a robust output module and adequate processor capacity are also essential.

However, things look rather different with the analogue system which is an important factor in a conventional servo drive - both technically and price wise.

This is illustrated by an example of an analogue set point: with a speed setting range of 1:5000 and an analogue +/-10V set point input, a resolution of 14bit is required.

If the setpoint input is digital, eg via a fieldbus, the user gains no benefit but still has to pay the cost of the unnecessary interface.

Another example is the resolver interface.

A resolver is a cheap and robust but very inaccurate measuring system.

To obtain satisfactory speed control, the resolver position must be calculated at a minimum resolution of 14bit.

Whether this is done with an integrated RDC (resolver to digital inverter) or with software support - the electronics for resolver evaluation are the cost of a servo drive might be achieved if the costly analogue technology could be eliminated as much as possible.

This can be done in most cases for the set point input by wide dissemination of the field buses.

Even in motor feedback which is currently dominated by resolvers and analogue sine-cos systems, there are signs of a new trend: innovative measuring systems that are connected to the drive via a fast-response, bidirectional serial interface are coming on the market.

The new Posidrive MDS 5000 servo drives from Stober are compact, modular and fully digital in design.

"Fully digital" mainly means consistently doing without high-quality cost-generating analogue technology as standard equipment.

The new drive has an inbuilt encoder interface that is digital only.

It enables four widely disseminated encoder standards to be connected: absolute encoder with EnDat interface; SSI encoder; TTL incremental encoder (5V, RS422); or HTL incremental encoder (24V).

Absolute encoders with a fast response bidirectional interface are the preferred choice for permanent-field servomotors.

They offer resolution of 17 to 22bit+ per revolution and the absolute position over 4096 revolutions in the multiturn version.

The EnDat protocol from the company Heidenhain is very well known in this connection.

Incremental and SSI encoders are frequently found in asynchronous systems.

Servomotor integration via a fast response bidirectional interface also has the advantage of simpler cabling.

There are only two signals, clock and data, (two strands each) to be run in the cable plus the power supply.

Removal of the analogue signals also reduces the shielding requirements.

Hardware and software modularity is consistent throughout the new servo drives, to give maximum adaptation of the function to the various application requirements.

Because in practice it is very important for a drive to be compact, a compromise was sought between minimal dimensions and the number of plug-in cards and options.

The result of this optimisation is inverters that are 70mm wide and 175mm deep (0-1.5kW size) or 100mm wide and 260mm deep (2-15kW size).

The input voltage range extends from 180 to 528V for the three-phase units; single-phase units are under preparation.

Each unit provides two universal slots for expansion cards and a special space for the switched-mode power supply (external 24V direct current supply or supply from the DC link) and a fail-safe stop option.

A line filter is fitted as standard in every inverter.

The LC display has plain text messages to help with fault diagnosis.

The integrated keypad can be used to input individual parameters such as the field bus address.

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