Product category:
Drives, motors and power transmission, couplings, clutches
News Release from: Stober Drives | Subject: Posidrive MDS 5000 servo drives
Edited by the Manufacturingtalk Editorial
Team on 06 June 2003
How servo drive technology helps reduce
costs
In a new generation of servo drives, the analogue system gives way to purely digital feedback.
When the economy falters, competitive pressures increase Which can have serious consequences for the life of a business
This article was originally published on Manufacturingtalk on 19 Dec 2002 at 8.00am (UK)
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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 analog system gives way to purely digital feedback.
By means of a changeover switch, a number of servo motors (also new) can be operated sequentially on a single drive.
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Planetary gear system options extended further
Less expensive spur gearing and other cost optimizations have produced an 'economy' planetary gear system while existing higher specification types have been further improved.
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Variable speed drives manufacturer offers an application-based drive solution and looks at and solves the electronic and mechanical problems as a whole.
The drives also provide an integrated IEEE 1394 (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 modularization 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 characterized by fast-response control circuits, a generously sized output module, a wide interface variety and a high-quality analog 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 analog system which is an important factor in a conventional servo drive - both technically and price wise.
This is illustrated by an example of an analog set point: With a speed setting range of 1:5000 and an analog +/- 10 V set point input, a resolution of 14 bits is required.
If the set point input is digital, e.g via a field bus, 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 14 bits.
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 analog 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 analog 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 analog 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 (5 V, RS422); * HTL incremental encoder (24 V).
Absolute encoders with a fast response bidirectional interface are the preferred choice for permanent-field servo motors.
They offer resolution of 17 to over 22 bits 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.
Servo motor 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 analog 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 optimization is inverters that are 70 mm wide and 175 mm deep (0 - 1.5 kW size) or 100 mm wide and 260 mm deep (2 - 15 kW size).
The input voltage range extends from 180 to 528 V 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 24 V 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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