Product category:
Stepper and Servo Drives, Motors, Controls
News Release from: Control Techniques | Subject: M'Ax digital servo drive
Edited by the Engineeringtalk Editorial
Team on 13 August 2001
Drive's 500-fold increase in feedback
resolution
High speed machinery manufacturers are set to enjoy greatly improved levels of competitiveness with the launch by Control Techniques of its latest M'Ax digital servo drive
High speed machinery manufacturers are set to enjoy greatly improved levels of competitiveness, and the machine users new levels of axis performance, with the launch by Control Techniques of its M'Ax true distributed digital servo control system with revolutionary Speed Loop Motor (SLM) technology SLM technology is a quantum leap forward in servo systems and offers major advantages over the existing technology
This article was originally published on Engineeringtalk on 30 Jan 2001 at 8.00am (UK)
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The possibility of signal degradation due to noise injection is reduced accordingly.
Furthermore, component, installation and maintenance costs are slashed as a result of reducing the total number of interconnections per machine axis by 65% (from 34 to 12), when compared to a standard servo encoder solution.
Secondly, the SLM system uses a combination of high resolution, motor-mounted SinCos encoders and DSP technology to achieve an application invariant 20 fold increase in position feedback resolution (over 8.3 million points per turn), and the capability for active torque compensation.
This is achieved by integrating speed and position control within the feedback system on-board the servomotor.
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As a result, the SLM is able to overcome the degradation in performance experienced when synchronising multiple servo axes on machines as operating speeds increase.
An additional benefit is that the SLM system performance is independent of the number of axes employed.
Also, the system cost can be optimised for the particular application The SLM system effectively overcomes the problems traditionally experienced with conventional high end servo technology which is nowadays approaching its limits due to the physical constraints imposed by high resolution position acquisition.
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In typical machining applications, the quality of the process depends largely on the smooth running of the motor and on a stable, dynamic response during system disturbances.
The ability to achieve smooth rotation of a motor depends, in turn, mainly on the quality of the feedback signal, mechanical non-linearities, the torque ripple introduced by the motor and the closed-loop controller algorithms.
By increasing the resolution of the position feedback signal from a motor, controller gain can generally be increased, yielding reduced position deviations due to system disturbances.
In addition, the resulting high resolution in position information also leads to the possibility of implementing a realistic torque compensation algorithm by computing torque (acceleration) feedback.
Since acceleration feedback involves the double differential of position information, a stable response from an acceleration control loop can only be realised if the initial position information has sufficient resolution and the control algorithm has a sufficiently high sampling rate.
The key, then, to achieving significant performance enhancement, is to obtain high quality, undegraded position feedback.
Help in this respect has arrived in the shape of Sine/Cosine (SinCos) encoders which are now commonly available and give resolutions in the region of the equivalent of 4 million pulses per revolution.
The problem with these components is that their signals have to be conditioned for transmission to drives and controllers.
If there were no need to transmit this data, often over significant distances, then the conditioning issue could be avoided.
One way in which this could be achieved is by locating the position loop and velocity control loop in or alongside the encoder.
This is exactly what has been done on the SLM system.
The availability of small signal processing components (such as DSPs and A to D converters) provides the opportunity to process the information at source, directly behind the motor-mounted SinCos encoder.
This both reduces the quantity and the cost of interconnections and eliminates any noise injection associated with systems that transport analogue signals along encoder cables back to a drive.
The analogue position information from the motor-mounted SinCos encoder is acquired by an A to D converter.
The simultaneous sampling of the sine and cosine channels with 12 bit resolution and a conversion time of 4ms enables the controller to acquire position information every 4ms.
Also a hardware counter tracks the number of SinCos cycles.
The controller combines this information to provide a position feedback resolution better than 8.3 million counts per revolution.
This information is then used to derive a high bandwidth acceleration feedback term to compensate for motor torque ripple and load disturbances.
The Speed Loop Motor (Unimotor SLM) is produced by combining a permanent magnet servomotor, a SinCos encoder and the necessary electronics to process and close the position and speed loops.
In order to operate as an ultra-high performance drive, a power stage; MultiAx three axis, or M'Ax single axis drive, and a programmable motion controller are required, in addition to the Unimotor SLM.
When coupled with the new M'Ax range of drives however, no motion controller is necessary and the M'Ax / SLM motor combination functions as a completely stand alone drive system.
The demands of the system dictate that each of the components must be capable of communicating with each other in a robust and reliable manner, and provide a data throughput rate sufficiently high to achieve the bandwidth required for servo applications.
This important issue has been addressed by the development of a dedicated ASIC, the so-called DriveLink ASIC, which forms the heart of all communications between individual parts of the overall system.
By placing a DriveLink ASIC in each of the system components (Motion Controller, Unimotor SLM and the drive), all communications are taken care of by the hardware protocol controller within the ASIC, which also has built-in error-detection, such that zero overhead is placed on any of the microcontrollers dedicated to the control algorithms.
The DriveLink implements synchronised fault-tolerant communication between the controller, the drive system components via a standard 2-wire RS485 physical layer at a data rate of up to 5M bits per second.
Typical cable lengths of up to 100m can be reliably implemented.
In addition, DriveLink enables users to optimise on the open architecture of the SLM system by enabling application specific control algorithms to be downloaded as required.
The deterministic nature of the DriveLink protocol synchronises three controllers within any axis i.e position interpolator, position/speed controller and the current loop.
It also synchronises individual axes to a primary trigger, and the whole system to within 50ns.
In order to eliminate "jitter" on synchronisation the parameter passing channel is implemented within the protocol, and a global hardware trigger facility which provides inter-axis synchronisation to within 10us. Request a free brochure from Control Techniques ...
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