Servo sets new standards

A new servo speed loop system has achieved a 20 fold increase in position feedback resolution and a 65% reduction in wiring costs. Design Engineering investigates.

With the launch of its M’Ax true distributed digital servo control system with Speed Loop Motor (SLM) technology, Control Techniques claims that machine users are now set to enjoy new levels of axis performance.

SLM technology offers certain advantages over existing servo system technologies. Firstly, the system employs a dedicated high-speed 2-wire data link which eliminates the need to transport multiple noise-sensitive analogue feedback signals. 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 57% (from 28 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 onboard the servomotor.

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.

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 this could be achieved is by locating the position loop and velocity control loop in or alongside the encoder.

On the SLM system, feedback information is processed at the source. 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 4 micro seconds enables the controller to acquire position information every 4 micro seconds. 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.

SLM SYSTEM DESCRIPTION

The Speed Loop Motor (Unimotor SLM) is produced by combining a permanent magnet servo motor, 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 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 5Mbits/s. 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 ie 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 10 micro seconds.

SYSTEM COMPARISONS

In order to make comparisons between the new SLM system and more conventional servo systems, typical system interconnections are shown in Figure 1 for a standard servo drive equipped with a typical feedback device. Figure 2 shows a typical connection arrangement for an SLM motor with a Sin/Cos encoder, connected to the M’Ax servo drive, and controlled from a controller also fitted with a DrivelinK ASIC. The benefits in system wiring due to the very significant reduction in system interconnections can be up to 16 cables per axis.

Information: Control Techniques Tel: 01582 567700