Servos help put Indian boat designs to the test

A recently completed project for the Naval Science and Technological Laboratory, at Visakhapatnam in India, has upgraded the marine research capabilities of the Indian Navy.

A recently completed project for the Naval Science and Technological Laboratory (NSTL), at Visakhapatnam in India, has upgraded the marine research capabilities of the Indian Navy.

The project, carried out by Manchester-based Cussons Technology, was for the installation of a new large amplitude horizontal planar motion mechanism (LAHPMM) on a high-speed towing tank carriage used for the testing of models in the development of new hulls for surface ships in a 500m-long water tank.

The Cussons LAHPMM provides a sophisticated control and data acquisition system for model testing at NSTL and is built around the real-time Autotest IV computer, with close control of seven axes of movement using a Delta Tau Turbo PMAC motion controller, and Control Techniques AC servo drives.

"We designed a system to translate data from our Autotest software into motion control in real time", explains Cussons' C and I Systems Specialist, Simon Dell.

"This demanded servo axes with fantastically fast response times - and we found this in the M'Ax with SLM in conjunction with the Turbo PMAC motion controller.

And we produced axis positioning accuracy better than the NSTL thought possible on this type of rig".

The LAHPMM produces a high precision prescribed motion to a captive surface ship model (typically 0.8 x 7m), while it is being driven through the water.

The sway, surge, yaw and propulsion axes are dedicated servo drives, each comprising the M'Ax single-axis servodrive, in conjunction with a 4.5kW Unimotor fitted with an SLM encoder, which allows the control of speed and torque to be carried out very quickly and in 24bit resolution.

Typical SLM counts per revolution are in the order of 8 million, giving accuracy that betters the NSTL's specifications on all axes.

SLM also considerably reduces the amount of cabling required.

The roll axis uses a Control Techniques Unidrive in servo mode, again in conjunction with a Unimotor, with a standard encoder and is fitted into the force measuring balance assembly.

This subsystem takes in a position demand signal from the host motion controller and can be turned on or off depending on the needs off the test sequence.

The rudder systems (port and starboard) are mounted externally to the drive cabinet but are still part of the main system.

Identical to the roll system, they act as self-contained positioning systems that accept a position demand from the host motion controller.

Each axis drive is "tuned" to fit the characteristics of its own axis, with particular attention to repeatability and speed of movement.

The sway, surge and yaw axes are in current mode, whilst the propulsion drive is in speed control.

The yaw axis is slightly different in that it uses two motors to counteract the inherent backlash in the gearbox and gear ring that is used.

This is achieved by imposing a small position difference to ensure that the gears are always fully meshed.

The axis position for the four major axes is fed back to the Delta Tau motion controller.

The Turbo PMAC controls the axes to a specified position with almost seamless steps, as each MC channel is tuned to give the best response to a position demand signal, giving startling accuracies.

"We have used Control Techniques drives for a wide range of applications over many years", adds Simon Dell, "and we like their consistent, schematic approach to programming the drives, whether AC, DC or servo.

For this project, there were a whole new set of problems to overcome, which we achieved with help from both Control Techniques and Delta Tau in the USA.

The combination of M'Ax with the Delta Tau PMAC worked better than we could have hoped and the client was absolutely delighted".

Limit switches on sway, surge and yaw prevents over-travel on these axes and additional optical sensors monitor the absolute position of each axis to allow the motion controller to "home" each axis (normally central).

As well as providing motion control, each M'Ax drive feeds back data, such as positions and velocities, for logging in Autotest and the drives can be safely disabled for maintenance or adjustments to the rig.

The SLM encoders facilitate almost real-time response, but are not compatible with the Delta Tau PMAC controller.

For this reason, the M'Ax drives are programmed to give a "simulated encoder" feedback, based on 16384 pulses per revolution, to the motion controller.

Each axis has a different gearbox and lead screw ratio so the distance travelled per number of pulses differed from axis to axis.

However, it was possible, within the motion controller, to specify the number of pulses per unit of distance or rotation, which allows the MC to work in real units, ie metres (sway and surge) and degrees (yaw).

"We are already working with Control Techniques on the next big project", adds Simon Dell, "and we're finding out the amazing capabilities of Unidrive SP".

Designed as a package in conjunction with Control Techniques' high performance Unimotor servomotors, M'Ax is the first servo system that can be used directly on load.

The Unimotor map details are automatically downloaded to the drive on power-up.

Built-in default values cope with inertia mismatch, machine friction factors and dynamic servo performance.

Its broad bandwidth and dynamic performance allows for rapidly varying machine inertias without a loss in performance.

Its plug-and-play feature covers more than 85% of applications.

Where optimisation is required, a two-minute adjustment of three settings of load inertia, friction factor and performance covers over 95% of user needs with the simple to use software package MAX-soft, a windows based package that gives infinite adjustment of all parameters, covering the final 5% - high-precision applications such as this.

Users simply dial in the machine details and the software produces the optimum settings to ensure top-notch performance.

It really can't be any easier than this.

But that's not all.

Stunning performance is achieved thanks to Control Techniques' revolutionary speed loop motor (SLM) technology.

This uses applications specific integrated circuits and motor-mounted sincos encoders to achieve an 8.3 million pulses per revolution, a 20-fold increase in position feedback resolution plus an industry leading 50ns synchronisation of axes - irrespective of the number of axes.

The very high resolution and speed of feedback means that the system is seeing changes that much faster.

Within the drive itself, the synchronisation of torque, speed and position loops produces a drive with very high gains and an ultralow torque ripple (the lowest currently achievable) resulting in the smoothest shaft control throughout the range down to zero speed.

Back to top