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Precision drive solves packaging bottleneck

A Control Techniques product story
Edited by the Engineeringtalk editorial team Jul 3, 2002

A recurring headache for Project Technician Brian Raynor at Johnson and Johnson's Wound Management Division has been cured thanks to the application of a Control Techniques Unidrive.

A recurring headache for Project Technician, Brian Raynor, at health care company Johnson and Johnson's Wound Management division, based at Gargrave near Skipton in Yorkshire, has been cured thanks to the application of a Control Techniques Unidrive.

A 28-position palletising line, which used photoelectric sensors to detect the position of the central trolley, was proving to be slow and unreliable with poor positioning causing frequent stoppages.

Now, with a Unidrive in servo mode providing the positioning, "it never varies", removing a production bottleneck in a very busy plant that makes wound dressings.

"The unreliability was proving to be a limiting factor on the capacity of the palletising line", says Raynor, "and was frequently causing a bottleneck, with boxes backing up the conveyor lines into the production area.

Clearly, it had to be sorted".

The Johnson and Johnson plant makes sterile wound dressings and theatre drapes, for the hospital market.

The palletising section can accept up to 28 different products at any one time, at a rate of 97 boxes per hour, providing pattern stacking onto pallets (up to 27 boxes on each pallet) using pick and place robots along two rows.

A central trolley provides collection of full pallets, delivery to the discharge point and return of empty pallets to available stacking points.

Full pallets go for cobalt sterilisation prior to final despatch.

The positioning of the trolley is crucial at each collection/discharge point, since the conveyors have to line up precisely for the operation to take place.

The previous system used open loop inverter control of the drive motor on the trolley with the positioning being determined by optical binary 'flags'.

Frequently the trolley over or under shot its target and hunted until to find its position - sometimes failing completely, necessitating manual intervention.

"We use many Control Techniques drives, particularly Unidrives, throughout the plant", says Raynor, "and had seen their positioning software demonstrated.

We thought it seemed ideal for the job and we've had no problems since it was installed.

It just never varies!" The solution proved to be simple and elegant, thanks to the on-board processing capability of the versatile Unidrive 'universal' drives from Control Techniques.

Two 4kW Unidrives were supplied (duty and standby), complete with a UD75 coprocessing plug-in unit, supplied with Control Techniques positioning software.

The UD75 plug-in applications module is one of a series of modules designed to provide both additional on-board processing capability and to add specific functionality to the Unidrive, making the drives capable of running customer programmable applications programs.

Bidirectional communications gives high-speed read/write access to speed and torque reference within the drive and the RS485 port is fully configurable.

External I/O can be added and a direct RS232 port is provided for programming and debugging, with ANSI communications and Modbus as standard.

For the control of the trolley, a 5Nm Unimotor servomotor, fitted with an absolute encoder, provides motive power for the trolley along its 20m track.

The Unimotor brushless AC servomotor, from Control Techniques Dynamics, is designed to supply high torque with either low or high rotor inertia and minimal cogging torque.

Thermal performance is improved by the unique, new "finned" motor housing, manufactured in high strength alloy casting to radiate and convect heat.

The single piece integral construction permits accurate bearing to housing alignment and maintains air gap concentricity giving excellent dynamic performance combined with exceptional reliability when used with modern, high frequency PWM (pulse width modulation) servo drives such as Unidrive.

A PLC controller provides overall control of the palletising section and sends a 5bit signal to the Unidrive, indicating its next position.

The drive then takes over, giving a precisely controlled acceleration and deceleration, stopping exactly on its designated position, determined by the count from the absolute encoder.

"It was very easy to set up", says Raynor.

"It's a standard programme on the plug-in module.

We simply had to program in the count number for each of the 28 positions and discharge position - and off it went.

It's removed a production headache for me and a production bottleneck for the plant".

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