Servo utility in ac drive cuts machine down to size
Machine builder, Sheppee International discovers how modern ac drives
can be programmed to provide simple, but precise motion control at low
cost and with minimal investment in additional hardware
The latest generation of hot glass container handling equipment from
Sheppee International uses the servo capabilities of a modern ac drive to
achieve reductions in both equipment size and cost. One of the company's
latest products, the ITD-8000 independent transfer drive, which
transports bottles through a 90o turn from one conveyor to another while
maintaining their separation, uses Control Techniques' Unidrive universal
ac drive and Unimotor servomotors in a combination that Sheppee's Richard
Moore describes as plug-and-go . The 4kW drive is housed within the
unit, so there's no requirement for additional external panel space and
cabling. Moreover, there's no PLC and (usually) no requirement for a
gearbox.
The drive is operated in closed loop servo mode, its reference being a
single pulse from the upstream bottle-forming machine every time a bottle
is produced. A 'virtual' axis is created in the Unidrive's processor and
the servomotor's running speed is synchronised to this pulse. The
pulse/running-speed relationship is also dependent on other parameters
such as transfer head 'finger' spacing or (if one is fitted) the gearbox
ratio. Programs for each type of transfer head can be stored in a PC and
downloaded to the Unidrive's co-processor module via a CT-Net high-speed
network.
The 1.5kW Unimotor is shorter than previous types used by Sheppee, and
this has allowed direct coupling to the IDT-8000's drive shaft. Despite
its low running speed, which can vary between zero and 250rpm, the motor
runs cool - crucial when operating in an area of the factory with a high
ambient temperature and in close proximity to glassware at around 500°C.
The drive system allows the torque limit to be pre-set for safety
reasons. If, for any reason, the system becomes jammed, the IDT-8000
stops, allowing the obstruction to be dealt with, then restarts, speeding
up until it is back in synchronism with the rest of the line.
So impressed is Sheppee with the performance of its transfer unit's drive
system, that it is currently considering using the Unidrive/Unimotor
combination in new and existing machines. These include a low-cost,
two-axis version of its three-axis Lehr loader, and the company's
Synchroflex multi-drive system, which now features Unidrives working in
digital lock mode.