TechArchive

Original article date: April 2000

Alan Quinn takes a look at a low-cost module for a drives systems which makes real-time control at high speed possible without a PLC

The on-going debate as to whether drive intelligence is better on-board, or provided remotely via a PLC or PC has been stimulated with the launch of Control Techniques’ new ASIC, the UD70.

The UD70 is a 32-bit, task-based option module that fits inside the Unidrive. It provides a low cost facility for a drives system designer to write application-specific programs and achieve peer-to-peer real-time control at high speed (up to 5Mbits/s) without the need to resort to a PLC or other standalone controller.

By enabling intelligence to be kept at a local level, the UD70 provides both savings on equipment costs and improved system redundancy. In contrast to PLCs, with their dependency on their scan times, the new UD70 allows real-time control operations and any fixed time-base calculations to be implemented easily. At the same time, the high speed of the network with the UD70 enables drives to communicate directly to one another in real time, thus improving system response. Moreover, with off-the-shelf programs running in the program modules at drive level, costs and development time are also reduced.

The UD70 has been adopted as the system controller ‘PLC’ in a new lift drive. Previously, lift control systems have relied heavily on standalone PLCs. Now, the UD70 can include the entire lift control system – and the system communications – in a single, compact module mounted on a Unidrive LFT.

Real-time response

Running at 16MHz, the microprocessor uses a dual port RAM to interface to the main processor inside the drive, providing bi-directional communications between the module and the drive’s internal processor. This facility enables the UD70 to read any parameter from the drive (including internal, virtual and I/O box parameters), which are essential for real-time response. It also enables the module to write to any read/write parameter within the drive, providing the capability to perform real-time calculations for time-critical process control systems.

In one of the first field applications for the UD70, the unit’s ability to read any drive parameter has been instrumental in achieving new levels of performance in real-time registration and web tensioning control on an 11-station off-line print finishing system for direct mail.

Drive control functions on the UD70 are programmed from a PC using SYPT (System Programming Tool), powerful Windows software compatible with IEC 1131-3 and connected to the network which simplifies and speeds up the configuration of data transfer between each node and allows the programmer to program every drive on the network from one point. Using IEC 1131-3 standards, SYPT can program in ladder logic or function block, and is also able to program in Drive Programming Language (DPL) – a reduced version of BASIC with real-time functions added.

The major advantage of DPL is that it allows higher level application programming. This feature was instrumental in its selection as the control language on the real-time registration and control system. DPL is simple to learn, but is not so basic that it has no power. The language gives primitive values in software which when compiled run extremely fast. This is key because speed is one of the most important factors in the majority of applications on which the UD70 is likely to be employed.

Tomorrow never dies

When James Bond and his female companion jump off a 32-storey building in the film ‘Tomorrow Never Dies’, the pair grab an advertising banner which rips and peels progressively, slowing their descent. They plummet some 15 floors and live to fight another day. Engineering stunts like this are all in a day’s work for Unusual Rigging, the majority of whose custom automation output is achieved using Control Techniques’ Unidrive.

Historically, such leaps have been achieved using air descenders, but these are slow and laborious and cause long delays between takes. In an attempt to overcome these problems an alternative system using two steel wire winches linked to harnesses was tried. The reason for using winches was that they can rewind quickly to allow for multiple takes. But in this case, the acceleration of the drum was so fast that there were problems getting the cable to come off without loops developing.

Unusual Rigging devised a take-off motor and pinch roller system controlled by a Unidrive with an on-board UD70 soft logic module. The pinch roller has its own encoder which is linked to the UD70 and the module also receives speed data from the winch encoder. By comparing the two streams of speed data, the Unidrive and UD70, in combination with the pinch roller, ensure that the tension in the cable never goes slack.

• Control Techniques

April 2000