Innovation in control

Fred Brown from PDD takes a look at some innovative techniques that were employed in the design of Eurotherm Controls’ process controllers

When Eurotherm Controls embarked on the development of a new range of process controllers, it wanted to build a modular range of units with a common mechanical format, enabling cost savings in manufacturing and servicing. In the development process, the company turned to PDD to create the range. The result was the Eurotherm Series 2000, a range of modular temperature and process controllers that have been designed with simplicity of build and installation in mind.

The man/machine interface for the process controllers, including key-handling and display message formats, had to be intuitive with identical mnemonics and key strokes across the whole range.

What is more, three levels of performance had to be available in four DIN sizes from 1/32 to 1/4. In those sizes, it was important that the OEM could choose from a standard controller, an advanced motorised valve controller, a setpoint programming controller and a controller with 16 setpoints.

These design criteria were met by innovative electronic and mechanical design. The main PCB electronic module common to all the designs in the range is structural, clipped together with a plastic skeleton to form a removable module. Attached to the front panel, the main PCB module can be removed for access and maintenance. Different functions, such as configurable dc outputs can be added into the main module by means of a set of electronic modules that plug into it.

Sealing the units was another important issue. The seal between the front panel and rear housing, and the unit and facia into which it is mounted, was vital to ensure that the units would provide IP65/NEMA 4X sealing against dust and water and conform to the EMC directives.

The front panel seal was developed using a two-shot moulding technique. Ensuring the seals remained in place during assembly was paramount to prevent mis-alignment and pinching which would render the seal ineffective. With the seals moulded in place and unable to move, installation is much simpler.

Because the rear housing of the controller was to be mounted through a facia panel, a ratchet moulding was required to provide pressure against the panel and thus retain the unit in a rack. During development, this component was prototyped through CNC machining and cast replicas from stereolithography masters to ensure the correct tension could be applied. The pitch of the teeth needed careful consideration – too fine and they would be susceptible to damage, too large and there would be too great a variation in the tension applied to the sprung arms.

The solution adopted for the ratchet consisted of two staggered rows of teeth, developed such that the pitch is effectively halved. Following the prototyping phase, the part was tooled in a metal safe condition to allow minor tweaking; this was not needed, the part was produced right first time.

In place of a traditional screw fixing, the front panel was physically clipped into the rear housing. When closing the unit, the clips provide a carefully controlled amount of pressure on the seal to ensure effective protection. Once a clip is popped open, a reverse angle prevents it from re-closing, thus allowing the two clips to be undone independently before removing the front panel and PCB structure.

Following approval of initial design concepts for the clip, 2D GAs and sections were drawn up to better understand the fit of components and provide defining data for the creation of a 3D CAD database. Visual detail and keypad configuration options were simulated in CorelDraw to assist with early assessment. Test rigs manufactured in representative materials allowed details to be tested and iterated.

Stereolithography was used to assess the major components; the depth of draw on the rear housing in particular prohibited the use of PDD’s preferred FineCut CNC machining methods. With the data files validated and production tooling commissioned, a tactile problem was identified with an area of the front keypad. Although a technically proven design, a slight sticking sensation could be discerned when pressing the keys, which, in the worst case, actually caused the keys to remain depressed following actuation – an intolerable situation. In an assembled condition, it was impossible to identify the cause of the problem, and the need to apply a real-life equivalent of 3D CAD’s dynamic clipping function – an ability to view sections right through the assembly – was evident.

The solution came about through setting the assembly in resin which, once set, could be machined as a solid block to reveal sections which could then be dimensionally analysed in a number of ways. Scanning the solid section using a conventional flatbed scanner at 1600dpi, and then enlarging and printing the images allowed a comparison to be made with 2D plots and dimensional analysis was achieved using a microscope to compare the measurements taken from the solid section with the 3D data file.

The analysis revealed a movement in the keypad following the welding of the external casing. In an assembled, but not welded, condition, the keypad is a snug fit in the retainer moulding; however, the welding caused a movement in the keypad, such that the critical angle of the collapsible webs was altered. Taking the web angle over 45 degrees reduced its ability to return the key to its relaxed position, resulting in sticking.

A modification was made to the mouldings such that the keys were physically constrained in their relaxed position, allowing the webs to perform properly. This extensive, considered analysis enabled a modification to be made to the component and prevented a potentially lengthy trial and error approach.

With over 400 production data files required to specify the product variants, maintaining control over the design data and controlling its issue to toolmakers was critical. SDRC’s Data Manager controlled the data libraries, with issue control monitored through a paper system requiring authorisation and signed acceptance of the data.

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Figure 1: The Eurotherm 2000 range. Three levels of performance are available in four DIN sizes from 1/32 to 1/4

Figure 2: (a) (top left) The front panel of the unit. (b) The front panel seal was developed using a two-shot moulding technique. Ensuring the seals remained in place during assembly and installation was paramount to preventing mis-alignment and pinching which would render the seal ineffective

Figure 3: Different functions can be easily added into the controller by means of a set of electronic modules that plug into it. Hence the unit can be easily custom configured by the user

Figure 4: The pitch of the teeth on the ratchet needed careful consideration. The solution finally adopted for the ratchet consisted of two staggered rows of teeth, developed such that the pitch is effectively halved