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Keypad considerations

Keypad considerations

A keypad for a variable-speed drive is a window into the drive's operation and so needs to be easy to use and able to present all the information clearly. Andy Preston of ABB looks at how many drive keypads are meeting machine builders' requirements.

Variable-speed drives are fairly anonymous boxes, with little external indication of what they are doing or whether they are working correctly at all. When it comes to controlling them and getting information from them, human operators turn to the drive's keypad, their window into drive's operation. Because of this central role, drive keypads need to be easy to use and understand, presenting often complex and extensive information and options in a very clear manner.

This is made more of a challenge by the small screens and small numbers of keys that can be fitted on a keypad (due to the need to operate with gloves on) and still make it a reasonable size that can be carried around and then replaced in the holder when needed. So what makes for a good keypad?

Just like modern mobile phones, variable speed drive keypads have inherent size and input/output limitations. But careful ergonomic design of a keypad can provide an efficient interface using programmable keys that call up different actions depending on context.

Some of the latest, most innovative new keypads make extensive use of icons. Main menu levels on the keypads can have associated icons, while graphics and small thumbnails can be used to display some parameter choices. These give quick recognition and understanding of the available options. However, the best keypad designs will not rely on these exclusively. They replace numerical codes and abbreviations with plain English, or a language of your choice, making it easier to understand what is going on and reducing the chance of potentially damaging and expensive mistakes.

Moving through the options and menus can be made easier by providing both left and right keys as well as the more usual up/down key, allowing numbers to be edited digit by digit, or words to be edited character by character. This gives very rapid and intuitive editing for all parameters and text values, while also making it easier to move through the assistants.

The screens themselves can be made bright and easy to read by offering both white text on black and black text on white, as well as several other contrasts to aid reading in dimly lit control rooms or machine rooms.

As well as being easy to read, the screen must also present information that is easy to edit, so that users can display just the information they need in the format that is most useful to them. On one of the latest keypads, for example, the user sees home screens on start-up, which can be easily edited to display the information and parameters the user requires most often. Each of the home screens can hold up to three easily selected parameters, which can be displayed in various formats, such as history trend graphs, amplitude loggers, peak load detectors and load profiles, or even take the place of an analogue meter with an animated meter image.

This feature lets users monitor their drive's operation, allowing processes to be optimised and trends to be used to diagnose process faults. Peak load and profile displays allow the motor load to be monitored effectively, allowing preventative maintenance to be conducted before a problem occurs.

One of the major needs that modern keypads address is to provide help with starting the drive for the first time and getting it configured to match the application. Start-up assistants can help guide the user through the basic set-up steps for the drive. By asking questions, these can guide the user through both start-up and commissioning using plain language text, rather than complex parameter numbers and codes.

A problem sometimes seen is that motor cables are wired up incorrectly. A set-up step can be included that checks motor direction to ensure correct motor wiring. Software controlled phase correction means heavy motor cables do not have to be disconnected and moved. A desirable final step would ask the user to make a back-up, ensuring that the basic drive set-up is stored straight away.  

For an OEM who buys 4,000 AC drives per annum, the time saved through using a start-up assistant feature can be significant. It can cut 15 minutes from the commissioning time per drive, saving 1,000 hours or 50% of an engineer's time if yearly average is 2,000 working hours. Or using the backup to parameterise the 'next' drive can save all of this start up time.

Another important aspect for users is to get the right operating parameters for the drive so that it can control the application in the way the user wants. A useful feature here is the ability to see just the modified parameters, making it easier to see the edits that have been made. Displaying the parameter set as a full list or 'by function' makes it quick and easy to find the parameter needed.

Being able to see the most frequently edited parameters on a favourites list allows the user to shrink the parameter list to a manageable length - it can also allow OEMs to generate special parameter selections for their customers. Parameters associated with energy saving should be grouped into their own menu, making it easier to discover, edit, read and configure them and so help contribute to using energy more efficiently.

There is nothing more frustrating and time consuming for drives' users than to lose the parameters and data from the drive. The ideal keypad would allow backup drive configurations to be stored, preferably more than one, allowing parameters to be restored following some problem or interruption. The backups would have date and time stamp information, allowing the user to tell when the backup was made. Users could look at what the backup contains and check the edits have been saved and are relevant before restoring the edits to the drive.

Modern variable-speed drives have many functions and offer many advantages to end users, but all need an excellent keypad if they are to fulfil their true potential. The new user interface HMI used on the new ABB ACS880 Industrial drive and all future drives of the ABB range, fulfills all of these criteria and more, making it an extremely user friendly product to operate and own.

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