Achieving maximum availability in any machine, system or component is a critical goal of optimum Overall Equipment Effectiveness (OEE) in a production environment. The more the impacts of both planned and unplanned downtime are understood, and its associated costs, the more accurate the through-life Return on Investment calculation.
The introduction of user-programmability in sensors, and particularly encoders, is one technology advance that impacts positively on machine availability. It goes without saying that devices need to be robust and reliable in themselves, but it is the additional value offered both in diagnostic monitoring and ease of replacement that can present significant opportunities.
Following initial capital outlay in any plant or machinery, the life-cycle costs rise steadily. Faults will always occur and components need to be replaced. While a system refurbishment or overhaul may reduce operating costs, the risks of failure will begin to rise again over time. Managing this process is key to profitable plant operation.
Availability can be expressed mathematically in terms of Mean Time Before Failure (MTBF) and Mean Time to Repair (MTTR):
Availability = MTBF / (MTBF + MTTR)
MTTR is a function of the time taken to identify, diagnose and repair/replace and recommission equipment. So, anything that can be done to shorten the MTTR – for example, keeping stock items of critical parts, clearly has a positive impact on overall availability.
In Failure Mode and Effects Analysis (FMEA), we commonly refer to three criteria to assess a problem – the severity or impact of the failure, frequency of the occurrence and also the probability of detection. Downtime is significantly affected by the duration of the fault i.e. the time it takes to identify and locate, access, source a replacement and reconfigure or commission it.
Before the advent of user-programmability, a unique encoder had to be specified and sourced each time a replacement was necessary, and for a multitude of different requirements. Mechanical considerations, coupled with different resolutions, frequency range and electrical interfacing requirements meant there was no such thing as a “one size fits all” solution.
Now, a modest investment in a smaller inventory of programmable encoders can be used as cover for a wide range of models and provide more immediate replacement of failed components, saving on expensive downtime.
Devices capable of having their configuration data uploaded and stored in a central PLC/PC, such as programmable Ethernet encoders, save significant time in recommissioning plant after failure or downtime.
Popular encoder programming options range from simple DIP switches and serial/USB interface to control system integration using a PLC or HMI unit. Now suppliers like SICK have made programming as user-friendly as it can be. For our AFS/AFM60 absolute encoders and the DFS family of incremental encoders, for example, our customers have the option of using a PC-based programming tool and a battery-operated hand-held programmer.
As discussed earlier, the time taken to diagnose a fault and replace a device has a significant effect on the machine availability. To facilitate easier diagnosis of a fault, today’s encoders have on-board LED’s and fault header data is also communicated to the PLC to advise if the encoder is in error. In the case of Ethernet devices the LED’s also indicate if there are upstream or downstream connection faults.
Cost analysis decision
Returning to the FMEA criteria of severity, occurrence and detection, it’s therefore easy to understand how investment in programmable encoders can help to safeguard machine availability, particularly in terms of reducing the time required to locate, identify, diagnose and rectify a fault.
The cost of downtime in any machine or system can vary tremendously depending on the application. That said, returning to the mathematic relationship between MTBF and MTTR demonstrates that reducing encoder replacement time – say from one or two weeks to half a day - is going to have a significant impact on availability. As the saying goes – you do the maths!