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10/08/2006
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Tom Shelley reports on a system that monitors an electric actuator and warns when maintenance is required
New actuators for hospital beds monitor duty cycles and can distinguish between light, heavy and abusive use - and predict when maintenance is required.
The Actuator lifetime monitoring device, supplied by Electro Mechanical Systems, depends on an Asic chip on a 25mm x 35mm PCB fitting inside existing SKF Magnetic actuators.
The motor's cycle time is first programmed in under full load, and then again under full load. The duration of a full cycle therefore indicates both that the cycle has occurred and whether it was under load or not - which affects the predicted lifetime. In addition, the Asic records excessive motor heat. This indicates that the actuator has been subject to abuse, which shortens its life expectancy by a known proportion.
The principle of logging duty cycles to predict when equipment should undergo maintenance is applicable to any kind of industrial equipment.
The system uses a traffic light style display on the actuator to tell the user how much of its lifetime remains before it needs to be serviced or replaced. Once it reaches the 'red' condition, the actuator sounds an audible alarm signal every time it is used.
The device is fitted to each actuator - which carries its own unique identity - rather than being fitted to the control box. This ensures that if actuators and control boxes are exchanged during service or repair, the usage record remains with the actuator.
It is possible to undertake more advanced diagnosis by downloading the information onto a PDA, laptop or mobile phone, from which the captured data can be downloaded for further analysis.
This functionality meets the requirements of the latest report on the failure of electrically operated patient hoists issued by the Medicines and Healthcare Products Regulatory Agency (MHRA).
The documents describe the types of failure and recommend that the life of a hoist actuator be dictated not by its age but by its duty cycle. The report states: "Establishing the life of an actuator is difficult because hoists do not have counters fitted to record the number of lifting cycles."
Due to varying weights of patients, it is also impossible to determine the actual work carried out by the actuator. It is recommended that equipment inspections take place every six months. These actuators ensure that, between inspections, the use of the actuator is constantly monitored - providing a greater assurance of safety and reliability.
Electro Mechanical Systems
Pointers
* System predicts when equipment is likely to require maintenance
* It can distinguish between loaded and unloaded duty cycles by measuring their lengths
* By measuring motor overheating, it can detect abuse - and adjust its remaining lifetime predictions accordingly
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Author
Tom Shelley
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