Room for improvement
Michael Kellett, technical director of Lammerholm Fleming, explains the development of his company's new hi-performance, low cost shock and vibration recorder.
In 1998 Lamerholm Fleming launched the RD298 ShockLog shock and vibration recorder. The RD298 is an event based Shock recorder featuring 3 built in piezo electric accelerometers and 2 Mbytes of flash memory. This instrument used the best available electronic technology to provide perhaps 80% of the functionality of the established aerospace recorders on the market but at about 20% of the price.
The good price performance ratio of the RD298 has allowed many more organisations to employ Shock recording as a technique to improve the design and packaging of their products or to monitor the handling of goods in transit. We decided to expand our product range to offer a lower cost recorder suitable for applications involving large scale monitoring of goods in transit and set about identifying the key 'cost of ownership' issues involved in shock recording.
It soon emerged that while the cost of purchase was important, the real barrier to mass applications was the cost of using a PC in the field to set up Shock recorders and extract data from them. So when we designed the RD317 we were determined to make it useable without ever directly connecting it to a PC.
To do this we added an iButton interface to the Shock recorder. iButtons are made by Dallas Semiconductor (www.ibutton.com) and are 16mm diameter metal pills containing memory, clocks, temperature recorders or other simple functions. There is a range of standard accessories which includes PC readers and software as well as convenient tags and holders. The iButton electrical interface is a fairly simple bi-directional asynchronous serial system which can easily be implemented in software. The operator just presses the iButton to a socket on the RD317 and data is transferred automatically, the whole transaction usually requiring no more than a few seconds.
The Shock recorder user can program low cost iButtons in the office using a PC and then use those buttons to set up and start an RD317 ShockLog recorder in the field. Once the mission is complete the user can download data from the RD317 to an iButton and read that data out into a PC when convenient. There is no need for a PC at the receiving point since the iButton can easily be posted back to a central location for data analysis.
We set about reducing the size of the recorder by changing the battery from a single C size cell used in the RD298 to a single AA cell. However, we did not wish to compromise on battery life so we also needed to reduce power consumption by a factor of four – down to less than 200uA average for a nominal 1 year endurance using a lithium battery.
At the same time we needed to reduce the cost of electronics and the size of the electronic package.
Power consumption was reduced by employing a 3V power rail throughout rather than the 5V used in the RD298. An Atmel mega103 processor (www.atmel.com) replaced the Hitachi H8/3337 (www.hitachi.com), reducing power consumption but at the expense of reduced processing power.
The RD298 uses an external 128k byte RAM buffer to store data which is then transferred to parallel access sector erase flash memory. The RD317 dispenses with the RAM buffer, using only the 4 k bytes in the mega103 and transfers data to a serial access Atmel DataFlash memory as it is acquired. A significant reduction in power consumption is achieved by eliminating the parallel data bus and the pcb layout becomes much more compact. A valuable side effect of the serial memory access is a reduction in radiated RF emissions which made it easier to gain EMC certification to use the RD317 in air cargo applications.
Because the DataFlash memory can be erased in small sections it becomes feasible to erase and re-record shock events during a mission so the RD317 can make effective use of a much smaller 0.5 Mbyte memory.
Finally, we reduced the cost of the analogue processing system by using lower cost op amps and active auto zeroing controlled by the processor to eliminate offset drifts with temperature. The active auto zero system injects trim voltages into the signal processing chain by means of Analog Devices (www.analog.com) digital pots.The component count was further reduced by building the RD317 with acceleration ranges fixed during manufacture and by removing the little used velocity converter.
The RD298 ShockLog recorder uses piezo electric accelerometers which offer the advantages of low cost, zero power consumption and good dynamic range. The contact wire for the ceramic piezo electric element is threaded through the centre which requires us to drill a small diameter hole in the ceramic material.
For the new RD317 recorder we reduced costs by re-designing the accelerometers to use standard piezo ceramic disks without a centre hole and value engineering the production process to reduce the manufacturing time of the sensors by 50%. A valuable spin-off from this work has been a measurable improvement in yield from the process — mainly derived from eliminating the difficult ceramic machining step. The ShockLog recorder control software runs on a standard PC and this was updated and modified to work with the slightly different feature set of the new design. The embedded software for the original RD298 product was written in C and about 50% of the code was ported to the Atmel processor with very few problems. The different nature of the hardware required us to re-write the data acquisition and storage sections of the code. All of the iButton interface code was written in C and even with the relatively slow processor clock (used to reduce power consumption) there was no problem operating the iButtons at full speed.
Even though only 2 years separated the design of the two products, significant improvements in power consumption were obtained — the RD317 uses 90% less power during event recording and 75% less when idling. The feature set of the RD317 has been tuned for cost sensitive applications by applying a combination of new technology, market sensitivity and traditional value engineering. The result is the ideal shock recorder for volume applications, a 50% cost reduction, a 33% weight reduction and an overall 75% reduction in current drain.
Lamerholm FlemingTel: 01438 728844e-mail: firstname.lastname@example.org