Programmable chips make agile controllers
Tom Shelley reports on an innovation that makes machine controllers faster to respond – and easier to fix
By basing a controller on a field programmable gate array (FPGA) chip, it can be made to run up to a thousand times faster than a conventional computer-derived controller. It can also help to beef up robustness and – using graphical programming – improves matters where benefits are to be found from tweaking one of the algorithms.
While National Instruments has been hawking the idea about for a while, users are clearly beginning to take it up. There are target applications in aerospace, mainly in simulation. But, because the technology is far cheaper than conventional control, it also looks appropriate for automotive and industrial uses, especially for motor control.
As well as the base technologies, which continue to be developed, NI and its customers have been learning new ways of making use of FPGA technologies, some of which were revealed at a recent Military/Aerospace Solutions Conference.
Mike Bailey, a systems engineer with NI, explained how its graphical programming technology could be used to join up logic blocks on the chip – so as to put more than one control algorithm on the same FPGA.
“This would allow true parallelism in the system,” he claimed. “No data bus gets in the way.” The result, he said, was the difference between a traditional computer-derived control system’s 25 microsecond response – with the possibilities for crashing – and an FPGA system’s 25 nanosecond response time, with no operating system to crash it.
“You can also have two or more separate logical operations on the same chip, so PID control can be run at, say, 200kHz, instead of 40kHz in a conventional solution,” he said.
In a demonstration, Bailey showed a motor control algorithm implemented on an FPGA that included a velocity loop, with input from an encoder, a current control loop and pulse width modulation generation. He first implemented it with a wrong parameter, so that the motor tended to hunt for the right speed, and then made a correction in the graphical programming and downloaded it to make the motor go to a set speed and run smoothly.
The point was thus made that control data can be implemented on chip and, if bug fixes or improvements are then developed, these can easily be implemented and, where necessary, downloaded over the Internet, with a minimum of delay and cost.
Ken McMullan, a software engineer with Goodrich Engine Control Systems, described using FPGAs to simulate an LVDT (linear variable differential transformer) and other sensors for a turboprop engine control system. Apart from cost saving, a strong reason for simulating sensors, rather than taking input from real ones, is that this allows studying what might happen in extreme conditions, without resorting to a real engine in a real aircraft. The approach, he said, came out of in-company research suggesting that LabView and FPGAs would greatly accelerate rapid prototyping.
Another serious commercial application is Product Technology Partners’ implementation of a LabView FPGA for servo hydraulic control in pharmaceutical tablet manufacture. PTP had already developed a successful controller for its pill-making machines, but decided to try a new controller, based on an FPGA card, to undertake both servo hydraulic control and fast data acquisition. This resulted in a controller that, with just two days of software development effort, was £20,000 cheaper per machine and performs with a significantly less error span.
* Sensor input processing and control algorithms can be implemented to run in parallel on the same FPGA chip, without interfering with each other
* With no operating system to crash and no databus required, process loop time can be greatly reduced and reliability enhanced
* Should problems come to light, they can easily be put right, using graphical programming, and downloading corrections and/or improvements
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