Bandwidth is a mathematical concept. It is the band around a desired operating value. We can consider the bandwidth of control in a process or the bandwidth of error in statistics. The concept applies to all sorts of things.
The speed bandwidth of your car might be described as zero to 100 miles per hour. You might want to include reverse speed, so it might be -40 miles per hour to +100 miles per hour. The bandwidth of error of your cruise control, which is an automatic speed control system, might be +/- .5 miles per hour, or at least something of that nature. If it were +/- 10 miles per hour, you would likely get a ticket for going too fast or too slow when your speed control was engaged. Not a good thing.
So the nature of bandwidth can have some important implications.
When controlling temperature of a process, like chill water in a cooling tower, the bandwidth of error in the control system can be a very significant factor in how much energy the system is using and how efficiently the process runs. If the control system bandwidth is +/-10 degrees, there is a great deal of energy lost purely as a result of the control system error. Simply changing the thermocouple and controller to +/-.5 degree accuracy ends up saving incredible amounts of power.
The same principles apply to mechanical situations. The bandwidth of control required for a mechanical system will have a lot to do with the technology required to control it, the cost and the overall performance. So it’s important to understand the required performance early in the design cycle of any new project.
This is a particularly valuable concept in selecting the right variable frequency drive for a new application. If the load is a fan or rotary pump, then the variability of the load can be easily identified. Large diameter fans have very large inertias, so the rate of change possible is very slow. A control bandwidth of a few seconds to change speed is probably sufficient in most cases.
In the world of variable frequency motor controls, the most basic drives have a 10 hertz bandwidth. This means that the drive will try to regulate speed at a set value to some specified percentage and be able to make adjustments 10 times a second.
What happens when there is a material handling system that needs to speed up and slow down to create gaps between packages on a conveyor? The maximum allowable speed change and the rate at which the changes have to be made, the bandwidth of the changing load, must be calculated to insure that the motor control is capable of meeting the desired conditions.
This same aspect is the means by which we can segment what technology of speed control is needed. At low frequency bandwidth, open loop variable frequency drives are used. As bandwidth in the load increases, bandwidth in the motor control increases as well. Variable frequency drives with feedback mechanisms on the motor and load are capable of performance to about 200 Hertz. Beyond this, a brushless servo is needed. In the servo arena, bandwidth of 2000 Hertz are available.
Filed Under: Mechatronic Tips