In the field of motors and controls one of the key parameters of merit is Bandwidth. This parameter can be applied to a number of issues related to motors and controls. Manufacturers of variable frequency drives and servo amplifiers often refer to this parameter, but it’s real importance is often lost in the massive amount of technical information used to describe these systems.
Bandwidth refers to the region in which a parameter occurs. It could be any parameter. The bandwidth of motor speed, for example, might be from zero to 1725 RPM in the case of an AC motor. Or it might be specified as minus 1725 to plus 1725 if the motor is capable of reversing direction.
Control system bandwidth can be described in relation to any parameter that is under system control. Most commonly, this is velocity control and the bandwidth is described with an additional parameter of accuracy. So a typical servo amplifier velocity bandwidth spec might read -100% to +100% of rated speed with a velocity accuracy of .1% of set speed. This means that for any motor attached to a given amplifier, the amplifier will control the motor speed to one tenth percent of set speed.
Not rated speed as in the case for many open loop speed controls. Many DC and AC systems use rated speed for the bandwidth parameter. So if you have a DC speed controller being used with a 2200 RPM motor, and they controller is rated for an open loop accuracy of 2% of rated speed, the motor’s actual speed could be off by 44RPM, which at 100 RPM commanded speed, would be an error of 44%. Not so great when you need accuracy. But for most simple speed control applications like conveyors, this is fine.
What is missing from all of these descriptions are the time allowed for the control system to control disturbances, also called dynamic response.
In addition, the most most important aspect is understanding the load’s demand for accuracy and speed of response. Since the appropriate specification of the product or technology is a function of the load requirement, then bandwidth and dynamics response of the load should be characterized early in the design of equipment to prevent mistakes from being made. You can spend way too much for too much technology, or you can end up building a piece of equipment that is incapable of meeting the performance requirement. Both situations are bad, the latter is worse.
More to come.
Filed Under: Mechatronic Tips