Why Motor Moves When Powered Up?
There are cases when a servo system is powered up, the motor starts to move undesirably. In some cases, it is just a jerk. In more serious cases, it hits a mechanical limit. This unexpected motion is totally undesirable in many applications, especially when safety is a concern.
One of the probable causes of this scenario is sharing of power supply between the controller and the motor amplifier, without any form of mechanism to inhabit the motor amplifier. When the power is active, both controller and amplifier are powered up at the same time. In many cases, the motion controller takes longer to initialise as compared to the motor amplifier, depending on the architecture. Without specific cares taken into the design to inhabit the motor amplifier while the controller is still initialising, the amplifier may be driving the motor undesirably until the controller takes over the control. This is most obvious if the motion controller is, for example a PCI card, plugged in a PC and the PC takes relatively long time to boot up and runs the controller.
There are a number of ways of installing motion controllers and motor amplifiers to ensure the above-mentioned problem will not surface.
Provide A Power-up Scheme
Separate the power supplies for controllers and motor amplifiers, and provide a power-up scheme. If you are certain that your controller is going to take relatively longer time to boot and initialise, as compared to your motor amplifiers, let your controller turns on the power supplies to your motor amplifiers after it has successfully initialised and takes control of the motion system. An example of such scheme is shown in Figure 1 below.
Figure 1: Separate power supplies for controllers and amplifier
During system power-up, the brushless servo amplifier is denied of V+ until the micro-controller turns it on. This design will ensure no energy flows into the servo motor unintentionally. Also note that the ground of the amplifier is separated from the rest of the digital ground for better noise immunity.
Use The Inhabit Input
Choose and use the inhabit pin that comes with some motor amplifiers. The default state of the inhabit pin must always be active to inhabit the motor amplifier during power-up. It will only be deactivated explicitly by the controller after initialisation. An example is shown in Figure 2 below.
Figure 2: Disable the amplifier using the Inhabit feature
The key message here is: do not leave things to chance. Have everything under control, even during power-up phase. It is a good practice to ensure all the lines in your circuits are at a deterministic state at all time. Not leaving these lines to "float" will save you all the nightmares.