Hey Jim, hey all,
I have no experience with the Masso controller so I don’t know which type of Emergency Stop it provides.
But the manufacturer of a machine (an entire machine, not the manufacturer of only a machine controller), in order to provide safety, has to implement the emergency stop type that is suited to meet the hazards that can occur during the operation of the machine. Such emergency stop standards are defined in IEC 60204-1. The updated version distinguishes between emergency stop and emergency power off. And cutting electricity from a device is only recommended when it is necessary.
There exists different stop categories. The most known is STO (Safe Torque Off). This is a category 0 emergency stop and it means that the power is cut from a motor so it immediately can no more exert any torque. VFDs are usually certified to provide the safety of STO when their STOP command is active – which is the reason why it is safe to change the bit in the collet with the VFD just in STOP mode.
But when you have STO as emergency stop on your CNC machine and subsequently on your milling motor, this does not mean that the motor really stops to turn immediately. It just means that it gets its power cut. A high-speed induction motor (that has no brushes) like a spindle may only come to a standstill after minutes. STO also means that the brake function of the VFD is not used, the spindle is simply left coasting.
With the stepper motors of a CNC machine, the situation is even worse if you just have STO as emergency stop mode: Today stepper motors are hybrid motors which mean their permanent magnets are usually not strong enough to hold a mechanical load when their power is cut and they loose their electromagnetic detent. What makes a stepper hold its position (and therefore enables the machine to enter a safe mode with no more parts moving e.g. when the door of your CNC machine is opened) is the holding torque, which can only be present if the stepper motor is still controlled by the controller and is provided with electric current and controlled by the loop.
Therefore IEC 60204-1 defines many more stop categories. A Safe Stop 1 (SS1) is defined as first bringing the machine to a controlled standstill, and only then enter STO (Safe Torque Off), which means powerless mode in the end, which prevents any torque being exerted. It first stops all motion before it enters the power cut mode. However this is still not the right stop category for a stepper motor, since it is still left powerless and may passively start to move by a load through gravity. Think of a robot arm, or a crane with a load on it. You don’t want it to let go of everything in case of an emergency, as this would be much too dangerous, just as you don’t want your heavy spindle to come down when emergency stop is pressed and makes a nice hole into your table or breaks the bit. It is clear that a stepper motor that receives no more power can not exert any torque, but it is also clear that because it looses its electromagnetic detent, it can start to passively move by the weight of a load attached to it, just like the load on a crane or, when thinking of a robot arm, by its own weight. Therefore you would need to have a Safe Stop 2 (SS2) mode, which brings the stepper motors to a controlled standstill and then maintains the power and the control by the controller in order to provide the holding torque to ensure the machine will not move anymore.
There are even more safety stop modes defined in IEC 60204-1. If someone is interested in the topic, search for “stop category” and “60204-1” in a WWW search engine to read more about it.
So when you design a machine that has moving parts, you have to choose the correct emergency stop category by thinking of all types of hazard that could occur.