ggom, (wish I knew your name - please fill out your personal profile so I can refer to you by at least your first name), what kind of motor are you talking about?
Even a 3 phase motor can be used in a servo system if the right electronics is used to drive it. A single phase AC motor will not work, because it has a centrifugal switch to switch in and out a start winding at a certain speed. The 2 phase will work because you can vary the phases to cause slight positional movements in rotation, with the right electronics to drive it. Most any DC is the commonly used type of motor for positioning type servo work because it easily reverses by swapping the polarity of the power to it The key here is the ability of the motor to be reversed easily and that does not have a starting winding. A 3 phase motor can be used if the right electronics are used to drive it. 3 phase motors are a great choice of rotating needs that require speed control without stopped positioning. These are rate servos and frequently used used in applications similar to lathes and drill presses.
When trying to use a stepping motor as well as an encoder, it's difficult to get the two set so that the encoder increments match the stepper motor increments. You will frequently not be able to stop the motor on a desired single count position of the encoder if they are not matched. The encoder will say it is not on position, but one more step of the stepper and it will be off position in the other direction. I've seen engineers try to use them together and it has never worked out well, unless a position that satisfies the encoder is several steps of the stepper motor in width, but this usually does not satisfy the desired system accuracy.
A linear motor will not have this problem if the servo amplifier (motor driver) has the gain setting high enough for it to stop within the one count of the encoder position.
If an incremental encoder is used with count up/count down electronics, the encoder will be much less expensive, but the system will need to go to a home position in order to establish a zero position, and then work from there, but a linear servo motor is the preferred choice whenever using an encoder because of the above problem.
Using stepper motors and no encoder will require that the stepper motor never miss a step. If it does, your parts will not be made correctly until the system is again sent back to home to re-establish zero. Missed steps occur if there is any sudden change in friction of the system or load, or the stepping rate is close to the maximum that the stepper and driver electronics can handle reliably. The faster that the motor is stepped, the less power it will have to prevent missed steps. It's just an electronic version of a ratchet wheel.
In any CNC machine, going to home on power up or when restarting a part mid-way through the program always needs software and/or hardware that raises the Z axis to home before moving the X and Y axis to home. If this function is not included, tool crashes will be frequent. If additional axis of movements are installed, they may also need to be homed before moving Z or X and Y to home.
I used to add a hardware button for the operator to push whenever access to robot stations was desired. This button would stop the robot program and record the last position in the program that the robot had completed and send the robot to a designated "safe position" with an included sequence to retract Z and anything else that was needed before it moved there. This position had a sensor that the robot end of arm tooling would activate. While this sensor was activated, a light indicating to the operator that it was "safe to enter" would light and the access door through the safety curtain could be opened for the operator to correct the problem, then close the door and push another button to continue the program from where it left off, including a sequence for the safe return to that point.
If the robot somehow moved off of this "safe position" sensor while the access door was open, the whole robot station would power off, and a complete program restart would be required, including the software/hardware sequence to move back to home safely. This usually also required scrapping, or hand finishing the part that the robot had been working on.
In any location with sudden power down conditions, you will need the program to be written to record each completed step somehow in a non-volatile memory, so that when power is returned and the tool has gone to home, the program can be restarted from the last completed position, in the program, usually repeating the work that may have been in process in the partially completed next step of the program. If the program is not written this way It will be necessary to start the program from the beginning, which will consume considerable additional time, and may also require scrapping the partially completed part, or possibly removing it to be completed manually. Getting the CNC station and the operator back in sync with each other is something that must be conveyed during operator training, and may need to be different for each program and part that is run. It cannot be left to chance.
I think this is enough education for one day. It's tough to get your head to absorb a lot of this all at once, especially when trying to do it without a "hands on" lab.
Please ask questions. The only dumb question is the one not asked.
Charley