I realised it might be more helpful if I explained my views on steppers, servos, open and closed loop control. These are just my views, I’m sure lots of people won’t agree or will point out key things I’ve missed
The dark ages
Back in the dark ages before cheap power electronics we used three phase or commutated motors to drive stuff that needed to move. They all have advantages & disadvantages but are very very bad at controlling position or accurately modulating speed. Some of these motor types exhibit basic feedback properties, for example the three phase spindle motors we run automatically adjust their current draw and torque based on how much load we put on the shaft (this happens as we slow the shaft down relative to the drive frequency). None of them has useful positional control though.
Stepper motors came along and were initially very expensive and not very strong, but we used them in machines, progressively more generally until they started to dominate in things like dot-matrix printers. Stepper motors were great in that we could achieve some basic positional control without needing expensive position sensors to feed-back the actual position. Steppers were bad in that we just have to believe they are where we asked them to be. Also they have a torque / speed curve that means they don’t go fast very well and making them accellerate things quickly without losing steps is tricky. Steppers also mostly sit there at full power to do nothing and stay still, if you turn off the power they flop around so you spend power to do nothing.
Standard stepper controls are “open loop” in that there is no sensing of the position of the motor or the thing being driven to feed back to the controller. So no adjustment of the commands to the motor based on what really happens. You ask, the controller asks, the stepper driver asks, the motor tries, you hope it all worked.
Then power electronics got a lot cheaper and volume manufacturing of stepper motors increased to the point where they became so cheap that their compromises and issues were outweighed by their price advantage, a perfectly usable stepper driver is $0.50 now and the motors are also cheap. This is arguably what enabled things like the Shapeoko for people without a Haas budget.
We have cheap motors with basic positional control (once homed after power up, our one bit of positional feedback control) and reasonable ability to hold position. With even cheaper driver electronics for commodity sizes and we’ve sort of got stuck in a rut because they were so ubiquitous and cheap. We sticky-tape over the issues by either selecting much bigger motors than we really need, which further limits the speed, or by limiting the rates of accelleration and max speed in our control software, it takes quite a bit of tuning to get this right.
Cheap position encoders - closed loop steppers
With the advent of relatively cheap rotary and linear position encoders (see the DRO) stepper motor controllers can now check whether the motor actually did what it was asked to do or not. This means they can, at least, stop and throw an alarm if they have missed steps, instead of continuing to trash your workpiece. More advanced closed loop stepper drivers can modify the drive current on the fly using the feedback, achieve better top speeds and accellerations etc. These are still quite limited in terms of their real control abilities though. There’s no reason they can’t do the magic stuff of servos, but economics may rule against them.
In the other corner we have the servo motor, commonly found in radio control toys doing things like steering. We send this a signal to say “23 degrees left of center” and the electronics inside the box check the position and then drive the cheap DC motor until it approaches that requested position. If you turn them away from position they’ll fight you to get back to it. This is “closed loop” control where the controller can sense the output condition and use that to modify the instructions to the motor. These little RC servos are cheap and not very powerful, but pretty reliable and if you knock them out of position they’ll try to get back.
The problem with closed loop control systems is math(s). There is some pretty well understood maths about how to build and tune a closed loop system so that it behaves reasonably well, for a given combination of controller, motor, drive system and load condition. In RC servos this exists as a “PID” controller, this uses a tunable combination of the accelleration, speed and absolute position to choose how to accellerate and decellerate the motor to effectively reach the target without overshoot, oscillation and other problems. RC nerds get quite excited about PID tuning.
In the modern servo motor world, where people like ClearPath have come along and made it consumer priced, what we have are big, powerful, cheap motors coupled up with high precision, relatively cheap position sensors, some smart drive electronics and the magic sauce. The magic is the controls tuning to make the whole thing usable instead of just a way to shake your machine apart.
What we get is a motor that can go fast, have huge holding torque, apply very large peak power when required, because it only needs to do that for peak, not continuously, maintain very precise speeds and positions and frequently integrates the driver and motor packages.
The bit that I’m impressed with is the advent of the “self tuning” servo motor, this is, to my view, the clever innovation for ClearPath and others. Instead of finding an engineer who studied enough control theory to build up a model of your system and calculate tuning parameters for your control loop they’ve managed to write smart software that uses the motor / encoder to test move the equipment as assembled, measure the results and come up with an “auto tune” for the installed system. This makes it possible for normal people like us to use closed-loop servo control on big heavy bits of equipment.
I also think that ClearPath’s hosting of their tuning smarts in the cloud is a very smart move for a long list of reasons.