One important decision you have to make when buying or building a CNC machine is to choose the type of CNC motors that will control your machine. Namely, you have to choose between stepper motors and servo motors.
Servo Motors and Stepper Motors: What Are They for Anyway?
Elevators, robots, conveyors, printers, hard disks, and not surprisingly CNC machines all need position control.
Most industrial motors can be used to achieve speed control. But in order to achieve position control, servo motors and stepper motors are the only types that can be used.
Why Does It Matter Whether You Choose CNC Servo Motors or Stepper Motors?
Not knowing whether your needs are easily met by stepper motors puts you in a tough position. You can unnecessarily cough up money for a servo system that brings abilities that you never use.
It is like buying a Lamborghini and only ever driving it under 20 MPH.
Then again, if you only focus on saving money, you may choose CNC stepper motors only to find out that for your purposes, they are unable to deliver the speed and reliability that you need.
In short, you need to take in mind four considerations:
- Torque and Speed
We will consider torque and speed together because torque depends on speed.
In a word, stepper motors are cheaper and simpler to control with relatively good reliability. On the other hand, servo motors can give you higher speed with perfect reliability.
Read on to get more details about each of these considerations.
Stepper Motors vs. Servo Motors: Torque and Speed Characteristics
Torque is a measure of how powerfully your motor shaft can turn; which usually depends on speed. Think of it as an indicator of how much weight your motor can lift.
Now, the faster a stepper motor turns, the less torque it can provide.
Often, at around 1000 RPM, your stepper motor can only give around half of its maximum torque; which as you guessed it, is when it is not moving at all.
This is called your stepper motor’s holding torque and that is what is advertised to you. When your motor is supposed to stand still, holding torque is the measure of how powerfully your motor can resist external forces trying to turn it.
All in all, you can never say how much torque a stepper motor will give you; you can only say how much torque it can offer at a given speed by looking at its diagram.
Since at high speeds, your stepper motor doesn’t have enough muscle to push your bit, it will miss steps and cut shorter than you expect.
On the other hand, servo motors will give almost a constant amount of torque up to several thousand RPM. So, you can say how much torque a servo motor has and you will be right for a wide range of speeds.
Servo motors eventually start to weaken above some speed, but their torque fades much slower than stepper motors. Some industrial servo motors can move heavy loads at 20000 RPM as if they are lifting a finger.
- Stepper motors are usually used with ball screws or lead screws. That is because screws will enhance torque while sacrificing speed.
- Servo motors are often used along with rack and pinion.
- Stepper motors are more suitable for smaller CNC machines doing lighter jobs at a lower speed.
- Large CNC machines and those working fast with tough materials are better off with servo motors.
Stepper Motors vs Servo Motors: Cost
Stepper motors are both cheaper and easier to control.
So, if you do not need that high-speed high-torque system, you better save money with stepper motors. Plus, if you are building a DIY CNC machine, stepper motors are far easier to implement.
What makes servo motors more expensive is not the motor itself; rather it is the whole entourage of sidekicks that the motor needs to be controlled. You will find more information about this under the next item.
Complexity: Controlling Stepper Motors Is Much Simpler Than Servo Motors
Imagine you want to hang a picture frame on your wall and you want it to be perfectly level. You will have someone stand back and check how you are doing as you are hanging the frame.
You are not accurate on your own, but you rely on someone else to give you feedback on your errors so you can correct them. This is what we call a “closed-loop control system” in engineering.
On the other hand, if someone has already calculated and outlined where the picture frame should hang, you can forget about any feedback. All you need to do is hang the frame in its place.
You do not need to check your work and use any feedback for your decision as you are hanging the frame. We call this approach an “open-loop control system.”
We use open-loop control when we are confident that our calculations are accurate, or when we do not need perfect reliability. That rids us of implementing a control system that monitors whether everything has turned out as we had expected.
Servo motors must be controlled with a closed-loop control system; while stepper motors can be controlled with just open-loop commands.
The reason is that stepper motors move exactly as commanded in discrete steps. You can give the same command to all the stepper motors on earth and they will react the same (provided that they have the same steps-per-revolution).
On the other hand, two exactly similar servo motors will react differently to the same command; so, they require closed-loop control.
The necessity of closed-loop control demands more devices to control servo motors. For example, they need encoders to sense shaft position and speed and send that feedback to the controller.
There are analog to digital converters, either in the encoder or the controller. This translates to added cost and complexity.
Reliability of Servo Motors vs. Stepper Motors
Here is a puzzle for you: which one do you think is more reliable, a stepper motor or a servo motor?
Sorry, this was a misleading question. Because what matters is that closed-loop control is much more reliable than open-loop control.
So, a closed-loop system with a servo motor is more reliable than an open-loop system with a stepper motor.
Interestingly, on their own, stepper motors are more accurate. That is why we can use them even with open-loop control. But a servo motor equipped with closed-loop control brings perfect reliability.
This reliability is a function of the control system that constantly monitors your operation and knows when something is wrong.
Imagine something gets jammed in your CNC machine and your stepper motor is missing steps. But with open-loop control, your oblivious controller keeps driving your stepper motors like a drunk maniac.
At times, there will be slips. Like when you have a dull bit or you are at a very high speed and your stepper motor does not have enough muscle to push the bit into your piece and it has not cut to the supposed place.
But your ignorant controller is like “everything is fine. I will never change my plans since what happens out there is none of my business.”
This is where you start earning back your money with servo motors. You will never need to throw away faulty productions.
Since a closed-loop control system monitors the situation constantly and moves your CNC machine based on the results, not calculations.
Additionally, it can react in case of accidents like when something gets jammed.
The Final Result
In summary, stepper motors and servo motors can both present position control for your CNC machine. Understanding them will allow you to manage your expectations and decide on the right choice.
You need to weigh the four considerations of cost, torque/speed characteristics, complexity, and reliability. Controlling stepper motors is easier and cheaper, while servo motors have better torque at higher speeds with the amazing reliability of closed-loop control.
So, which one will you choose for your CNC machine?
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