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Servo or Stepper?

Oct
01
by Mike Fahey

A question often asked is this: which is better, servo or stepper? Perhaps a more precise question is: for which applications are servos the best choice and for which are steppers the best choice? As with many things, the answer is not always simple and there are many factors to consider. The common answer is: servos are better for high speed applications and steppers are better for low speed. Then there is also the standard reply that servos are closed loop, so if precision and certainty of position is critical they are preferable to steppers.

With advances in technology, the choices have become less distinct. Today, Applied Motion offers both servo and stepper solutions, because the optimal choice is application driven. While all servos by definition are closed loop, we offer steppers in both open loop and closed loop versions. So the need for closed loop no longer dictates a “true” servo be used.

A good place to start when selecting a motor - whether servo or stepper - is to determine the motor’s speed, torque, and resolution requirements for the application. For positioning applications, to determine motor shaft speed, one could start with the time allowed to make the position move, and then work back through the mechanism to determine what that dictactes in terms of the motor move profile. Selections made for the mechanism will also directly influence the torque and resolution requirements at the motor shaft.

Let’s consider an example to illustrate some of the choices involved. It is common to see both stepper motors and servo motors used in conjuction with lead screw and ball screw type linear actuators. In fact lead screw actuators are often the mechanism of choice for the x,y,z gantries that are used in many 3D printers. So lets focus on the x axis of our imaginary 3D printer.

Lets make some assumptions:

1. Our parts are no bigger than 5 inches diameter

2. We need resolution of 0.001 inch

3. Time to make a 5 inch move should be less than 1 second

Assume a lead screw with a 1.0 inch lead, it would take 5 revolutions to travel the 5 inch move distance. The chart below taken from our Q programming editor software shows the move profile assumptions.

Based on the 1.0 inch lead assumption, we can derive the motor shaft accel/decel and max speed parameters needed to meet the 5 inch in less than 1 second requirement. Also, if we assume the stepper is set for 8,000 steps per revolution, the linear resolution will be 0.000125 inch per step, much finer than the 0.001 requested.

Now lets get back to the stepper vs servo comparison. Looking at the speed torque curve of Applied Motion Products Integrated Stepper Motor pn STM24IP-3EE at 12 RPS, we have a maximum 225 oz in of torque available when in stall prevent mode.

Brand A SM23375DT is the highest torque size 23 integrated servo motor. It has a peak torque of 157 oz inch and continuous torque of 83 oz inch.

 

In conclusion, the "servos are for high speed and steppers are for low speed" rule of thumb is based on realistic assumptions. The example discussed here hopefully illustrates the importance of quantifying what is low speed and what is high speed.

Applied Motion STM24 series integrated motors are available in model types that include:

RS232/RS485

Modbus RTU

Modbus TCP

Ethernet IP

Applied Motion SCL over Ethernet

CanOpen

IP65

They have non volatile memory, can be programmed for host control or standalone operation, and are compatible with most PLC’s such as Allen Bradley, Panasonic, Mitsubishi, Omron, Automation Direct and many others. They are also compatible with most HMI’s including Proface, Red Lion, Maple Systems, and EZ automation.

STM24 series are easy to configure (no tuning) and are a great solution for multiple axis distributed motion control. Contact Applied Motion Products and we will help you discover all the ways the STM24 series can be used to satisfy your application requirements. We can help you find the best solution for any application, whether servo or stepper.

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