Steppers and servos can be used with Bimba OLE actuators. They are the best motor technology for position control. The choice of which one to use depends on speed, torque, price and often delivery time. The following discussion explains the differences between the two technologies.
Stepper Motor Design
Stepper motors are called "digital motors" because they move in steps, like the hands on a clock. When the first coil is energized, the rotor teeth align with the teeth in the first stator winding and hold position. When the second winding is energized, the teeth in the rotor move slightly and align with the second stator winding and hold position. The total movement in this example is one complete step.
Typically there are 200 steps per revolution at 1.8° per step. The development of electronic stepper motor controllers has made stepper motors possible. Electronic devices need to power the windings at the right time with the right voltage and current, in the right phase, and in the right order. Controllers have evolved to be able to move stepper motors at up to 20,000 steps per revolution, providing 100 times finer motion (0.018° per step).
Stepper Motor System Components
In order to understand the performance of stepper motors, it is helpful to understand the components of a stepper motor based system.
Motor and Actuator: Consider the motor and actuator together as the equivalent of an arm. It will not move without power.
- Output: The thrust and speed of the moving load.
- Input: Power from the stepper driver.
Actuator: The device that provides the muscle to move the arm is called an actuator.
- Output: The motor windings are supplied with power in the correct amount and in the correct sequence to move the actuator "arm" in the desired direction. Actuators require a power source, such as a DC power supply. The voltage output of the DC power supply is transmitted directly to the motor windings through the actuator. The current and voltage across the motor windings is proportional to the torque output of the motor and therefore proportional to the thrust output of the actuator. In general, the higher the voltage and current, the higher the thrust. Above a certain point, an increase in current will only increase the motor winding temperature, not the torque output.
- INPUT: Accepts stepper pulses and direction commands from the controller. Each electronic stepping pulse changes the way the motor windings are energized, which in turn causes the motor to turn one step, which moves the actuator.
Controller: think of the controller as the brain that directs the muscles.
- Outputs: Step and direction pulses are provided to the drive. Various outputs are also provided to the PLC, including communications over buses (RS232, RS485, Ethernet IP, Profibus, etc.), in-place signals, alarms, and fault outputs.
- Inputs: Intelligent and systematic management of inputs from various sources.
Magnetic switches
Sensors elsewhere in the machine
Programmable controller outputs
Bus communications from PLCs
Encoder signals from motors
Encoder: The encoder is the "eye" of the system. It tells the controller if its commands have been executed. The controller compares the stepper pulses it sends to the drive with the pulses it receives from the encoder. If the pulses it sends are equal to the pulses it receives, the controller knows its command has been executed. If they are not equal, the controller adjusts the motor until it receives the correct number of encoder pulses.
For ease of discussion, the system block diagram conveniently separates all the functional components. It is common for manufacturers to combine functions. For example, controllers often have built-in drives and some have built-in power supplies. Some motors have built-in encoders, and some have built-in drives and controllers. Some PLCs have built-in controllers.
Stepper motor performance
Stepper motors have an inherent advantage:
- Stepper motors are the least expensive position control motor technology.
- Stepper motors can be used in open loop without the need for an encoder.
- Stepper motors provide very high torque at low speeds.
- NEMA frame sizes are available with standardized mounting and shaft diameters.
Stepper motors have inherent disadvantages:
- While stepper motors provide high torque at low speeds, the torque output decreases rapidly with increasing speed.
- Stepper motors can become desynchronized; that is, out of sync with the stepper pulses coming from the controller. In other words, the stepper pulses from the controller are converted into electrical energy for the motor's windings, but the motor does not rotate. This happens when the torque required to move a load exceeds the motor's torque capacity at the desired speed. To avoid this problem:
- Increase the motor size. Size the motor to provide double the maximum required torque.
- Reduce the speed to the point where the motor torque is sufficient.
- Increase the voltage to the motor windings (large power supply).
- Increase the current to the motor windings (large power supply).
- Use an encoder to monitor position and correct missed steps.
- Difficult to synchronize because error correction is done at the end of the stroke, not continuously in real time.
Servo Motor Design
By definition, a servo is any device with feedback. Servomotors have coils on the stator that are energized by a servo driver or amplifier in a sequence that causes the magnetic rotor to rotate (spin). In order for the controller to determine which winding to energize next, it must know the current position of the rotor. This sensing is accomplished using Hall sensors or dual function encoders.
Servo Motor System Components
The block diagram of a servo system is the same as that of a stepper motor system, although the components are technically different. Servo amplifiers are functionally equivalent to stepper motor drivers. As with stepper motors, functions are usually combined by the manufacturer.
Servo Motor Performance
Servos have an inherent advantage:
- High speed
- High acceleration
- High accuracy
- Higher torque at higher speeds
- Multiple motors can be easily synchronized (real-time error correction)
Servos have inherent disadvantages:
- More complex system than stepper motors
- Motor must be tuned for optimum performance
- More expensive system than stepper motors
- Less torque at lowest speeds than stepper motors
- Non-standard mounting geometry (varies by manufacturer) and shaft diameter
- Less torque at low speeds than stepper motors of equivalent size
Implications for use with electric actuators
Whichever technology is used, it must be matched to the actuator design.
- A low-speed servo may not achieve the same low-speed actuator performance as a stepper motor.
- Replacing a stepper motor with a servo on the same actuator will not necessarily produce more thrust at higher speeds, as other actuator components may limit performance.
Depending on the application, there are advantages to each of the two motor technologies. The following table can be used as a guide.
| stepper motor | servo motor | Meaning of actuator |
|---|---|---|
| Provides more torque at low speeds | Provides more torque at low speeds | Special actuator assemblies may be required that including couplers, screws and nuts. |
| Not that expensive. | more expensive | Steppers minimize costs. |
| Can be synchronized with medium precision | Can be synchronized with high precision | Servos are preferred for multi-axis applications. Special controllers are required. |
| End-of-trip error correction | Instant error correction | Servos are preferred for precise positioning. A backlash nut may be required to improve accuracy. |
| No encoder required | Encoder required | |
| simple system | complex system | |
| May be out of sync. | It's impossible to lose sync. | |
| No adjustment required | Adjustments are critical | If the servo is not properly adjusted, the performance of the servo actuator may be seriously affected. |
| Standard sizes for easy installation and interchangeability | Lack of dimensional standardization poses a challenge | Before deciding on servo compatibility, make sure the servo will be mounted to the actuator. Special motor mounts and couplers may be required. |