What is a Stepper Actuator?
A stepper actuator is a mechanical device which produces force, as well as motion along a straight path. A stepper actuator uses the core principles of a stepper motor, with some slight modifications. With the stepper actuator, the shaft of a normal stepper motor is replaced with a precision lead screw, and the rotor is tapped to convert it to a precision nut that is adjusted to the lead screw. As the rotor rotates, the lead screw rotates up and down the precision nut, allowing for linear motion. Minimizing outside mechanical systems to convert rotary into linear motion, greatly simplifies rotary to linear applications. The stepper actuator design allows for high resolution and accuracy, while minimizing extra mechanical components.
Block Diagram of a Stepper Motor System
Physical Properties of a Stepper Actuator
The physical properties of stepper actuators are made up of the same core properties of a stepper motor, with some modifications. The shaft of a normal stepper motor is replaced with a precision lead screw and the rotor is tapped precision nut that interacts with the lead screw to allow for linear motion. The stator and rotor laminations are comprised of silicon steel which allows for a higher electrical resistivity and lower core loss. There are a variety of magnets used: ferrite plastic, ferrite sintered and Nd-Fe-B (neodymium magnet).
Figure 2: Physical components of a PM stepper actuator with a threaded shaft and a mounting plate.
Figure 3: Illustration of the threaded shaft with the pitch and lead.
How do Stepper Actuators Work?
A stepper actuator is driven by a stepper motor driver and/or controller, which provides the instructions to manipulate the stepper actuator to start or stop. The driver and/or controller sends the proper signal pulses to the windings of the stepper actuator, causing the rotor (precision nut) to rotate and the lead screw to extend or retract. By the use of instructions, a stepper motor controller designates how far and how fast the stepper actuator should extend or retract. A controller can be pre-programmed or controlled in real time by inputs predefined on the stepper drive or controller.
How are Stepper Actuators Controlled?
Stepper actuator motors are typically controlled by a driver and controller. The amount, speed, and direction of rotation of stepper actuator motors are determined by the configuration of digital control devices. The main types of control devices for stepper actuator motors are: stepper actuator motors control links and stepper actuator controllers. The stepper driver accepts the clock pulses and direction signals and translates these signals into appropriate phase currents for the stepper actuator. The stepper controller creates the clock pulses and the direction signals for the Actuator Motors. The computer or PLC (Programmable Logic Controller) sends out commands to the controller. An HMI can also be added for user-friendly applications.
There are three basic types of stepper actuators. The stepper actuator motor types vary by their construction and in how they function. Each type of stepper actuator offers a solution to an application, in different ways. The three basic types of stepper actuators include the Variable Reluctance, Permanent Magnet and Hybrid Actuators.
Variable Reluctance (VR) Stepper Actuators
VR stepper actuators are known for having soft iron multiple rotors and a wound stator. The VR stepper actuators generally operate in step angles from 5 to 15 degrees, at relatively high step rates. They also possess no detent torque. In Figure 5, when phase A is energized, four rotor teeth line up with the four stator teeth of phase A by magnetic attraction. The next step is taken when A is turned off and phase B is energized, rotating the rotor clockwise 15 degrees; Continuing the sequence, C is turned on next and then A again. Counterclockwise rotation is achieved when the phase order is reversed.
Permanent Magnet (PM) Stepper Actuators
Permanent Magnet Stepper Actuators differ from variable reluctance stepper actuators by having permanent magnet rotors with no teeth. These rotors are magnetized perpendicular to the axis. When the four phases are energized in sequence, the rotor rotates as it is attracted to the magnetic poles. The motor shown in Figure 6 will take 90 degree steps as the windings are energized in sequence ABCD. Permanent magnet stepper actuators generally has step angles of 45 to 90 degrees and tends to step at relatively low rates, but produce high torque and excellent damping characteristics.
Hybrid Stepper Actuators
Hybrid Stepper Actuators combine qualities from the both permanent magnet and variable reluctance stepper actuator motors. The Hybrid stepper actuator motors have some of the desirable features of each. These stepper actuator motors have a high detent torque, excellent holding and dynamic torque, and they can operate in high step speeds. Step angles of 0.9 to 5.0 degrees are normally seen in hybrid stepper actuator motors. Bipolar windings are generally supplied to these stepper actuator motors, so a single power supply can be used to power the stepper actuator motors. The rotor will rotate in increments of 1.8 degrees, if the phases are energized one at a time in the order they are indicated at. These Actuator Motors can be driven in two phases at a time to yield more torque. Hybrid stepper actuator motors can also be driven by one, then two, then one phase to produce half-steps of 0.9 degree increments. Typically hybrid stepper actuators are more costly due to the combined characteristics.
How to Select a Stepper Actuator
In determining the right stepper actuator for a specific application it is necessary to understand stroke length. The stroke length allows the device to handle short or long operational distances. Most, but not all, stepper actuators have a stroke length that allows the actuator to move a specific amount. For example, a small actuator could move a curtain for a window, while a large actuator could move a curtain for a movie screen. As a result, a longer stroke length will increase the price of a linear actuator. Extra length is rarely required, so users should select stepper actuators that exactly fit the application.
Understanding the duty cycle, the elapsed time between operations, will provide a reasonable approximation of the expected lifetime of a stepper actuator. The duty cycle can be based on units of hours per day, minutes per hour, or strokes per minute. By managing the duty cycle one can increase the lifetime and the necessary maintenance required for the stepper actuator.
One of the criteria for selecting a stepper actuator is to identify the amount of force required of the application. Defining the load required for the application can help identify the proper size and capabilities of the stepper actuator. The orientation also plays a key role in selecting the proper stepper actuator.
IMPORTANT NOTE: With a stepper actuator in a horizontal position, the overall load capabilities must compensate for the frictional force. With a stepper actuator in a vertical position, the load necessary is that of the weight due to the gravitational force.
Mechanical Power: The necessary requirement in calculating mechanical power is based on the linear force required to move the load multiplied by the speed at which the load will be moved.
Electrical Power: The electrical power is obtained through performance graphs illustrating force vs. speed and force vs. current, both of which are good representations of the performance of a stepper linear actuator. The performance graphs are specific to each specific stepper actuator model.
Stepper Actuator Modes
There are three excitation modes that are commonly used with stepper actuator motors: full-step, half-step and microstep. The following will explain the basics of the different modes; for more information, see Anaheim Automation's videos on Microstepping and the Full-Step vs Half-Step Excitation.
Stepper Actuator - Full-Step
In full step operation, stepper actuator motors step through the normal step angle e.g. 200 step/revolution motors take 1.8 steps while in half step operation, 0.9 steps are taken. There are two kinds of full-step modes. Single-phase full-step excitation, where stepper actuator motors are operated with only one phase energized at-a-time. This mode should only be used where torque and speed performance are not important, e.g. where the motor is operated at a fixed speed and load conditions are well defined. Problems with resonance can prohibit operation at some speeds. This type of mode requires the least amount of power from the drive power supply of any of the excitation modes. Dual phase full-step excitation is where the stepper actuator motors are operated with two phases energized at-a-time. This mode provides good torque and speed performance with a minimum of resonance problems. Dual excitation, provides about 30 to 40 percent more torque than single excitation, but does require twice the power from the drive power supply.
Stepper Actuator - Half-Step
Stepper actuator motors have half-step excitation, alternate single and dual-phase operation resulting in steps one half the normal step size. This half-step mode provides twice the resolution. While the motor torque output varies on alternate steps, this is more than offset by the need to step through only half the angle. This mode has become a commonly used mode by Anaheim Automation because it offers almost complete freedom from resonance problems, and is cost-effective. Stepper actuator motors can be operated over a wide range of speeds and used to drive almost any load commonly encountered.
Stepper Actuator - Micro-Step
In Actuator Motors microstep mode, a stepper actuator motor's natural step angle can be divided into much smaller angles. For example, a standard 1.8 degree motor has 200 steps-per-revolution. If the motor is microstepped with a 'divide-by-10', then each microstep would move the motor 0.18 degrees and there would be 2,000 steps-per-revolution. Typically, micro-step modes range from divide-by-2 to divide-by-256 (51,200 steps per revolution for a 1.8 degree motor). The micro-steps are produced by proportioning the current in the two windings according to sine and cosine functions. This mode is only used where smoother motion or more resolution is required. Generally, the greater the divisor, the more costly the microstep driver will be. Also, some microstep drivers have a fixed divisor (least costly) while other have a selectable divisor (most costly).
Advantages of Stepper Actuators
|• Minimize mechanical components|
|• Cost-effective design|
|• High resolution and accuracy|
|• High force output|
|• Long stroke lengths|
|• Many options in construction and functionality available|
Disadvantages of Stepper Actuators
|• Parts prone to wear|
|Mechanical||- Inexpensive, cost-effective
- No power source needed
- Self contained
- Identical behavior extending or retracting
|- Manual operation only
- No automation
|Electro-mechanical||- Inexpensive, cost-effective
- Position feedback
- Identical behavior extending or retracting
|- Many moving parts which are prone to wear|
|Linear Motor||- Simple design
- High speeds
- Self contained
- Identical behavior extending or retracting
|- Low force|
|Piezoelectric||-Very small motions possible
- Good in compression only
|- Requires position feedback for repeatability
- Low speeds
- High voltages necessary
|Hydraulic||- Very high forces possible
- Moderate speed levels
- High accelerations
- High shock loads
- High stiffness
|- Can leak
- Requires position feedback for repeatability
- External hydraulic pump necessary
- High maintenance
|Pneumatic||- Low stiffness
- High shock loads
- High acceleration
- High speeds
|- Precise position control impossible except at full stops
- High maintenance
Applications of a Stepper Actuator
Although stepper actuators have been overshadowed in the past by servo systems for motion control, in many applications it has emerged as the preferred technology. The major contributing factor in this trend is the prevalence of digital control, and improved microprocessor.
With the increased demand for smaller, low cost machines, stepper actuators have been designed to reduce size and costs of motion control systems. Stepper actuators applications are used in both commercial and domestic industries. Stepper actuators are used in printing and labeling, packaging, electronic manufacturing, medical industries, laboratory instrumentation, CNC machinery, robotics, X-Y tables, valve control, and many other applications requiring precise linear motion control. The ingenuity and further advances in digital technology from researchers will continue to extend the list of applications in which stepper actuators will be used.
Where are Stepper Actuators Used?
Figure 4: Applications of Stepper Actuators
Cost of a Stepper Actuator
The price of a stepper actuator varies and is typically affected by size, function, accuracy, stroke length and linear force. The type of stepper actuator can greatly increase in the price and vary from manufacturer to manufacturer. Below is a list of stepper actuators products offered by Anaheim Automation. A comprehensive overview, as well as 1 pc. and 50 pc. pricing is available at www.AnaheimAutomation.com for each of the offered types:
|• Hybrid Non-Captive Linear Actuator|
|• Hybrid Threaded Shaft Linear Actuator|
|• Permanent Magnet (PM) Non-Captive Actuator|
|• Permanent Magnet (PM) Captive Actuator|
Environmental Considerations for Stepper Actuators
The following environmental and safety considerations must be observed during all phases of operation, service and repair of a stepper linear actuator system. Failure to comply with these precautions violates safety standards of design, manufacture, and intended use of the product. Anaheim Automation, Inc. assumes no liability for the customer's failure to comply with these requirements. Even well built products, operated or installed improperly, can be hazardous. Safety precautions must be observed by the user with respect to the load and operating environment. The customer is responsible for proper selection, installation and operation of the products purchased from Anaheim Automation, Inc.
• Use caution when handling, testing, and adjusting during installation, set-up and operation
• Service should not be performed with power is applied
• Any exposed circuitry should be properly guarded or enclosed to prevent unauthorized human contact
and moisture with live circuitry
• All products should be securely mounted and adequately grounded
• Provide adequate air flow and heat dissipation
• Avoid operating in vibration and shock environments
• Do not operate in the presence of flammable gases, vapors, liquids or dust
Lifetime of a Stepper Actuator
The lifetime of a stepper actuator is dependent on several factors such as duty cycle, load and temperature. Duty cycle represents the elapsed time between operations of the stepper actuator in hours per day, minutes per hour or strokes per minute. By staying within the limitations of the load capacity the stepper actuator will help maintain its durability and lifetime. Exposing the stepper actuator to above ambient temperatures can greatly reduce the overall performance and life of the motor. By managing the duty cycle, load and temperature can maximize the lifetime of the stepper linear actuator motor. Always use a stepper actuator within the manufacturer's specifications.
Lifetime of a Stepper Actuator
Plinear = Distance needed to travel * Force = Force * Velocity (Nm/s)
Time to travel to distance X
Speedlinear = Distance needed to travel (in/sec)
Time to travel distance X
|Lead Screw Threads Per Inch:||10|
|Number of Starts:||2|
|Turns Per Inch:||5||10 Threads Per Inch ÷ 2 start, so it takes 5 revolutions to move 1 inch|
|IPM (Inches Per Minute):||200||= 1000 RPM ÷ 5 Revs Per Inch|
Stepper Actuator Glossary
Acme Screw – a rolled thread on a straight shaft designed to have contact with a nut on the faces of the thread and have 29° thread angles. These types of screws are used to withstand shock and vibrations in applications.
Axial Load – load where the center of gravity runs through the axis of the actuator screw.
Ball Screw – translates rotational motion to linear motion with little friction. A helical shaft provides tracks for ball bearings which act as a precision screw. They are highly efficient and are used for high loads and speeds.
Cycle – complete sequence of extension and retraction by the actuator.
Duty Cycle – percentage of time an actuator is in motion relative to total time.
Eccentric Load – load whose center of gravity does not go through the screw axis. These types of loads cause binding and shorten the lifetime of an actuator.
End of Stroke Limit Switches – end stroke limit switches allow for the full stroke of an actuator to be used and once the end of the stroke has been reached, will shut off the power.
Lead – the linear distance the screw travels in one revolution.
Load – amount of weight which can be moved by the actuator.
Linear Actuator – electromechanical system that converts rotational motion into linear movement.
Linear Step Increment – linear travel movement produced by the lead screw by each step of the rotor.
Peak Load – a momentary maximum load that the actuator can control.
Rotary Actuators – provide rotary output to position a load, turn a winch, or rotate a gear/ sprocket.
Screw – converts rotational motion into linear motion and rotational torque into linear force. A common screw is a cylindrical shaft with helical groves called threads.
Screw Pitch – distance between adjacent threads.
Screws Lead – linear distance the screw travels in one revolution.
Static Load – maximum load an actuator can hold when not in operation mode.
Stroke Length – the allowed linear distance the actuator can move.
Tension Load – the load that pulls an actuator along the axis of its screw.
Trapezoidal Screw – thread profile is of a trapezoid with a thread angle of 30°. They offer high strength, can be used for large loads, offer high accuracy. These types of screws are used to withstand shock and vibrations in applications.
Thrust – measurement of linear force.
Frequently Asked Questions
Does the lead screw rotate in a Non-Captive Linear Actuator?
No, non-captive linear actuators must be secured to a non-rotating unit. Once the unit is secure, the lead screw will actuate back and forth without rotating. Non-captive linear actuators are designed for longer stroke lengths.
My stepper motor/actuator appears to be getting extremely hot when in the holding position.
Is this a normal occurrence?
With rated current as holding current, the motor/actuator will have a 75 degree rise in temperature. This is normal operation temperature of the actuator. Actuators have use Class B insulation which allows the system to have a 130 degree temperature rating. To minimize the heat rise in the actuator reduce the holding current to 75 percent of the rated current.
What is the difference between Captive and Non-Captive Actuators?
Captive Actuators have a built in anti-rotational mechanism through the use of a splinted output shaft that allows it to extend and retract as a whole unit. These types of actuators are designed for shorter stroke lengths.
Non-Captive Actuators have a lead screw going through the motor without any stroke limitations but must be securely assembled on a unit which will not rotate. This allows the lead screw to extend and retract without rotating. These types of actuators are used for longer stroke lengths.
What is an External Linear Actuator?
External Linear Actuator uses a lead screw and nut combination that extends out from the motor. As the lead screw rotates, linear motion is created as the nut motions back and forth.
Figure 5: Non-captive and external shaft stepper actuators
What is the difference between static and dynamic load?
Static load, also referred to as holding load, is the force that can be applied to the actuator when it is in not in operation mode. Dynamic load is the force applied while the stepper actuator is in motion.
What are the common factors of failures for actuators?
Stepper actuator failures can be a result of improper loading, excessive duty cycle, exposure to harsh environments and failure to set limit switches.
What is Anti-Backlash?
Anti-backlash is typically produced due to the clearances between a screw and a mating nut. In applications where loads may be in either direction, backlash can result from these clearances creating unacceptable movement in the controlled mechanism as loads change. These applications are common in the paper, plastic, film, sheet metal forming processes, satellite, or other load-reversing applications. Such applications may be subjected to extreme vibrations. These vibrations can produce constant movement between the screw and lifting nut which can hammer the threads and cause premature wear.
Troubleshooting a Stepper Actuator
Problem: Stepper actuator is not moving
Solution: If the stepper actuator is not moving, please remove the power source first. Confirm with the wiring diagram that the stepper actuator is correctly assigned to the proper terminal block pins. If the wiring configuration is correct, check for loose cable wires from the terminal block of the driver. If there are any loose cable wires, please secure those wires to the terminal block. Next check if the stepper actuator is hot. If so, please refer to the ‘stepper actuator is getting hot' section below.
If the wire configuration is correct, no loose wiring is observed and the stepper actuator is not hot, the unit may be damaged.
Problem: Stepper actuator is getting hot
Solution: Excessive heat is caused by not enough air flow to the system. Installing a fan near the system or by moving the unit to an area with more air flow can help lower the overall temperature. Decreasing the duty cycle can help prevent the unit from overheating and causing it to malfunction.
Problem: Environmental factors are less than ideal.
Solution: Environmental factors such as welding, chemical vapors, moisture, humidity, dust, metal debris, etc., can damage the electronic components and the stepper actuator. Protect drivers, controllers and stepper actuators from environments that are corrosive, contain voltage spikes, or prevent good ventilation.
Problem: The stepper actuator has a shorted winding or a short to the motor case.
Solution: It is likely you have a defective stepper actuator. Do not attempt to repair stepper actuators. Opening the stepper actuator may cause the actuator to lose its magnetism, causing poor performance. Opening of the stepper actuator case will also void your warranty. The stepper actuator motor windings can be tested with an ohmmeter. Contact the factory if you suspect a defective stepper actuator that is still under warranty.
Required Maintenance for a Stepper Actuator
The stepper actuator components which are prone to wear are the lead-screw and the precision nut. Both components, after time will begin to show effects of grinding and coming into contact with one another. By limiting the exposure of the stepper actuator to dust, debris and harsh environmental conditions can lower the required maintenance on the actuator. Also, by lowering the duty cycle, hours used a day, can greatly reduce the required maintenance of the actuator and increase its lifetime.
Anaheim Automation offers a selection of customizations for stepper actuators. The list of modifications includes, but not limited to: shaft, brake, oil seal for IP65 rating, mounting dimensions, speed, torque and voltage. Along with stepper actuators, Anaheim Automation carries a comprehensive line of drivers and controllers, power supplies, gear motors, gearboxes and integrated stepper motor/driver packages. Additionally, Anaheim Automation offers optical and magnetic encoders for feedback control of position and speed, brakes, HMI, couplings, cables and connectors, linear guides and X-Y tables.
For other applications requiring a motor, please consider Anaheim Automations extensive line of stepper motors, brushless DC, brush DC, Servo or AC motors along with compatible drivers/controllers.
Stepper Actuator Quiz
1. The lifetime of a stepper linear actuator is dependent on which of the following?
|A. Duty Cycle|
|D. Both A and B|
2. Which of the following statement is false?
|A. Non-Captive Actuators need to be securely assembled to a non-rotating unit|
|B. Captive Actuators are used to shorter stroke lengths|
|C. Non-Captive Actuators are limited in stroke length|
3. List some of the common failures of actuators below:
|- Improper loading|
|- Exposure to harsh environments|
|- Excessive duty cycle|
|- No limit switches|
4. List some applications stepper actuators can be used in:
|- Printing and Labeling|
|- Electronic Manufacturing|
|- Laboratory Instrumentation|
|- CNC Machinery|
5. Calculate the Inches per Minute give that lead screw thread per inch is 20 and the number of starts is 5. The speed of the motor is 5000 RPM.
|Turns per Inch = Thread per Inch / Number of Starts
= 4 turns per inch
|Inches per Minute = Speed of Motor / Turns per Inch
= 5000 / 4
= 1250 inches per minute
6. What criteria's are necessary to consider when selecting a stepper motor?
|A. Linear force|
|B. Stroke length|
|D. Duty Cycle|
|E. All of the above|
7. If an application using a stepper actuator required feedback control, which device would be used?
|B. Linear Guide|