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Closed loop-capable stepper motors merge the benefits of stepper and servo motor technology. They run more quietly and have a lower resonance than stepper motors, provide position feedback and control, feature short settling times, and exhibit no step loss at all. They are an alternative to stepper motors if energy efficiency, smooth running and a high load tolerance are required.Compared to servo motors, they have advantages due to their high torque at low speeds, short settling times, correct positioning without back swing and a lower price for sizes that are often smaller.

The closed-loop method is also referred to as a sinusoidal commutation via an encoder with a field-oriented control. The heart of closed-loop technology is power-adjusted current control and feedback of control signals. Through the encoder, the rotor position is recorded and sinusoidal phase currents are generated in the motor coils. Vector control of the magnetic field ensures that the magnetic field of the stator is always perpendicular to that of the rotor and that the field strength corresponds precisely to the required torque. The current level thus controlled in the windings provides a uniform motor force and results in an especially smooth-running motor that can be precisely regulated.

 

True/pseudo closed loop

There are stepping motors that dress themselves up as being closed loops and work with encoders but do not provide any field-oriented control with sinusoidally commutated current control. They only check the step position, and cannot correct step angle errors during operation. True closed loop with field-oriented control compensates step angle errors during a run and corrects load angle errors within a full step.

linear stepping motor linear actuator is essentially rotary stepper motor “unwrapped” to operate in straight line. Linear motor operates on electromagnetic principle and consists of moving “forcer” and stationary platen. The platen is passive toothed steel bar (stainless is available) extending over desired length of travel. Forcer incorporates electromagnetic modules and bearings and moves bi-directionally along the platen.

linear stepper motor is an turn-key linear actuator available with either mechanical roller bearing or air bearings.

Side and bottom mechanical bearings are built into forcer and do not require any adjustments by the user over the lifetime of the motor. They are permanently lubricated and exhibit very little friction.

Air bearing operates by floating the forcer on high pressure air introduced through orifices in the forcer. Air bearing motors can operate continuously at high speed without wear. Air bearing permit smaller air gap resulting in larger motor forces.

linear stepping motor linear actuators are micro-stepped by proportioning currents in two phases of the forcer, much same as in rotary stepper motors. When micro-stepper linear stepper motors following benefits are achieved:

– higher resolution for positioning

– smoothness at slow speeds

– wider speed range

Manufacturers apply the term “closed-loop stepper” to a wide array of controls. Here, we’ll spell out how the three most common closed-loop stepper control schemes work and highlight their advantages and disadvantages.

Are all closed-loop stepper systems created equal?
No. Some manufacturers give the closed-loop stepper motor systems similar-sounding descriptions, which confuses the marketplace. As proof of the confusion, it’s not uncommon that a designer requests one capability and actually needs another.

What are the most common closed-loop stepper systems?
There are three common types: Closed-loop stepper with step-loss compensation; closed-loop stepper with load position control; and closed-loop stepper servo control. Stepper-drive manufacturers call them all “closed loop” but the three have distinct functionalities.

What are the functionalities of these closed-loop stepper systems?
Closed-loop stepper with step-loss compensation is the most common type of closed-loop stepper control. The stepper drive operates as a micro-stepping drive and typically receives pulse and direction commands to move to the desired position. An encoder tracks shaft or load position. If lost steps are detected, a compensation algorithm inserts additional steps so that the motor shaft (or load) arrives at the desired position. Typically, the Closed-loop stepper-motor driver has settings for two currents: The motor gets running current when in motion and gets resting current when stopped.

Like servo motors, china stepper motors are available in both rotary and linear designs. When an application requires force (rather than torque) output and can operate in open loop control, a linear stepper motor is often the preferred solution. Although linear stepper motors are available in both variable reluctance and hybrid designs, the more common version is hybrid linear stepper motors.

In a hybrid linear stepper motor, the base, or platen, is a passive steel or stainless steel plate with slots milled into it. The forcer contains motor windings, permanent magnets, and laminations with slotted teeth that serve to concentrate the flux that’s created when current is applied to the coils. The teeth of the forcer and the platen are staggered by ¼ tooth pitch in relation to one another to ensure that constant attraction is maintained and that the next set of teeth will come into alignment as current is switched in the coils. This means that for each full step of the motor, the forcer moves along the platen by ¼ tooth pitch.

Whereas variable reluctance linear stepper motors for sale can only operate in full step mode, hybrid versions can operate in either full step or microstepping modes. Microstepping, which divides the step angle into smaller increments, enables higher resolution motion and better control of speed and force. Because each phase of the motor is driven with (theoretically) ideal sine waves, 90 degrees apart, microstepping also allows the current to increase in one winding as it decreases in the other, providing smoother operation at low speeds than can be achieved with full- or half-step operation.

For guiding the load on hybrid linear stepper motors, either mechanical roller bearings or air bearings are typically used. (Because the platen in a hybrid linear stepper motor is passive, it can serve as the air bearing surface.) The magnetic flux between the forcer and platen creates a strong magnetic attraction, so these support bearings actually serve two purposes – to guide and support the load and to maintain the correct air gap between the forcer and the platen.

Like other linear motor designs, hybrid linear stepper motors can incorporate multiple forcers onto one platen, with each forcer moving independently. In addition to smooth low-speed operation (obtained with microstepping control), they are also able to achieve very high speeds and accelerations with high resolution and low to moderate force generation.

With simple mechanical construction and easy setup (no servo tuning required), hybrid linear stepper motors are ideal for applications that can operate in open-loop mode and that require either high speed with low force production or very smooth motion at low speed.

 

When the stepper motor receives the final pulse signal, (either one or from a continuous train), it will stop rotating. However, complete rest will not occur until all the oscillations have stopped. The time it takes from the application of the last pulse received until the stepper motor comes to a complete rest is known as settling time. (See graph below). Resonance occurs when the stepper motor suddenly makes large oscillations, or the output torque suddenly drops at one certain pulse rate or numerous small regions of pulse rates. The stepper motor will stop (stall), may miss steps or reverse direction from the commanded direction. This phenomenon occurs when the natural frequency of the stepper motor coincides with the frequency of the input pulse rate. This generally occurs around 100 – 200 pulses per second in a full-step operation, and also at higher pulse rates. Microstepping half-step operation, or electrical or mechanical damping, can reduce resonance issues. Microstepping has a large effect on settling time and resonance due to the smaller angular displacement taken per pulse. See Figure below.

Resonance Characteristics
Since a hybrid stepper motor system is a discreet increment positioning system, it is subject to the effect of resonance. Where the system is operated at this given frequency, it may begin oscillating. The primary resonance frequency occurs at about one revolution per second. Oscillating will cause a loss of effective torque and may result in a loss of synchronism. Settling time and resonance can be best dealt with by dampening the stepper motor’s oscillations through mechanical means. Mechanically, a friction or viscous damper may be mounted on the stepper motor to smooth out the desired motion.

Methods for Changing or Reducing Resonance Points:
• Use of Gearboxes or Pulley Ratios
• Utilize Microstep Drive Techniques
• Change System Inertia
• Accelerate Through Resonance Speed Ranges
• Correct Coupling Compliance

General Stepper Motor Driver Safety Considerations

In this post, I’ll describe the process I use for sizing gearboxes and geared stepper motors.

To make the selection, I am using KEB’s software sizing program called KEB-DRIVE. KEB-DRIVE is free and easy to use. If interested to follow along, you can download a copy of the software.

2. Select the correct gear technology for the application
Configurations in KEB-Drive start at the top left. On the left, you’ll see drop-downs to select different gear types and sizes.

3.Motor Selection (Size, voltage, frequency)
Working to the right, I then select the size of the motor I want. Options for both Induction motors and AC Servo motors are listed. Here is a comparison of the advantages between servo and induction motors.

4.Adjust the Torque/Speed selection
Is it a speed reducer? Or a torque Increaser? It’s both – higher gear ratios will provide lower output speeds and higher torques. Use the drop down to see all the different possible configurations with the selected gearbox/motor combo.

5. What is the gearing Service Factor and why is it important?
The gearing service factor (SF) is the ratio between the:
A SF of 1.0 means the gears will have a nominal output torque equal to that of their rating. Selecting a motor/gear configuration with a SF of less than 1.0 is not advised. This means the gears will be undersized when operated at the nominal point. This could also indicate that the motor selected is too large.

6.Select gearmotor options (mounting style)
This section allows a user to select how the geared motor will be mounted. The flexibility of mounting is one reason that the KEB integral gearmotor solution has been so popular. Users can select a unit with an output shaft. Or a shaft mounted unit with a hollow bore. Mounting feet and mounting flanges can also be selected.

Geared stepper motors are perfect solutions for low speed and high torque positioning applications. GEMS provide NEMA size stepper motors that are paired with planetary gearbox and spur gearbox. Our design incorporates a square bodied motor and an round shape planetary stepper motor gearbox into a compact and cost-effective package. Our geared stepper motors are offered in six NEMA frame sizes (from NEMA 8 to NEMA 42) and each NEMA size motor has an integrated stepper motor gearbox with a wide range of gear ratios (from 1:3.7 to 1:369) so as to deliver any desired torque and speed combinations for your applications. The dual shaft version is available for you to install the brake, shaft coupler or an encoder where you need to keep track of the shaft position. 8-lead motor is aslo available upon request for all possible wiring configurations: bipolar, unipolar, series, or parallel. For higher speed and better speed control capability, please check out our brushless gear motors.

1.8° NEMA 8 GEARED STEPPER MOTOR WITH PLANETARY GEARBOX
NEMA8 is our smallest size of geared stepper motor. The 22mm diameter planetary stepper motor gearbox has gear ratio from 1:3.7 to 1:369. The gearbox output torque is up to 143 oz-in (1 Nm).

 

TABLE 1. MOTOR SPECIFICATIONS
Model Length L Rated Current Resistance Inductance Holding Torque Holding Torque Rotor Interia Lead wires Weight
mm A Ω/Phase mH/Phase Oz.in N.m g.cm2 g
17hs19-2004s 30 0.5 4.8 1.3 2.6 0.018 2.5 4 50
17hs162004s 42 0.5 7.5 2.4 3.6 0.025 4.5 4 90
TABLE 2. GEARBOX SPECIFICATIONS
Number of gear trains 1 2 3 4
Gear ratio 3.7, 5.2 14, 19, 27 51, 71, 100, 139 189, 264, 369
Length L2 mm 24.4 33 41.5 49.8
Rated output torque N.m 0.6 1 1.6 2
Max output torque N.m 1.8 3 4.8 6
Efficiency % 90 81 73 66
Weight g 35 45 55 65

First let’s explore how high performance closed-loop stepper motor system compare to traditional open-loop stepper systems in terms of torque and efficiency.

There’s superior performance from closed-loop stepper systems over open-loop setups as demonstrated in laboratory test results comparing the two systems’ acceleration (torque), efficiency (power consumption), position error (accuracy), heat generation, and noise levels. Just consider the relationship between torque and acceleration. Torque-speed curves show the peak and continuous torque ranges of a closed-loop stepper system alongside the usable torque range of an open-loop stepper system. Very often, torque in the real world translates into acceleration — so motors with greater torque can accelerate a given load faster.

To test this difference in torque performance in the lab, equally sized open-loop and closed-loop step motor systems get identical inertial loads. Programming commands the two systems to perform identical move profiles, except that acceleration rate and top speed are slowly increased in each system until they make positioning errors.

Here we have a move profile comparison between a open-loop vs. closed-loop system. This is a comparison between that from a StepSERVO closed-loop system and that from an open-loop system. The closed-loop system (due to its higher torque producing capability) gets a maximum acceleration rate of 2,000 rev/sec2 and a top speed of 20 rev/sec (1,200 rpm)
as seen here.

Say that the open-loop system gets a maximum acceleration rate of 1,000 rev/sec2 and a top speed of 10 rev/sec (600 rpm). This top speed of 10 rev/sec correlates to where the flat portion of the torque-speed curve ends. The closed-loop system (due to its higher torque producing capability) gets a maximum acceleration rate of 2,000 rev/sec2 and a top speed of 20 rev/sec (1,200 rpm). This is double the performance of the open-loop system and cuts the move time nearly in half — from 110 msec down to 60 msec.

For applications requiring high throughput (such as indexing, edge guide positioning and pick-and-place systems) the closed-loop stepper motor driver system provides a clear performance advantage.

Amount, speed, and direction of rotation of a stepper motor are determined by the appropriate configurations of digital control devices. Selecting the most compatible stepper motor, driver, and/or controller, can save the user money and be a less cumbersome motion control solution. Anaheim Automation categorizes the major types of digital control devices as:

• Stepper Motor Drivers – offered in full-step, half-step and micro-step analog stepper driver
• Stepper Motor Controllers (sometimes referred to as Control Links – controllers indexers, and pulse generators sold separately or in Drivers Packs
• Stepper Motor Driver Packs – packaged units that include drivers and optional controller, with a matched power supply (most models are enclosed units that are fan-cooled)
• Integrated Stepper Motor/Driver/Controllers – packaged at the end of a stepper motor are drivers and simple controllers (only available for high-torque stepper motors)

These devices are employed as shown in Figure 5. The stepper motor driver accepts clock pulses and direction signals and translates these signals into appropriate phase currents in the stepper motor. The Indexer creates the clock pulses and direction signals, “translates” them into power to energize the stepper motor windings. The computer or PLC (programmable logic controller) sends commands to the indexer/controller.

Load characteristics, performance requirements, and mechanical design including coupling techniques must be thoroughly considered before the designer can effectively select the most suitable stepper motor and driver combination for a specific application. Failure to do so may result in poor system performance, or cost more than necessary. The following factors must be considered in order to obtain an optimum stepper motor motion control solution:

Why do you use a stepper motor?

The following safety considerations must be observed during all phases of operation, service and repair. Failure to comply with these precautions violates the safety standards of design, manufacture and intended use of the product(s). Anaheim Automation assumes no liability for the customer’s failure to comply with these requirements, and advises the misuse of its stepper motor products will void the warranty. Even well-designed and manufactured products, operated and installed improperly, can be hazardous. Safety precautions must be observed by the user with respect to the load and operating environment. Described briefly are general Safety Considerations. Please refer to the Environment Consideration’s Guide for more details.

•Use caution when handling, testing, servicing and adjusting during all phases of installation and operation of a stepper motor system
•No service/maintenance should be performed with power applied
•Exposed circuitry should be properly guarded or enclosed to prevent unauthorized human contact with live circuitry
•Stepper motor drivers/controllers should be securely mounted and adequately grounded
•Provide adequate air flow and heat dissipation for all stepper motor system components
•Do not operate a stepper motor system in the presence of flammable gases, dust, vapor or liquids

IMPORTANT NOTE: The customer is responsible for the proper selection, installation, and operation of the stepper motor products purchased from Anaheim Automation. The customer must determine the fitness of selected products for a specific application. Although it is the company’s intention to provide sound advice and accurate documentation, Anaheim Automation assumes no liability in the suggestions it offers.