Stepper Drives - Motion Control Tips

What is current decay (aka recirculating current) in a stepper drive?

December 27, 2018 By Danielle Collins Leave a Comment

A common type of stepper drive is the chopper drive, so named because it rapidly switches the output voltage from the drive on and off — “chopping” it — to provide constant current to the motor. Chopper drives use one H-bridge per motor phase to drive voltage to the motor windings. A sense resistor at the bottom of the H-bridge monitors the current. When a set current level is reached, the H-bridge changes state to stop the current supply to the motor. After a set period of time (referred to as the “off time”), a new cycle begins, and the H-bridge once again drives current to the winding.

Although the H-bridge doesn’t supply current during the off time, current is maintained in the motor coils. While this stored current dissipates, or decays, it must have a path to flow through, or the field-effect transistors (FETs) in the H-bridge will be damaged. One option for a safe current path is to use freewheeling diodes in parallel with the FETs, but these add bulk and cost. Instead, the FETs themselves can be manipulated in one of two ways to provide a safe path for the decaying current.

An H-bridge is a circuit that contains four independently controlled switching elements (FETs) that direct the current flow to the load (in this case, the stepper motor).

In an H-bridge, diagonal switches are turned on to allow current to flow through. Alternating the sides of the “on” switches changes the motor’s direction of rotation.
Image credit: Texas Instruments

Turning on two FETs on the same side of the H-bridge causes very high current (referred to as “shoot through”) that could damage the FETs. Therefore, when the FETs on one side of the H-bridge need to be turned on sequentially, a method referred to as “break before make” is used.

This method disables the first FET for a set amount of time (typically a few hundred nanoseconds) before the second one is enabled, preventing shoot-through. During this brief “dead time,” when neither FET is enabled, internal body diodes on the FETs carry the current.

Current decay: Fast or slow

The two methods of current decay using the FETs are termed “fast decay” and “slow decay.” It’s important to note that, in this context, the terms “fast” and “slow” refer to the speed at which current decays, and are inversely related to how quickly the motor stops.

With fast decay, negative voltage is applied to the winding by switching on the opposite FETs and reversing the direction of current flow through the H-bridge. Fast decay enables the motor to respond quickly to changing step inputs, but it results in a slow stop of the motor. The downside of fast decay is that it causes high current ripple.

Although fast decay doesn’t provide truly recirculating current (current flows back to the power supply), it is sometimes referred to as a type of recirculating current.

With slow decay, two of the FETs (either both high-side or both low-side) in the H-bridge are turned on. This shorts the motor winding and allows the current to recirculate and slowly decay according to the motor’s L/R time constant. In a DC motor, the slow decay method causes the motor to stop rapidly because it shorts the motor’s back EMF.

Recirculating the current through the H-bridge provides slow decay (middle), while providing reverse current through the H-bridge to the power supply allows fast decay (right).
Image credit: Monolithic Power Systems

Microstepping drives find a compromise

Microstepping drives, which drive the motor windings with sinusoidal currents, typically use a method referred to as “mixed decay.” During the rising part of the current waveform, slow decay is sufficient to simulate the current sine wave as microsteps are issued, while providing high efficiency and low current ripple. But when the waveform is decreasing, slow decay doesn’t allow current to decrease fast enough to maintain the sinusoidal waveform, and fast decay would introduce unacceptable current ripple.

Fast decay produces significant current ripple, so microstepping applications use mixed decay.
Image credit: STMicroelectronics

Mixed decay solves this problem by beginning the downward part of the current waveform with fast decay enabled, and then switching to slow decay. This maintains the sinusoidal shape of the current while minimizing current ripple.

Feature image credit: STMicroelectronics

New stepper drives from Applied Motion Products

August 8, 2018 By Mike Santora Leave a Comment

Applied Motion Products has introduced a new series of stepper drives that support a range of industrial Ethernet and Fieldbus network protocols including EtherCAT, EtherNet/IP, CANopen, Modbus, Ethernet and RS-485, as well as a proprietary Serial Command Language (SCL) for efficient network communications over Ethernet (UDP or TCP) and RS-485.

By accepting high-level commands via a network connection, STF Stepper Drives provide system designers and machine builders the flexibility to control step motors using the network protocol of their choice. These advanced stepper drives simplify the programming by consolidating motion control functions into the application software of the primary logic or machine controller, rather than in individual axes.

These DC-powered microstepping drives also run stored programs created with Applied Motion Products’ Q Programming language. Q Programming provides a range of motion profiles, multi-tasking, math functions, conditional processing, data register manipulation and more in a robust yet simple, text-based programming language. Operators can configure drives and create Q programs using STF Configurator software available as a free download from Applied Motion’s website.

Designed to work with a range of 2-phase step motors, STF Stepper Drives offer advanced current control with an anti-resonance algorithm that electronically dampens motor and system resonances to improve motor smoothness and maximize torque over a wide speed range. Torque ripple smoothing decreases motor noise and vibrations. With STF Stepper Drives, step motors perform faster, quieter, smoother, and more accurately for today’s high-speed machine and process applications.

Applied Motion Products offers the STF Stepper Drives in three power ratings:
• STF03: 12 to 48 VDC input, up to 3.0 A/phase (peak-of-sine) current output. Lowest power of the series; suitable for smaller motors starting at NEMA 8 frame size.
• STF06: 12 to 48 VDC input, up to a 6.0A/phase (peak-of-sine) current output. Mid-range power within the series.
• STF10: 24 to 70 VDC input power, up to a 10.0 A/phase (peak-of-sine) current output. Highest power for largest step motors including high torque NEMA 34 frame size.

All STF Stepper Drives feature two communication ports (dual-port) for daisy-chain connections of multiple drives to the central processor, HMI or PLC.

Applied Motion Products
www.applied-motion.com

New CVK-SC Stepper Motor Speed Control System from Oriental Motor

August 6, 2018 By Danielle Collins Leave a Comment

CVK-SC series — 0.72º, 24 VDC input stepper motor speed control

Oriental Motor is pleased to introduce the new CVK-SC series stepper motor speed control system. Specifically designed for common speed control applications with constant duty, the CVK-SC series features ease of use, holding torque when stopped, and increased repeat positioning accuracy. This speed control system offers a simple configuration consisting of a stepper motor, driver, and programmable controller.

The CVK-SC series’ operating speed, acceleration and deceleration time, and running current can be set via the driver switches, and simply turning the FWD (RVS) input to ON or OFF allows for easy control. With a speed range of 0.02 to 600 r/min, two speed settings, and no additional controller required, the CVK-SC series is ideal for speed control applications that demand more precise performance but require simple control at the same time.

The CVK-SC series speed control stepper motors utilize our latest 5-phase stepper motor technology, allowing for more torque, increased overhung loads, significantly reduced vibration and noise technology. And there is no need for a separate pulse generator or controller as they are both built-in. The CVK-SC series is available in three motor sizes and two driver types with 24 VDC power input.

Oriental Motor

Three ways to reduce heat when operating a step motor

June 29, 2018 By Lisa Eitel Leave a Comment

Reduce motor heat by reducing idle and running current — or switch to closed-loop control.

Eric Rice | Applied Motion Products

Step motors are widely used motion-control devices because of their simple setup and accurate positioning with an open-loop configuration. But problems associated with heat generation can arise. That’s because the drive that powers an open-loop step motor doesn’t use feedback to control the amount of current provided to that motor. Instead, the drive provides full current at all times … regardless of torque demand on the motor. So the motor consumes power at excessive rates …. wasting energy and generating avoidable amounts of heat.

In some applications, operators run open-loop step motors with as much current as possible to generate the most torque possible. That ensures the motor will reach its position in the application. But as current increases, so does motor temperature. If motor temperature rises too high, windings short and the motor ceases to function. So design engineers (and motor end users) must find ways to maximize motor torque while avoiding motor overheating.

One industry approach is to set the drive to give the motor its rated current. Doing that will make the motor hot but not too hot. It’s normal for step motor temperatures to reach 70°, 80°, or even 90°C. While these temperatures make it too hot to touch the motor, the motor itself is unharmed.

But how else can design engineers (and motor end users) reduce motor heat? Here we’ll explore three more ways:

Reduce idle (holding) current
Reduce running current
Switch to closed-loop control

First — Reduce the idle (holding) current for the motor

Many motion control applications need the most torque when the motor accelerates and decelerates to meet its mark. But when the motor idles, or holds position, it requires much less torque. These instances are a good time to reduce motor current.

Most modern stepper drives do this automatically. For example, the drive might reduce idle current to 50% of the running current. More sophisticated stepper drives let design engineers program the idle current to any value between 0 and 100%. If a step motor spends even a modest amount of time sitting still, reducing idle current has a significant impact on motor heat.

Second — Reduce running current

Many design engineers err on the side of caution when selecting a step motor — often choosing a motor with more than enough torque (rather than just enough) for the application at hand. In these scenarios, it may be possible to reduce the motor’s running current with some trial-and-error testing. Basically, this requires reducing running current by a small amount and monitoring the motion output. If this trial run proves successful, reduce running current again and retest motor performance.

Continue making adjustments until the motor mispositions or stalls. Then slightly increase the current so the motor resumes accurate positioning. In some cases — especially when a step motor is oversized for an application — the engineer can reduce running current enough to measurably reduce the temperature of the motor.

Third — Switch to a closed-loop step motor system

The most dramatic reduction in heat generation occurs when replacing an open-loop step motor system with a closed-loop step motor system. Step motors are operable within a servo control loop when outfitted with a high-resolution encoder and powered by a suitable closed-loop drive. This relatively new system configuration necessitates that the engineer replaces the application’s motor and drive. But the magnitude of temperature reduction is usually worth it.

To review, a closed-loop step motor system uses feedback loops to accurately control step motor current, velocity, and position. The current loop ensures that the current provided to the motor by the drive is only the amount needed to meet torque demand. When the motor isn’t producing torque (or producing anything less than maximum torque) current to the motor automatically drops accordingly. This closed-loop control scheme significantly improves motor temperature … reducing it by as much as 50% or more in some applications.

Additional benefits of closed-loop steppers are quicker acceleration and higher throughput (up to 50% more torque than rated holding torque) as well as quieter operation (even to 10 dB quieter) because there’s less current in the motor windings. Plus without stalls, system accuracy is higher.

Closed-loop step motors incorporate a high-resolution encoder and are driven by an integrated, closed-loop drive that automatically reduces current to the motor when the load isn’t demanding torque.

Test results confirm that closed-loop step motors draw current only when needed, so they run cooler and with less power consumption than open-loop options. Consider actual tests that run both the open-loop and closed loop variations to get the same motion profile:

Acceleration = 100 rev/sec² and deceleration = 100 rev/sec²
Distance = 5 rev and speed = 10 rev/sec
Dwell time = 0.1 sec

Both carry a set load inertia and rotor inertia and operate from a 48-Vdc power supply. Results in the graph below show that the average power consumption of an open-loop stepper system delivering such output uses 43.8 W.

In contrast, a comparable closed-loop stepper system consumes only 14.2 W to deliver the same routine. That reduced power consumption translates into less heat generation and lower energy bills.

Results from tests conducted by Applied Motion Products confirm that closed-loop stepper systems consume less current than comparable open-loop stepper systems — and stay cooler too.

Applied Motion Products | applied-motion.com

Eric Rice graduated with an Electrical Engineering degree from the University of Illinois Urbana-Champaign.

He joined Applied Motion Products as an applications engineer in 1997 and specializes in step motors, servo motors, drives, and controls.

New series of stepper drives for ac-powered applications from Applied Motion Products

May 8, 2018 By Miles Budimir Leave a Comment

Applied Motion Products expands its family of STR Stepper Drives with two new ac-powered units for use in industrial applications powered by ac supply voltages. Offering the benefits of low noise, smooth motion, and high torque over wide speed ranges, the STRAC Stepper Drives bring high-performance motion control to basic step and direction applications. Units are ideal for larger motors and applications requiring maximum power.

Like their dc-powered counterparts, STRAC Stepper Drives feature advanced microstepping performance and current control with anti-resonance that improves motor smoothness and torque over a wide speed range. Units operate on ac supply voltages of 90 to 240 Vac and wire directly to 120 or 220 Vac line voltages to eliminate dc power supply requirements. The drives internally convert ac line voltages to high dc bus voltages to drive large step motors at maximum power.

Available in two models, the STRAC2 provides up to 2.2 A/phase while the STRAC8 provides up to 8.0 A/phase. Every stepper drive in the STR Series operates in step and direction or pulse/pulse control mode. Selecting between the two modes is simple by moving a jumper located under the drive cover or via dip switch on the front panel. Each drive microsteps to 25,000 steps/rev with a 1.8° step motor (1/125 step) even when command pulses are low resolution – thanks to a proprietary step smoothing filter. Users can set up drive parameters – including motor selection, current, and step resolution – using dip or rotary switches. No software is required.

The STRAC Stepper Drives are UL Recognized, CE certified, and RoHS compliant. Applied Motion Products offers a full complement of NEMA 23 and 34 frame step motors optimized for use with these new ac-powered drives.

For more information on the STRAC Stepper Drives, visit www.applied-motion.com.

Stepper drives: What’s the difference between an L/R drive and a chopper drive?

April 19, 2018 By Danielle Collins Leave a Comment

There are two fundamental types of stepper drives: L/R drives and chopper drives.

L/R stepper drives are also referred to as “constant voltage” drives, because they supply constant voltage to the motor windings. The current produced in the windings depends on the motor’s time constant, which is the relationship between its inductance and resistance (L/R). The time constant causes current to increase slowly with each voltage pulse. So for very high pulse rates (i.e. high motor speeds), full current, and thus full rated torque, may not be reached. This limits the use of L/R drives to primarily low-speed applications.

Torque vs. pulse rate for a typical unipolar L/R drive with a 15 degree step angle.
Image credit: AMETEK Inc.

Current in the windings can be increased by using higher supply voltage, but motor temperature rise then becomes a problem. When the applied voltage is high, the “off” time (when no power is applied) must be long enough to prevent the motor from overheating. Another option is to add resistors in series to improve the time constant (L/2R or L/4R, for example) and increase the voltage. But resistors waste power through heat generation, so efficiency is reduced.

Resistance (R) affects the maximum current in the windings, according to Ohm’s law, I = V/R.

Inductance (L) affects the rate of change of current in the windings, according to the relationship, dI/dt = V/L.

Because of the torque and speed limitations of L/R stepper drives, an increasing number of stepper motor applications now use chopper drives. These drives are also referred to as “constant current” drives because they supply constant current to the motor windings, by “chopping” the output voltage (i.e. turning the output voltage on-and-off very rapidly).

With each motor step, a very high voltage (typically eight times the motor’s nominal voltage) is supplied to the motor windings. This high voltage gives a very short current rise time, according to the relationship for an inductor (dI/dt = V/L), and it causes higher current to be produced, according to Ohm’s law (I = V/R).

Torque vs. pulse rate for a typical bipolar chopper drive with a 15 degree step angle.
Image credit: AMETEK Inc.

Voltage chopping also modulates the width of the output pulses (pulse width modulation) and is typically done at a frequency of 20 kHz or higher — above the audible range. The impedance in the windings varies with the motor speed, so it plays a role in determining the voltage on-time.

At slow motor speeds — and low impedance — the chopper drive provides a short on-time for the voltage, producing a small pulse width. Alternatively, at high speeds —and high winding impedance — the chopper drive provides a long on-time for the voltage, producing a large pulse width and allowing time for the current to rise sufficiently to produce the rated torque.

Relationship between voltage and current in a (chopper) drive.
Image credit: Oriental Motor USA Corp.

A current-sensing resistor placed in series with each winding regulates the current and ensures the shortest time possible for current to build up and decline. As the current increases, voltage develops across the resistor. This voltage is monitored by a comparator, and at a predetermined reference voltage, the output voltage from the drive is turned off (chopped) until the next pulse occurs. This allows current to build and decline as the voltage is switched on and off.

By controlling the duty cycle of the drive through chopping, the average voltage and current are kept equal to the motor’s nominal voltage and current ratings. The result is precise torque control, and more importantly, higher torque production at high motor speeds.

Microstepping drives are common versions of chopper drives.

Feature image credit: AMETEK Inc.

Stepper drive is internationally certified for use in hazardous locations

March 6, 2018 By Paul Heney Leave a Comment

Applied Motion has announced the first and only step motor drive internationally certified for use in hazardous locations. The STAC6-Q-H Stepper Drive is ATEX and IECEx certified for Class I, Zone 2 locations, and UL certified for Class 1, Div. 2, Group C & D locations; Class 1, Zone 2 locations. The HazLoc Step Motor Drive is approved for use in hazardous industrial applications throughout the USA, Canada, Europe and the Middle East.

Addressing the need for step motors in hazardous locations in the oil, gas, paint, and printing industries, the STAC6-Q-H Stepper Drive operates in environments where flammable gases, vapors or liquids may be present in the air in quantities to produce explosive or ignitable mixtures. The STAC6-Q-H is currently in operation driving a step motor to control the braking and speed of the drill string of on oil rig.

Powered by single-phase 120 VAC that generates a nominal 160 VDC internal bus, the STAC6-Q-H provides superior performance when paired with high-quality NEMA 23, 24, 34 or 42 frame step motors. An advanced anti-resonance algorithm ensures optimal torque over a wide speed range, while microstepping to 51,200 steps/rev provides for smooth, high-resolution positioning.

An encoder feedback option is available for enhanced system accuracy using Applied Motion’s proprietary Stall Detection and Stall Prevention features, which overcome limitations of open loop step motor systems. The drive is capable of running stored programs created with Applied Motion’s Q programming language, and has sufficient input/output signals to connect to multiple external devices or sensors.

For more information on Applied Motion’s STAC6-Q-H Stepper Drive for Hazardous Locations contact the company for technical support at 1-800-525-1609.

Applied Motion Products
www.applied-motion.com

Absolute type AZ Series stepper motor driver with EtherCAT from Oriental Motor

November 22, 2017 By Lisa Eitel Leave a Comment

Oriental Motor now sells a DC-input AZ Series Multi-Axis Drivers with built-in EtherCAT communications. These are based on closed-loop absolute-type AZ Series driver.

The AZ Series Multi-Axis Drivers with EtherCAT is a space saving, DC input, multi-axes controller and driver designed for applications demanding 2 or more axis of control and integrating EtherCAT communications from an upper lever controller.
These new drivers come in 2 axes, 3 axes or 4 axes configuration. It is compatible with all of the DC input AZ Series family of products, including DC input stepper motors, linear and rotary actuators. The advantages of consolidating up to 4 drivers into 1 unit includes space saving benefits, reduced cost and simplified wiring.

The AZ Series Multi-Axis Drivers supports EtherCAT communication and is compatible with the CiA402 drive profile.
Learn more about the AZ Series Multi-Axis Driver with EtherCAT at the deep link on orientalmotor.com.

Since its founding in Japan in 1885, Oriental Motor has been a world leader in motion systems. For over a century we have concentrated on technological advancement and product design improvement—an emphasis evident in the sophisticated devices we market today. Oriental Motor’s sales and service network is international, with offices throughout North America, Europe and Asia. Domestically, ORIENTAL MOTOR U.S.A. CORP. was established in 1978.

Controller-drivers steer stepper-motor-driven sample-transfer machine

November 9, 2017 By Lisa Eitel Leave a Comment

Gatan Inc. makes specimen-preparation equipment and recently built an alpha prototype of a new machine. The iPrep prototype mounts to a scanning electron microscope (SEM) and shuttles samples back and forth between a processing chamber and SEM receiving dock. Within the processing chamber, two broad-beam ion guns polish samples.

The iPrep’s automated transfer mechanism allows use of SEM-related imaging techniques for 3D reconstructions of sample areas with exact control over slice thickness.

This is Gatan’s alpha prototype of an iPrep machine that next to a scanning electron microscope. It is a high-vacuum-compatible receiving dock — one that can withstand E-6 Torr — with a kinematic mechanism that lets samples get mounted with accuracy to better than 1 µm every time. In this application, the workflow also consists of dozens of very specific operations … all needing reliable handling thousands of times. The iPrep from Gatan balances several parameters to deliver on these design objectives.

The iPrep machine employs an AllMotion controller to command a transfer arm’s end effector to accurately open and close … and move a sample carrier to the polishing chamber and SEM. Another AllMotion EZHR23EN48V controller is part of a kinematic mounting system on the receiving dock inside the SEM. Stepper motors drive the end effector and receiving dock.

AllMotion EZHR23EN48V controller+drivers have selectable step resolution from half stepping to to 1/256 steps … and can deliver to 20 million microsteps per second.

The EZHR23EN48Vs are controller-drivers measuring 2.25 by 2.25-in. and bolt to the backs of the stepper motors they control. The controllers use a single four-wire bus with two power wires and two for communication. Simple commands are made via any serial terminal program or from the EZStepper Windows application.

Because every operation the iPrep performs occurs in a vacuum chamber, it’s impossible to hear or see when things go wrong. So Gatan relies entirely on sensor data (leveraging EZHR23EN48V wiring for OptoSwitch inputs) to identify problems in the chamber. If this does occur, workflow is halted for maintenance.

The main specifications for the system lie in the repeatability of placement in the SEM. 3D reconstruction only works with very low shift and rotation between placements. “Our tolerance for shift between images in the SEM (after transferring) is on the order of several nanometers,” said Thijs Hosman, project manager and research scientist for Gatan. Plus incorporating shift detection algorithms can compensate for some of the shift between images … but the mechanical placement accuracy still must be on the order of 1 µm. The tolerance for rotation is about 0.05° between images. The distance between the preparation chamber and the SEM is on the order of 1 meter.

New AC-powered STR stepper drives from Applied Motion Products

September 22, 2017 By Lisa Eitel Leave a Comment

Applied Motion Products, Inc. now sells an expanded line of STR Stepper Drives with the release of new drives for use in AC powered applications. STR stepper drives are designed for smooth motion and high torque over a wide speed range, bringing high performance motion control to basic step and direction applications. Two new AC powered STRAC drives expand the offering to cover larger motors and applications where maximum power is required.

Operating on AC supply voltages of 90 to 240 AC, these new STRAC drives can be wired directly to 120 or 220 VAC line voltages, eliminating the need for a DC power supply. AC line voltages are internally converted by the drives to high DC bus voltages that enable larger step motors to be driven with maximum power. These drives are designed for demanding applications where high torque and high speed are necessary.

The STRAC2 provides up to 2.2 A per phase while the STRAC8 provides up to 8.0 A/phase. All STR drives are great for OEM applications needing basic step and direction control of a 2-phase step motor. STR drives feature advanced microstepping performance and sophisticated current control with anti-resonance. Anti-resonance electronically damps motor and system resonances, which improves smoothness of motion and torque over a wide speed range.

“Step and direction drives make up more than 80% of the stepper drive market because customers want drives that are easy to configure and that can be commanded via simple, digital control interfaces. The addition of the cost effective, AC powered STRAC drives to the already strong STR offering means a more complete series of step and direction drives for the majority of users,” said Dennis Joyce of Applied Motion Products.

Every STR step motor drive operates in step and direction or pulse/pulse control mode. Selecting between these two modes is done by moving a jumper located under the cover of the drive. Each STR drive can microstep up to 20,000 steps/rev with a 1.8° step motor (1/100 step) and can even microstep the step motor when the command pulses are low resolution. This feature is called Microstep Emulation. All drive setup is done via dip or rotary switches on the side of the drive, including motor selection, running current, idle current, and step resolution.

Because of the high internal bus voltage of STRAC drives, step motors capable of withstanding high voltage should be used. Applied Motion Products offers a full complement of NEMA 23 and 34 frame step motors optimized for use with these new AC powered drives. Like the STRAC drives, many of these motors are UL recognized, CE certified, and RoHS compliant. To review, the feature of the stepper drives include: