要約

Stepper motors are mainly used because of their great reliability at low cost.

However, this reliability was often bought at the expense of a characteristic noise level. Even though the days when stepper motors were operated in full or half-step operation in voltage-controlled operation are long gone, the stepper motor still has the flaw of noise development. To address this problem, ADI Trinamic has developed stepper motor driver devices that reduce noise to a minimum without additional development effort and, above all, without additional costs.

Content

• Stepper motors in new fields of application
• From full-step to micro-step operation
• From "Unipolar Unregulated" to Bipolar with Current Control
• From conventional power chopper to stealthChop^TM
• Comparative measurement with different control methods
• Current reduction to reduce volume
• Recommendations for an acoustically (and thermally) optimized layout
• Result

Stepper motors in new fields of application

Around humans, more and more tasks are being automated. Motors are used in many of these applications. Especially when these movements are carried out in the vicinity of humans, there are increased requirements for noise. The simplest type of motorization is to use a DC motor. Here it is sufficient to apply a voltage and the motor rotates. A DC motor is very inexpensive to manufacture, so it is used in many applications. However, it also has some serious disadvantages: A DC motor has a very low torque at low speeds. Therefore, for applications in the lower speed range, a gearbox must be used, which causes costs, noise and losses and forms a mechanical weak point. For positioning tasks, a feedback device is also required, e.g. potentiometer, encoder, resolver, which in turn causes costs and weakens reliability. Abrasive carbons are used to commutate the engine, which wear out and produce abrasion in the process. Sparks generated during commutation sometimes generate undesirable electromagnetic interference (EMC).

In contrast, the torque characteristic of a stepper motor has the maximum value at standstill and then drops when the speed increases. Therefore, it can often be used as a direct drive without a gearbox. Position feedback is usually not necessary (open-loop). A stepper motor is electronically commutated.

If a low noise level is added as a further advantage, further fields of application open up.

A market that is currently growing very strongly is 3D printers, which are also in the process of conquering private households. Since 3D printing can take many hours and often runs overnight, noise is particularly critical here. Other devices such as office printers or copiers should also be mentioned here.

Another fast-growing area is home and building automation. Movable surveillance cameras (PTZ) should be as inaudible as possible. The same applies to the controlled exhaust flaps of a central air conditioning/ventilation system.

Classic applications such as laboratory automation and medical technology also benefit from silent and low-vibration drives. Even in industry, noise is an issue, e.g. with control valves. Since the noise also reduces vibrations and thus signs of wear, the new technologies are also very interesting for conventional applications.

From full-step to micro-step operation

The simplest stepper motor consists of four coils, two of which are connected in series and opposite each other, and a rotatable magnet as a rotor. The two coils connected in series are called "phase".

If the two phases are now supplied with a current, magnetic poles form on the coils and the rotor magnet aligns itself to a stable position between the two phases. This position is called the "full-step position". If the current direction is reversed at one of the phases, the rotor magnet rotates 45° to the next full-step position. He takes a "step".

After four full steps, a period is over. In the simple 2-phase stepper motor from the example, a period of 4 steps corresponds to one revolution. In practice, multi-pole stepper motors with up to 400 steps per revolution are usually used. Here, too, there is the same electrical state at every fourth full step. Thus, a multi-pole stepper motor has several stable positions on one revolution. Therefore, in the event of overload or external influences, "step losses" can occur.

The rotor of the stepper motor, together with the stator, forms a spring-mass system with its own resonant frequency. The inertia of the rotor causes it to transient to the desired position. The larger the stride size, the higher the overshoot and undershoot.

A stepper motor builds up torque by twisting the rotor and the electrically controlled magnetic field of the stator against each other by up to one full step. Thus, the torque of the system depends on the load angle and thus on the rotor position. This, in turn, means that deviations from the desired position also result in restrictions in the desired torque. Large transient processes, such as those that occur particularly close to the resonance frequency, therefore lead to considerable torque drops.

Large transient processes, such as those that occur particularly close to the resonance frequency, therefore lead to considerable torque drops. If the rotor exceeds a distance of more than two full steps from the actual target position during settling, it jumps to the next stable position (step loss). As a result, stepper motor drives sometimes behave erratically in full-step operation close to the resonance frequency of the system.

A slight improvement in this behavior is already given in half-step operation. Here, one of the phases is de-energized between two full steps. This results in a doubling of the number of positions and a halving of the step size.

The optimum running behavior is achieved with the microstep control. Here, the phases are not only switched on and off, but also charged with different current values.

The two phases are controlled with an approximated sine wave offset by 90°. A full step is divided into smaller units. These are called "microsteps". The microstep resolution is the number of intermediate positions into which a full step is divided.

The higher the microstep resolution, the cleaner the theoretical sinusoidal shape of the phase current.

Since the distance traveled in micro mode is significantly reduced compared to full-step operation, the transient effects are also significantly lower. Figure 8 shows how the position deviations are significantly reduced when switching from full step to microstep.

The state of the art in integrated stepper motor driver ICs is currently a micro resolution of 256. In practice, however, the specification of sinusoidal setpoints is not sufficient to obtain a sinusoidal phase current. The switching of inductors as well as other interference effects require further measures in order to maintain a very smooth operation.

From "Unipolar Unregulated" to Bipolar with Current Control

In the past, unregulated unipolar control was often used, in which the center tap of the two coils of a phase was firmly connected to ground and then the coils were alternately switched to operating voltage.

An advantage of this solution is that only 4 switches are needed for control. However, since only half a phase is energized at a time, the motor is never fully utilized.

Another disadvantage is the regenerative voltage generated when the motor is rotated: the back-EMF. This is higher the faster the engine turns. Since it counteracts the operating voltage, the faster the motor rotates, the lower the effective voltage and thus the phase current. Therefore, in this mode, the torque decreases very quickly as the speed increases.

For this reason, regulated bipolar driver stages are now used in applications that have slightly higher requirements. Here, an H-bridge is used for each engine phase. The phase current is measured via a resistor and the switches – usually MOSFETs – are controlled with a PWM in such a way that the desired phase current is achieved.

The desired motor current is therefore imprinted and is therefore no longer directly dependent on the voltage. The increasing back-EMF with increasing speed thus has a significantly lower influence and the torque curve drops much more slowly. This PWM current control is also called a chopper. In practice, there are different chopper variants that regulate the current differently.

Since inductors are switched in the stepper motor in which the current flow cannot be changed at any speed, the different chopper variants use different mechanisms to take this into account.

In the ON state, the MOSFETs are driven so that the current flows through the motor phase in the desired direction. When switching off, it is then possible to de-flow the coil as quickly as possible by reversing the polarity of the bridge (Fast Decay). If the current in the coil is to decrease as slowly as possible, the two lower switches are closed so that the current can circulate (slow decay).

Chopper variants that use both switch-off modes are called mixed-decay choppers. If the proportions are set automatically, it is automatic mixed decay.

There are choppers in which the switching frequency is constant and the duty cycle changes. In other variants, the switch-off time is constant and the frequency is variable.

Many drivers available on the market today use a mixed-decay chopper with a constant switch-off time (constant toff).

The switch-on phase (ON) lasts until the desired target current is reached. This is followed by a fast decay phase, followed by a slow decay phase. As can be seen from Figure 13, this results in an average current that is always lower than the target current. The current ripple is not symmetrical.

The effect is an unclean zero crossing. At low speeds, this is even visible in the movement: the motor takes just under two full steps and then stops for a short time. At medium speeds, this causes an audible hum and vibration of the motor.

At higher speeds, the back-EMF often comes into play, which significantly deforms the falling edge of the sine wave if the mixed-decay settings are not optimized.

ADI Trinamic's patented spreadCycle^TM Chopper process offers a variety of improvements.

Here, the switch-on phase is followed by slow decay, then fast decay followed by another slow decay phase. This results in a symmetrical current ripple. The average current corresponds to the target current. Due to the adjustable hysteresis points, the fast-decay component optimizes itself, so that a clean sinusoidal phase current is always achieved.

Furthermore, the lowest possible fast-decay content helps to ensure that the motor is not unnecessarily heated by current ripples. A cleanly regulating chopper is the prerequisite for a sensible use of high microstep resolutions. Tests have shown that the quality of the chopper is more important for smoothness than the microstep resolution. A motor operated with 16 microsteps and spreadCycle is already quieter and less vibrational than with 256 microsteps and an unoptimized mixed-decay chopper with a constant switch-off time.

For comparison, measurements were made with three commercially available drivers. Evaluation boards were used in the delivered basic setting.

The tests were carried out under the following conditions:

• Motor voltage: 24V
• Phase Current: 1A (rms)
• NEMA 17 Hybrid Stepper Motor : QSH4218-47-20-044

The following were tested with the highest possible microstep resolution: ADI Trinamic TMC262: 256, driver A: 16, driver T: 32 [microsteps / full step]

With driver A, there is a significant distortion of the sine wave at most speeds. These are due to a non-optimal mixed decay.

In the magnification, the microstep steps of driver A are clearly visible.

In the case of driver T, the high current ripple and the unclean zero crossing are noticeable.

The ADI Trinamic TMC26x with spreadCycle Chopper produces the cleanest sine wave at all speeds.

At the highest speed (858 rpm), the current curves of all drivers are distorted. This is normal, as the drivers cannot reach the target current due to the back EMF. By increasing the operating voltage, the curve would approach a sinusoidal shape again.

All deviations from the ideal sine wave are noticeable in practice by vibrations and thus by noise!

In many applications, a satisfactory noise level can already be achieved by using the spreadCycle chopper. The spreadCycle Chopper also offers the possibility to use helpful functions such as the sensorless load detection stallGuard2 or the sensorless load-dependent current adjustment coolStep.

From conventional power chopper to stealthChop^TM

However, the basic process of the power chopper has its limits. Due to the principle, the method has distortions of the sinusoidal shape due to current ripple and therefore cannot be absolutely silent.

Ideally, it should be controlled with a pure analogue signal, but the amplifiers have far too high a power dissipation for this. Occasionally, such amplifiers are used, e.g. in electron microscopes, where even the slightest external interference falsifies the actual measured value.

The voltage-controlled operation also achieves very good events. Here, similar to unregulated unipolar operation, the voltage is specified and the motor current is adjusted via the complex winding resistance. If this voltage is varied via a PWM, microstepping operation is also possible. There are a few built-in drivers on the market that work on this principle. The disadvantage here is also that there is no current measurement and control. This means that the driver must be precisely tuned to the complex motor resistance and operating voltage. Also, the back-EMF cannot be compensated. Therefore, this mode of operation is only suitable for low speeds.

Under the name stealthChop, ADI Trinamic has improved voltage-controlled operation and combined it with current control. This allows the system to adjust itself to the motor and operating voltage and to compensate for the back-EMF as the speed increases.

The result is a torque curve similar to that in current chopper mode.

Since the switching frequencies are far above the audible range and the sinusoid has hardly any distortion, this operating mode is largely silent. Only noises from mechanical sources such as ball bearings, belt drives or gearboxes are still audible.

In the stealthChop method, one current measurement per full step is taken during a motor movement. At a typical acceleration, it takes about 10 full steps for the current to be readjusted. This means that there is a slight reduction in dynamics compared to the power chopper operation.

For new driver generations, the technology has been further developed into stealthChop2. Here, the internal resistance and the back-EMF constant of the motor are measured and stored during the first run. With this data, the current is immediately readjusted so that it no longer changes during acceleration. The motor is always optimally energized.

For a comparison, a motor was operated with stealthChop and spreadCycle.

In the case of the phase current, spreadCycle only shows a slightly larger ripple. The situation is different when looking at the individual chopper cycles.

Figure 19 compares the coil current curve (green) and the associated output voltages (yellow, blue) at the coil terminals for stealthChop and spreadCycle. The engine is at a standstill. stealthChop shows a much lower current ripple than spreadCycle, since no fast-decay cycles are required due to its principle, i.e. the driver only switches between ON and slow-decay. With the cycle-based control chopper spreadCycle, you can see that due to slight measurement noise, the cycle duration varies slightly from one cycle to the next, while stealthChop delivers an absolutely uniform signal.

With spreadCycle, the current is measured during each chopper cycle and the target current is adjusted. Even disturbances of a few millivolts, which can be found in almost every complex system, cause current changes and thus a change in chopper frequency. This jitter can lead to chirping or whistling noises due to magnetostriction in the stator.

In contrast, stealthChop operates at a fixed chopper frequency and changes are only caused by varying the current setpoint.

Comparative measurement with different control methods

In order to show how the different types of control affect the torque curve of a stepper motor, test series were carried out on a motor test bench. The ADI Trinamic TMC5130 was used as the driver, which supports the chopper variants stealthChop, spreadCycle and Automatic Mixed Decay with constant switch-off time.

It is important to understand that this measurement is only an example. The mechanical structure (the spring-mass system) has a significant influence on the measurement. Therefore, measurements with different setups cannot be directly compared with each other. This must be taken into account when evaluating the torque curves declared by manufacturers. The behavior in the practical application is then different again.

In the lower rev range, where the motor has its natural resonances, the torque with stealthChop is highest and has no significant drops. In the medium and high speed range, spreadCycle offers the best results. This is because a voltage-controlled chopper can dampen system resonances worse than a chopper like spreadCycle, which readjusts every cycle. The microstep resolution has at most a selective effect on the torque. So there is nothing wrong with always using the full microstep resolution of 256. The ADI Trinamic drivers have a built-in microstep multiplier (microPlyer^TM) that internally generates intermediate steps to output 256 microsteps even at lower input resolutions.

In order to take advantage of the respective torque advantages of stealthChop and spreadCycle, it may be useful to switch the chopper mode at a certain speed. At higher speeds, noise from the mechanics usually predominates, so the smoothness of stealthChop is not essential. However, in belt-driven applications, it has been shown that the belt dampens the system to such an extent that stealthChop can also show its advantages in the higher speed range.

Another reason to switch to spreadCycle at higher speeds are additional helpful functions such as the sensorless load detection stallGuard2^TM or the sensorless load-dependent current adjustment coolStep^TM. These procedures can only be used in conjunction with spreadCycle.

Current reduction to reduce volume

Another rather simple way to reduce the noise of a stepper motor is to lower the motor current. At first glance, this seems trivial, but here, too, modern motor drivers offer intelligent solutions. The basis here is very simple: If a spring-mass system is excited with less energy, it also oscillates less.

In practice, work is often carried out according to the principle: if the stepper motor system does not run stably, the current is increased. But there are cases where exactly the opposite helps. By reducing the current, the system is less prone to oscillating and the problems do not occur.

A stepper motor driver with a current chopper injects a constant current into the windings. This is largely independent of the required load. With this current, there is a load limit depending on the motor. If this is reached, the rotor can no longer follow the magnetic field and the motor loses steps or stops (stall). To prevent this from happening, a safety margin of about 20 - 50% is planned in practice. This means that the motor constantly receives too much energy, which increases the tendency to oscillate and thus the noise level (in addition to energy consumption and engine heating).

ADI Trinamic's intelligent stepper motor drivers feature the stallGuard2 sensorless load detection system. This allows the current load on the motor to be determined very finely. The load-dependent current adjustment coolStep uses this value and lowers the current depending on the load on the motor. In this way, the motor only gets the power it needs to reach the current load. The engine receives less energy – noise is reduced.

For the use of stallGuard2 and coolStep, a serial interface is required for parameterization. Therefore, these functions are reserved for the intelligent "smart drivers".

ADI Trinamic also offers a function for the simple drivers with clock and direction control to reduce the current. At a standstill, the motor current can be automatically reduced to e.g. 1/3. This saves up to 70% energy and drastically reduces system heating.

Recommendations for an acoustically (and thermally) optimized layout

As already mentioned above, optimal current control is the prerequisite for low-noise operation. Therefore, special attention is required when connecting the measuring resistors and the ground connections.

The pictured ideal circuit shows only the driver, the RSense measuring resistors and the ground connections. In practice, the parasitic properties of conductors and components have an influence on the precision of the measured voltage. The TMC260 is considered as an example. However, the basic considerations apply to all stepper motor drivers.

In the following example, only the parasitic resistances are considered. Since the current chopper switches the motor current within a few 10ns, the dynamic behavior of the circuit also plays a significant role and is crucial for the quality of chopper operation. Therefore, in practice, the parasitic inductances must also be kept as low as possible.

The example of a bad layout shows that the traces alone increase the value for the measuring resistance by 25%, which causes a corresponding reduction in the maximum current. The common ground connection causes dynamic problems because the measurements influence each other.

To improve this layout, three essential steps are necessary:

1. Reduce parasitic resistances and inductances in the phase current paths
2. Separating the phase current and measurement paths (BRx & SRx)
3. Disconnect the ground connections to the measuring resistors and to the IC

Implementation:

1. Make the lines to the measuring resistors as short as possible. Use of a ground plane with many vias. Keep traces as wide as possible.
2. Separation of the motor and test leads at the measuring resistor.
3. Use of a ground plane with many vias.

In practice, it has been shown that the use of a 2-layer board is possible, but this places very high demands on the layout. A thermally and electrically good layout can hardly be ensured with two layers. A 4-layer layout allows the use of large ground planes and simplifies the use of short conductor tracks.

Result

Strength lies in calmness. The new control methods not only dampen the vibrations and resonances directly on the motor to an inaudible level, they also ensure that the coils no longer report due to their magnetically induced deformation. This means that the drives, which are considered loud, now work amazingly quietly. Thanks to complete evaluation boards, the design-in of the stepper motor drivers is done quickly. The ears of the users will thank you!

Author

Dipl.-Inform. Bernhard Dwersteg is co-founder, partner and head of IC design at ADI TRINAMIC Motion Control in Hamburg.

Dipl.-Ing. (FH) Jonas P. Proeger is Marketing Director at ADI TRINAMIC Motion Control in Hamburg.

Trademark

spreadCycle^TM, stealthChop^TM, stealthChop2^TM, stallGuard2^TM, coolStep^TM, microPlyer^TM are registered trademarks of ADI TRINAMIC Motion Control GmbH & Co. KG

Company profile MEV Elektronik Service GmbH

MEV is a distributor/stocking rep. and manufacturer representative for electronic components, modules and systems. When looking after customers in Germany as well as in Central and Eastern Europe, the focus is on optimal technical support and advice from 14 engineers. MEV focuses on state-of-the-art applications in leading market segments. Customers have access to the in-house laboratory and demonstration rooms in which the best possible technical solutions can be developed together with the help of FAEs, especially in the areas of power management, motion control and optoelectronics.

In addition to design-in-oriented distribution, MEV also focuses on logistics concepts and services that are precisely tailored to the respective needs of customers. The open, honest and reliable partnership is the basis for successful cooperation with customers and suppliers, as well as within the team.

MEV Elektronik Service GmbH
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D-49176 Hilter
info@mev-elektronik.com
www.mev-elektronik.com

Company Profile ADI TRINAMIC Motion Control GmbH & Co.KG

ADI Trinamic, headquartered in Hamburg, Germany, supplies integrated circuits and modules for motor control to customers worldwide. ADI Trinamic's application-driven approach and in-depth understanding of applications make it possible to provide solutions that simplify and shorten the design phase, enabling significant savings in development labor and costs and total cost of ownership. ADI Trinamic's customers benefit from the company's extensive know-how in the field of engine physics as well as the portfolio of intellectual property rights (IP), which have been developed from years of application experience. Product development at ADI Trinamic focuses entirely on miniaturization, increasing efficiency as well as diagnostics and protection functions that ensure the reliability of the overall system.

ADI Trinamic's mission statement is to provide energy-efficient solutions. Industry-leading technologies, such as the patented coolStep^TM products, offer not only user-friendliness and the precision of stepper motors, but also energy savings.

The ADI TRINAMIC Motion Control Language (TMCL) simplifies the development of motion control applications and enables shorter development cycles and time-to-market.

With its more than 20-year history and the traditional German ownership structure with all shares in private hands, ADI Trinamic can guarantee long-term availability.

ADI Trinamic products are sold through a worldwide distribution network.

ADI TRINAMIC Motion Control GmbH & Co. KG