Is a stepper or servo best for torque optimization?

A machine builder wants to know the tradeoffs between stepper motors and servo motors for its CNC machines

We've been using stepper motors in the CNC machines we build. However, we sometimes lose steps because of the heavy table. We’ve considered putting a feedback device on the stepper application to optimize torque, but is there a better solution? Would a servo motor solve this? What are the tradeoffs?

 

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  • <p>Steppers and servos have a number of similarities and a couple of key differences. Steppers and servos are both synchronous motors. The construction of both motor technologies consists of a rotor with permanent magnets and a stator with coiled windings. Both systems operate by applying a dc voltage to the stator windings in a specific pattern that results in movement of the rotor. Both technologies are capable of position and speed control.</p> <p>The two main differences between stepper and servo systems involve the use of a feedback device and the complexity of the amplifier electronics. A servo by definition is a closed-loop system utilizing a feedback device. Steppers are open-loop systems with no feedback. The amplifier electronics utilized in a servo system are typically much more complex than that of a stepper system. While a stepper amplifier simply sends full rated current to each winding set, a servo amplifier regulates the current levels it sends to the motor windings. In a servo system, only the current demanded by the application is used. Since current is proportional to torque, the control loop in the servo amplifier that regulates current is called the torque loop. The servo amplifier also employs velocity and position control loops. The ability of a servo amplifier to close the torque, velocity and position control loops ensures that synchronism in the motor can be maintained. Since a stepper system has no feedback and no control loops, motor stalling can occur when torque demand exceeds available torque at any given speed. There are also some inherent performance differences between steppers and servos based on the motor design. Stepper motors by design have a larger number of poles and a higher winding inductance than servo motors. As a result, available torque of stepper motors drops off much more quickly than that of servo motors as speed increases (given the same dc bus voltage).</p> <p>Another inherent disadvantage of the stepper motor design is the existence of two distinct instability regions. There is a low-speed region of instability (typically between 100 and 300 full steps per second or 30 to 90 rpm) that results from the excitation of the natural frequency of the motor. When the motor is operated in this region, there will be a large velocity ripple and a potential loss of steps (and loss of position). The effect of low-speed instability can be minimized through the use of a technique called microstepping. There is also a mid-range instability that also results from the excitation of the natural frequency of the motor. This typically occurs at the speed where motor output torque is one-half of the full running torque of the motor. Mid-range instability can cause motor stalling, velocity ripple and loss of steps (and loss of position). There are some electronic damping techniques that can minimize the effects of mid-range instability. The best practice when using stepper systems is to steer clear of operating in these two speed ranges.</p> <p>If you are using stepper-motor technology, there are a number of reasons to upgrade to a servo system.</p> <p>• Increased machine throughput: When you upgrade to a servo system, the output torque capacity at higher speeds opens up dramatically. As a result, you can utilize the additional torque at higher speeds to modify the application move profile.</p> <p>• Increased accuracy and repeatability: The accuracy of a stepper system is limited by the number of physical full steps per revolution (there are 200 full steps per revolution in a traditional two-phase stepper motor). The repeatability of a stepper system will vary with the amount of frictional load in the system. Because a servo system employs a feedback device, it can achieve much higher levels of accuracy and repeatability. The position control loop in the servo amplifier will assure that the servo gets to the position that has been commanded, regardless of changing conditions.</p> <p>• Less down time: Stepper systems are ideal for applications where the conditions do not change. In the real world, there are not many applications where conditions do not change over time. The characteristics of mechanical components can vary with temperature and time (increasing or decreasing frictional loads). Users of the machine can introduce loads or duty cycles that fall outside of the specifications of the machine. Since stepper motors stall when their rated torque is exceeded, machine throughput can be affected when changing conditions get introduced. Servo systems have the ability to warn the user when changing conditions are introduced. Since the servo amplifier keeps track of torque, speed and position via the control loops, this information can be used to prevent stoppages in production.</p> <p>• Higher efficiency: Steppers operate by sending full rated current in sequence to the motor windings while the motor is moving, regardless of the application requirements. Most steppers have an idle current reduction setting, which allows the current level to be reduced automatically when the motor is not moving (reduced level is typically 50% of full rated current). Servo motors only use the current required in the application at any given point in time. The step motor uses full rated current while the motor is moving and uses half rated current while the motor is idle. The servo motor uses much less energy as it only uses the current required.</p> <p>• Energy savings: A significant amount of energy can be saved with a servo when compared to a stepper system. Given the following repeating move profile (acceleration = 85 ms; constant velocity = 1,000 ms (at 2,000 rpm); deceleration = 85 ms; dwell = 1,170 ms; stepper current at dwell = ½ rated stall current of motor; total duration of test = 10 min), the servo motor maintains a relatively low temperature (30 °C) while the stepper motor maintains a much higher temperature (70 °C).</p> <p>There are also a number of arguments against upgrading from a stepper to a servo.</p> <p>• Cost: Servo systems are typically more expensive than stepper systems. This is due to the addition of a feedback device on the servo motor (and consequently a feedback cable) and more expensive amplifier electronics. A number of low-cost servo options have entered the market in recent years. In many cases, the cost difference between stepper and servo is not significant. Upgrading to a servo can be as little as a 10% increase over a comparable stepper system.</p> <p>• Commissioning time: Since a servo is a closed-loop system with multiple control loops, there is usually additional setup time required for adjustment of tuning gains. Some servo products, like Yaskawa’s Junma servo series, have been designed specifically to replace stepper technology. Part of this design is reducing the effects of servo complexity to the user. With the Junma servo system, load inertia is detected automatically and adaptive tuning algorithms calculate optimum gains for the control loops without any user interaction. The amplifier comes with a pulse and direction input so in many instances, the same controller and motion program used with an existing stepper system can be used with the replacement servo system.</p> <p>• Application requirements: Some applications do not justify an upgrade to servo technology. In fact, there are applications where a stepper system is a better fit. Stepper systems work best in applications where the conditions are unchanging and predictable. The majority of applications have some level of variability. In most cases, upgrading to a servo will allow for higher throughput and will eliminate issues typically seen with steppers like lost steps, low and mid-range instability and stalling.</p> <p>If your current machine design employs a stepper system, it only makes sense to investigate options for upgrading to servo. With today’s technology and pricing levels, you can make a significant improvement to your machine design without a major impact to your bottom line. </p> <p>Scott Carlberg, product marketing manager, Yaskawa America, www.yaskawa.com</p>

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  • <p>If you are losing steps it could be a sizing issue. A larger stepper could solve the problem. It is typical to purposely oversize steppers to avoid this. Also a lot of applications will give the motor a few more pulses on the return stroke to ensure it gets back to a known position (i.e. drive it into a hard stop). Although this will not help with the loss of pulses encountered on the out stroke, it will prevent any positional error from building up and will ensure subsequent moves start from a known position. You see this type of thing in vending machines that use steppers to dispense products. Steppers operate in peak current all the time so they would not be as efficient as a servo motor which only uses as much current as it needs to do the work (i.e. as you twist harder on the shaft, more current will be required to hold the motor in position). I would recommend using a servo rather than adding an encoder to a stepper. Servos will provide more accuracy, precision and performance (i.e. get into position quicker and more accurately) but they are a bit more complicated and require tuning of the servo drive’s current, velocity, and position loops. Most servo manufacturers have software that will do a lot of the tuning for you. If sized properly, the improved performance generally outweighs the added complexity. Since a servo motor only uses current when necessary and can position quicker and more accurately than a stepper, you can reap the benefits of better efficiency and quicker cycle times (more throughput). Steppers are used in many applications because they are generally considered simpler and less expensive. As technology has evolved over the years, the price difference between servo and stepper has gotten smaller and is no longer the determining factor it once was. Because of the added performance that can be gained, I would definitely recommend looking to servo, as a solution. Below I have summarized some characteristics of both technologies. </p> <p>Steppers are used for positioning applications – typically open loop – there are less than 10% running closed loop. Open loop means that steppers can lose steps – especially when the load increases – thus they should be used in constant (or near constant) loads. Steppers typically have resonant speed areas, which should be avoided (i.e. do not operate in). The peak torque and the continuous torque is the same for steppers. Usually stepper manufacturers recommend using a safety factor on the amount of torque for an application – somewhere between 50-100%. </p> <p>Servos are used for positioning applications, and position very fast due to lower rotor inertia. Note the linear characteristic and speed controllability is down to zero rpm. The motor has higher speed capability. (i.e. application gets to speed faster) Servos provide higher peak torque – therefore higher acceleration capability – i.e. the machine positions faster – it can make parts faster. </p> <p>Steppers are less expensive as they do not usually include the feedback device. Servos are “more complex” since they operate in closed loop with feedback. Steppers are typically used for lower power - in applications like: X-Y tables, packaging, labeling, semiconductor equipment, and instrumentation. As long as steppers are operated within their design limits, they perform adequately. Servos are used in the same applications as steppers, however in the moderate to higher power range. </p> <p>Bob Merrill, Product Manager - Servo Motors, Baldor Electric Co, www.baldor.com</p>

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  • <p>The simple thing to do would be to make the move at a slightly reduced motion profile and give a little more margin for the speed-torque operation point. That would probably eliminate the “missed” steps issue. However, it is not typical for a step motor to miss just a few steps; they normally stall if they lose steps, in which case the motion would not be just a few steps short. If you have enough safety margin in the torque/speed performance, you should not require a closed-loop/servo and you should never have position errors. </p> <p>You may want to consider how you might upgrade systems already in the field. Adding an encoder to the existing stepper system would require a change to the control, programming and wiring, and would add cost to your stepper system, essentially converting it to a servo system.</p> <p>A true servo has encoder feedback as an active part of the control, so it will measure and adjust or correct for error during the motion. If the motor is lagging behind, the control will tend to push more current to provide higher torque in order to overcome the error. This is a more dynamic and real-time error correction during the motion, as opposed to correction for position error after the move has been done, as with a stepper system.</p> <p>A servo system could solve your problem if it is sized properly for your application. Of course, the control/drive and other changes will need to be made to your system if you choose this option. Consider cost vs. performance, as it ultimately comes down to return on investment in designing a new system.</p> <p>Gary Whiteman Applications Engineer Nippon Pulse America, Inc. www.nipponpulse.com</p>

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  • <p>Continuing to use a stepper motor but adding encoder feedback is probably the best solution. It sounds like the stepper motor is basically meeting the application requirements with the exception of lost step counts from time to time. The lost counts may indicate moments where the accelerations are too aggressive. If these accelerations are required for the application, then a motor with more torque may be required, but for the sake of this response I will assume that isn't the case.</p> <p>Using encoder feedback (and the corresponding drive) will add cost, but will prevent the loss of steps. The benefits of using a servo motor are generally seen in the dynamics of the motor in terms of smoother motions. Servos can also achieve higher speeds. Neither of these seems to be an important factor here, so the addition cost of the servo motor and associated drive do not seem worth it.</p> <p>The last consideration is the type of encoder used. An incremental encoder provides the lowest cost solution, but will require the machine to be homed when powered up. If a multi-turn absolute encoder is used, however, the machine would only need to be homed once and then would always retain its position. The absolute encoder will add cost and the associated stepper drive would likely be more expensive.</p> <p>Tom Worsnopp, Product Manager of Servopneumatics and PLCs, Festo, www.festo.com/us </p>

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  • <p>This answer was submitted by Gil Guajardo, product marketing manager at Bimba.</p> <p>A stepper motor is an exceptional motor selection for torque optimization. And often is the best selection for a CNC machine application or any motion control application. Similarly servo motors may often be the optimal motor drive technology in an abundance of these same applications. The ultimate answer lies in the details of the application and the solution parameters that the right solution requires. To that end, I cover below key considerations that a motion control designer must take into account and consider to help answer your question of finding the best alternative for moving heavy loads reliably and with longevity.</p> <p>• stepper motor torque capability is superior for a similar size servo motor</p> <p>• stepper motor inherent torque advantage makes it well suited for heavy loads</p> <p>• ideal for positioning heavy loads at low speeds</p> <p>• outstanding repeatability</p> <p>• proper stepper motor sizing upfront will ensure no “lost steps”</p> <p>• ideal for well-defined static loads</p> <p>• no feedback solution offers less cost and complexity than a servo motor solution</p> <p>• precise positioning via digital input pulses</p> <p>• optional encoder enhances reliability and prevents potential damage due to “lost steps”</p> <p>• encoder detection of missteps and stalls is quickly known and can be addressed</p> <p>While certainly helpful to prevent missteps and faulting, an encoder, in it of itself, will not increase torque or optimize torque. Instead, it is the stepper drive that contains advanced algorithms that help to optimize the torque utilization. Typical drives achieve this by using the encoder to monitor the lead angle of the motor, a measure of torque utilization. If the motion profile begins to demand more torque than the motor can produce, the velocity is automatically reduced before the motor stalls. Again, it is the stepper drive that copes with the torque optimization using information provided from the encoder.</p> <p>A servo motor is not always a better solution than a stepper solution for a well-defined heavy load application. Generally speaking, a comparable size servo motor does not have more torque capability than a stepper motor. However, there are distinct advantages that a servo motor offers not available with stepper motor. These advantages include…</p> <p>• inherent encoder prevents missteps</p> <p>• additional “Peak” current curve offers enhanced torque and hence acceleration capability</p> <p>• constant speed performance curve offers clear advantage over “slower” stepper motors</p> <p>• faster positioning capability</p> <p>• servo motor inherent design adapts to changing dynamic loads</p> <p>• advanced algorithms including torque control offer more precise control</p> <p>• servo motors run more efficiently and cooler than stepper motors leading to energy savings</p> <p>• servo motor require “tuning” which provides precise, repeatable and defined motion profiles</p> <p>• servo drives allow adjusting of the position error fault leading to greater precision and position control</p> <p>As demonstrated above, a servo motor can offer many advantages but to be fair there are some potentially negative characteristics one must be cognizant of when considering a drive motor for any application. For a servo motor they include: additional Cost, increased complexity, longer implementation time, and tuning. But, these facts may be only a small hindrance to obtaining peak and optimal performance. In summary, the answer to your question is “maybe”. Depending on the application circumstances, a stepper is often the best solution. In some cases a stepper with an encoder is the answer. And, yet in other cases, a servo motor is the clear and best selection. A thorough review of the application with all the application parameters, variables and expectations taken into account is the right first step to arriving at the best motor drive solution. And, help and guidance in making certain that all varying parameters are considered in the beginning stages.</p> <p>Gil Guajardo, product marketing manager, Bimba Manufacturing, www.bimba.com</p>

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  • <p>While stepper motors still clearly have their place in machine design, such as in lower-accuracy point-to-point applications, servomotors do a much better job addressing the motion complexity and requirements common to most CNC applications. As was indicated in the reader’s question, adding a feedback device may help to solve some of your step loss issues; however, implementing the feedback device adds the need for a more expensive amplifier, as well. This adds cost and complexity to the system while taking up valuable cabinet space. Even with the added feedback device, stepper motors are also more likely to stall out, due to added difficulty when dealing with changes in friction or material mass. The overall cost and complexity here could match or exceed that of a servo alternative.</p> <p>The choice of a servomotor solution, in lieu of stepper motors, does carry with it a higher initial cost. However, once you consider additional the components and engineering work required to use a stepper system in a more complex application, the cost gap narrows considerably. Most importantly, you should consider the ability of a servo system to run the machine with higher performance and efficiency. The resulting increased production throughput and lower energy costs translate into a rapid ROI that will quickly become evident. EtherCAT I/O terminals are available from Beckhoff, such as the EL7211 with integrated servo drive functionality, representing a compact and cost effective means to add servo drive functionality to a CNC application, which also lowers the machine footprint. The terminals in the EL72xx range are rated for use at 50 V DC and from 2.8 ARMS to 4.5 ARMS continuous output current. The Beckhoff servo system also scales up to cover much higher power and load requirements with full sized servo drive options (1.5 to 170 A rated continuous current and 100 to 480 V DC voltage range).</p> <p>Should a machine builder or integrator be fully set on using a stepper system, companies like Beckhoff also offer further solutions such as EtherCAT I/O terminals for stepper motors with integrated vector control to eliminate stalling issues. For CNC applications and other complex equipment however, we typically recommend an all-encompassing servo system as the more reliable solution with higher performance and precision. More information can be found by visiting www.beckhoff.us/compact-drive-technology </p> <p>Robert L. Swalley, Motors and Drives Product Specialist, Beckhoff Automation </p>

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  • <p>Submitted by: Paul Coughlin, Team Lead Technical Support, KOLLMORGEN</p> <p>CNC machines are excellent designs for the use of stepper motors. The stepper motor is compact and powerful, offering high torque up to mid-level speeds of 1,500 RPM. Whereas the servo system boasts high speed, accuracy and efficiencies, you pay for these items. Usually with higher costs in the controllers, but certainly in the advanced technology of feedbacks. The choice is never ending, with BiSS, EnDat, Hiperface, Absolute, Multi Turn, Single Turn, Resolvers, Incremental encoders. All require unique set up to the servo amplifier, multiple wires and proper shielding.</p> <p>By contrast the stepper motor has only 4 wires and a ground. It requires no tuning and offers out of the box compatibility to most stepper drives. Due to its open loop design, meaning no feedback, it offers precise repetitive motion at a relatively low cost. Additionally, the motor offers higher Holding Torque, without the concern of dithering. During stand still the stepper motor offers up to 20% higher torque and remains rock solid, whereas the servo motor can jitter in its servo loop. When properly sized, utilizing a safety margin of 50%, the stepper motor can be not only repeatable, but accurate with positioning.</p> <p>True, there are two areas of the performance curve to be concerned about. The first is at relatively low speeds of 30 – 100 RPM, which is the natural frequency. However this is mostly a thing of the past. Over the years microstepping drives, and advances in the balancing of the drive currents within the two stepper motor windings, have filtered this resonance area out, or at least reduced it to a relatively negligible concern. The advances in microstepping technology allow the stepper motor not only to accelerate through this low end resonance, but to actually run within this area of resonance. </p> <p>The other area of concern is what is commonly known as mid-range instability. However, once again, the advancement of the stepper drive technology has been offering numerous methods to automatically correct for this resonant phenomenon since the mid-1980s. Simply varying the pulse width discourages oscillation, while still maintaining the needed position requirements by stepping on the leading edge.</p> <p>Another great advantage stepper motors have over servo motors is the commonality of the mounting. Almost all stepper motors use the Nema mounting dimensions for pilot, mounting holes and sometimes shaft sizes. I say sometimes, because the industrial stepper motors of today offer much higher torque than the motors of only a few decades ago. Therefore the shaft diameters have increased in size to be able to transmit these higher torques. </p> <p>This commonality to design means quick replacement should something with the motor go wrong.</p> <p>High torque, low speed, compact size, commonality of mounting, repeatability and ease of connection make stepper motors a great choice CNC machines.</p> <p>However, if the CNC has variable loads and/or vibrations, or need precise velocity control, then servo motors are a must. The open loop design of the stepper motor is at a disadvantage when load characteristics are shifting and cannot be stabilized. </p>

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  • <p>The ideal motor choice ultimately depends on your application requirements. If the motor is not properly sized, synchronization may be lost regardless of servo motors or stepper motors. Oriental Motor offers both stepper and servo options. Generally, servo motors feature high speed, high efficiency, and less noise due to lower winding inductance, lower current utilization, and less pole counts respectively. However, servo systems are more complex and costly since encoder feedback and a PID loop for velocity, torque, and position control are required. A stepper motor system can be open-loop or closed-loop. Both stepper systems would be less complex than a servo motor system. Since stepper motors typically utilize full current regardless of load, they feature the highest torque per volume but at a cost of operating temperature and therefore duty cycle. Servo motors only utilize the current necessary for the load, and since current is proportional to temperature, servo motors typically run cooler. Adding a position feedback device to the stepper motor to close the loop itself does not help optimize torque. The primary task of an encoder on a stepper motor is to provide pulses for position verification. Something else is necessary in order to optimize the torque. We do offer a patented technology on our Alphastep AR closed-loop stepper motor and driver series called "Keep-In-Step (KIS)" that optimizes torque on stepper motors based on rotor position feedback as well as self-correction on the fly to keep synchronization with input pulses. The Alphastep AR series also offers patented motor technology to enable higher efficiency, lower heat generation, and continuous duty operation. With products like the AR series, we have resolved the disadvantage of limited duty cycle for stepper motors.</p> <p>Traditionally, servo motors are better for very fast and long moves while a step motor is better for quick and short moves. For applications such as CNC machines where positioning accuracy, synchronization between axes, and timing are critical, step motors may be a better choice because its response time is quicker than servo motors. The high pole count toothed rotor from the step motor allows the operator to command it to rotate to a very specific target position and immediately stop, settle, and hold. The structure of a servo motor is similar to a brushless DC motor based on a 3-phase winding and a permanent magnet rotor. It does not have a high pole count toothed rotor like a stepper motor, so it has to move to a position close to the target position, then "hunt" with the encoder until the positioning error is minimized to an acceptable level. This effect can potentially increase delays in between moves in the same axis or multiple axes. Because a servo motor has a built-in encoder (and positioning accuracy depends on encoder resolution), the servo motor driver (or amplifier) will also need to be more complex in order to quickly interpret the encoder signals and work with a PID loop. A typical step motor system has much less components than a step motor system and does not utilize a PID loop. It's important to point out that if a step motor is sized properly for the application requirements, even without an encoder to close the loop, the step motor would not miss steps. We offer closed-loop stepper motor systems for users who want the peace of mind of verifying if a move has completed or not.</p> <p>For the rest of the article with visual aids and test data, please click the following link: <a href="http://forum.orientalmotor.com/viewtopic.php?f=18&amp;t=978">http://forum.orientalmotor.com/viewtopic.php?f=18&amp;t=978</a></p> <p>Johann Tang, Technical Support Engineering Supervisor, Oriental Motor USA, <a href="http://www.orientalmotor.com">http://www.orientalmotor.com</a></p>

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  • <p>Abe Amirana, Director, Teknic, Inc.</p> <p>Great question! As previously mentioned, a stepper motor is an open-loop, undamped, spring mass system. Historically, these motors do have some advantages but, in the end, adding an encoder would only verify that you’ve stalled or lost steps (the electronics would typically “check” at the end of a move). However, in a CNC application, minimizing position error at all times is critical as positional errors will leave a permanent distortion in a machined part (unlike a “point-to-point” application where position error, enroute, is of lesser concern). </p> <p>Before I address your question let me be upfront and state that, here at Teknic, we don’t make stepper motors, only servos – but I’ll try and answer your question as objectively as possible.</p> <p>So what to use to solve your problem? Personally I’d love to see some more data here and that’s one of the fundamental problems with a stepper – it’s open loop and you never really know just how much torque you are using. Because if you ever “touch” the line of the stepper torque/speed curve, you seriously risk losing steps or stalling – that’s why most stepper manufacturers will recommend that you derate their stepper torque/speed curve (50% as a rule of thumb). But other than run an axis repeatedly, there’s little a machine designer can do to really figure out how much torque they need/use versus what the motor can produce. In other words, what’s my design margin (some will emulate a highly accelerated long term test or use current probes but these techniques are far from foolproof).</p> <p>A good, high end servo will typically use an internal processor (usually a DSP) and will execute a servo compensator along with a digital, “vector” torque algorithm – some fancy language for basically guaranteeing that the proper torque is always delivered, at the right moment, to faithfully follow your command (minimizing position error at all times so you get a nice, smooth cut on your parts, regardless of friction). But the servo’s inherent diagnostic tools also provide a window into your mechanics, which let’s you see, in real time, how much torque the motor is using, its positional accuracy (both on-the-fly and end of move settling) as well as your overall design margin (it’s also really interesting to see the command that the CNC controller is sending). </p> <p>I won’t belabor many of the points which were brought up elsewhere regarding the pros/cons of steppers versus servos. They’re largely correct, that is: Servos are fully closed loop devices – they are designed to check continuously and deliver torque to minimize positional errors regardless of friction or load. Servos run cooler (they are significantly more efficient), quieter (audibly), smoother (you won’t experience any resonance throughout their entire, specified speed range) and they have substantially more power in the same frame size. I emphasize power here (being the product of torque and rotational motor speed). The same sized servo (let’s say a single-stack, Nema-23 motor) will have anywhere from 3 to 11 times the power of a comparably sized stepper motor! That is, if the mechanical gearing is appropriately selected, you can move a substantially larger inertial load, generate a much higher cutting force or simply cut faster (assuming your tool bit can handle this). You also don’t need to derate the servo’s torque/speed curve and when you compare the curve to that of a stepper you’ll see that that servo’s peak torque is often available to almost 4,000 rpm (typically limited by the system’s primary input bus voltage). </p> <p>The two other points worth mentioning are cost and complexity – historically steppers are lower cost and easy to deploy (however, no one ever mentions how hard it is to track down a stepper axis that is intermittently losing steps or stalling) while servos are known for their higher cost and increased complexity. But the gap has closed rather substantially in both regards. New, fully integrated servos (which include the digital drive, the high resolution encoder and the brushless motor all within a compact package) have reduced wiring, require less cabinet space, and include advanced autotuning algorithms, making setup and deployment far simpler than ever before. I would check out Teknic’s new ClearPath, All-in-One integrated motors and compare pricing (it’s all online) at <a href="http://www.teknic.com/clearpath"> www.teknic.com/clearpath</a>. Focus on the “Step/Dir” modes as they are a drop-in replacement for a stepper motor.</p> <p>Hope you found this helpful, best of success with your machine!</p> <p>- Abe Amirana, Director, Teknic, Inc. - 5/5/2015 </p>

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  • <p> I would break this problem down into the following questions:</p> <p>1. Can I alter the current setup or machine settings to solve the problem? 2. Will stepper feedback really fix my problem? 3. Can I simply use a larger stepper? 4. Can a servo fix my problem and increase the machine performance? 5. Are servos the only alternative?</p> <p>1. Can I alter the current setup or machine settings to solve the problem?</p> <p>When you exceed a stepper's available torque, you lose the “step” (the shaft didn't move the step). Are you exceeding the torque during acceleration, run or deceleration? Perhaps slowing the accel/decel rate or the run speed can avoid the error. If the cycle time of the machine becomes too large, the existing hardware is undersized.</p> <p>2. Will stepper feedback really fix my problem?</p> <p>Feedback will not really make the stepper "stronger". The controller will see the stepper is out of position (slipped) and will try to correct. Since the stepper is already slipping, the correction will not work until the motor torque is higher than the load (typically at the end of the move). The result: corrected position at the cost of longer time-to-target. (Same result as point 1, but with the added cost of feedback).</p> <p>3. Can I simply use a larger stepper?</p> <p>If a larger/stronger stepper is available, this may be the simplest option (no training on new products, no new vendors, etc.). When upsizing motors, however, make sure the added torque is worth it - larger motors have larger inertias. The ideal situation is a larger stepper of the same frame size, but longer, as this adds the least amount of inertia. Work with your supplier on proper sizing.</p> <p>4. Can a servo fix my problem and increase the machine performance?</p> <p>A properly sized servo will solve the problem. It will provide the required torque and the closed loop positioning to eliminate errors. If a switch to servo makes sense, then take the opportunity to improve the machine - you are already forced to switch hardware at additional cost. Recover this cost increase by providing a machine with higher performance. Can you move faster, have higher accuracies or even have a quieter machine?</p> <p>5. Are servos the only alternative?</p> <p>Servo means controlled by feedback. In the automation world, it is commonly accepted that servos are permanent magnet (typically rare-earth) synchronous motors with feedback. They are designed to be dynamic - long and skinny for high torque with low inertia. Several different feedback styles are available, depending on brand and product lines: resolver, absolute, SinCos, SSI, Hiperface, EnDat and Biss. Feedback devices that support absolute positions can remove homing routines at machine power on.</p> <p>Vector systems are used for larger and heavier applications. Vector motors are AC induction motors with improved winding insulation and cooling, along with a feedback encoder. These motors are typically bigger in diameter, have higher inertias and are used in less dynamic applications. Again, several feedback options are available: encoder, absolute, SinCos, SSI, Hiperface, EnDat and Biss.</p> <p>Linear motors are a great option for some applications. Linear motors can be thought of as an unrolled, flat servo motor. No rack and pinion gears, no gearboxes, just direct drive.</p> <p> If a redesign is in your future, work with a supplier who offers more than one solution. Suppliers with only one solution will only offer what they can sell and you may not get the best solution for your money and machine.</p> <p>Scott Cunningham - Engineering Manager, KEB America, Inc., www.kebamerica.com </p>

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  • <p>Since you say "heavy table" I have to assume that it is during acceleration that you loose steps. Assuming that the machine has no mechanical problems I would consider the following: a) The motor does not have the torque required to push the tool and to accelerate the table . Increase the acceleration time and use a "S" curve profile(cycloidal profile is one of the best) , if the problem goes away the motor is probably undersized to accelerate at a fast rate. (F=feed force + m*a) b) The power supply can be a problem especially if one supply is used to supply current to more than one motor and the drives have current modulation. If one of the motor current goes from half current to full current the PS voltage drops and for an instant the motor does not have enough current to step. This is more true if the PS is a switching PS, the addition of a large tank capacitor will help. Replacing the stepper with a servo will not better the situation if the server, also, is undersized. Server can supply for short time higher torque if the reducer can take it. Certainly the servo will recoup the steps but the profile while accelerating can be out of tolerance if the torque is insufficient. The feedback is better but does not make things better if the sizing is not correct. The final cost of the systems against the performance should dictate what to use. In the cost include maintenance, service , loss of production during downtime, scrap cost, personnel qualifications and more importantly the loss of a customer.</p>

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  • <p>Stepping motors require proper sizing to prevent step loss. If the stepper drive or controller accepts quadrature inputs, then a $50 encoder should do the job. However, there are some newer stepping motor controllers that do full closed loop torque control at a lower price point than a brushless dc (or PMSM) motor. </p>

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