Open-loop stepper motor versus closed-loop stepper motor systems

By Jeff Kordik, CTO • Applied Motion Products Inc. || Step motor systems are a bedrock of the motion control industry. We’ll look at the differences between open-loop system vs. closed-loop system and also explain the latest developments making step motor systems even faster, quieter, and more energy efficient than ever before.

Step motor systems have come a long way from the early days of voltage drives and full stepping. First came PWM drives and microstepping and then digital signal processors (DSPs) and anti-resonance algorithms. Now, new closed-loop stepper technology is ensuring that step motors continue to be a cornerstone of the motion control industry for years to come.

Whether the motion is linear or rotary, two top considerations that dictate which motor and drive systems are most suitable are torque and efficiency. This applies whether the final application is an automated assembly system, a material handling machine, a 3D printer, a Cartesian positioner, a peristaltic pump, or one of countless other applications in which step motors are a preferred technology.

The latest development in stepper systems is the application of low cost, high resolution feedback devices and advanced DSPs to close the loop on stepper motion. Such controls boost closed-loop stepper performance to outperform open-loop systems. As we’ll see, one such closed-loop system implements on an integrated motor design that includes a feedback device, driver and controller boards, power, communication and I/O electronics, and system connectors on the motor’s side and back.

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

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

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

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

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

Open-loop vs. Closed-loop efficiency

To measure the relative efficiency of an open-loop vs. closed-loop system, assume we repeat the same test with the same two motors of equal size. This time we have the closed-loop and open-loop motors run side by side with the same inertial loads but run programming that holds the move profiles constant and equal, so that both systems perform the same amount of work.

While the two motors index the same move profile repeatedly, current draw from the dc power supply feeding the two systems is measured and power consumption is calculated. As can be seen in value plots, average power consumption of the open-loop stepper system is 43.8 watts, while that of the closed-loop system is only one third as much — 14.2 watts on average. This dramatic difference in power consumption clearly shows the higher efficiency operation of the closed-loop system. Any user looking to increase the system efficiency of their open-loop stepper system can now consider a simple upgrade to a closed-loop system and expect significantly lower consumption.

How to address motor heating

A natural extension of the power consumption tests is the investigation of motor heating. Open-loop stepper systems are simple beasts. One simply sets the drive for the motor’s rated current and the drive will do its best to supply that current to the motor at all times, whether the resulting torque is needed or not. This often causes the generation of heat instead of energy towards the application function — and is the reason why open-loop stepper systems typically run hotter than closed-loop counterparts. It also means that machine designers must take additional steps to deal with this heat, often by including special guarding around step motors that will run in the vicinity of human operators, or by installing additional cooling systems such as fans.

Consider the results of a motor heating test conducted in a laboratory using the same open-loop and closed-loop systems as above. In this test, the two systems again produce the same amount of work driving the same inertial loads, and are allowed to operate until they reach thermal equilibrium. The open-loop system reaches a case temperature of 76.0° C, whereas the closed-loop system reaches thermal equilibrium at a case temperate of only 36.9° C — less than half that of the open-loop system. This significant reduction in motor heating can mean lower component costs for machine builders, because they can omit extra guarding and cooling subsystems.

Noisy motors no more

Another common complaint about open-loop stepper systems is that they are known to make quite a bit of audible noise. In certain environments, such as laboratories, hospitals, and offices, this noise can pose a real problem for machine designers.

The noise emitted by step motors arises from high electrical frequency and rapid flux changes in the stator teeth, and because open-loop systems are operated at full rated current regardless of load. Closed-loop stepper systems, on the other hand, supply the motor with just enough current to control the load and this results in much less audible noise.

To produce the test results shown in the plot of acoustic noise accompanying this article, the acoustic noise of each system is measured in a soundproof chamber. The closed-loop system is dramatically quieter than the open-loop option at speeds from 0 to 20 rev/sec. This speed range coincides with the real-world speed range of applications where step motor systems are most often used, meaning that the vast majority of step motor applications could benefit from reduced motor noise if switched to closed-loop systems.

Better motor accuracy to eliminate position errors

Open-loop step motor systems are prized for their ability to precisely position loads without a feedback mechanism or closed-loop control system, but only if the open-loop system has sufficient torque margin such that position errors will not occur during normal operation. For improved accuracy, and for a more robust system design, closing the servo position loop around feedback from the high resolution encoder allows closed-loop systems to compensate automatically for increases in torque demand that would otherwise lead to position errors in open-loop systems. This greatly improves overall system accuracy, particularly for highly dynamic applications such as pick-and-place systems and 3D printers where short, fast moves and frequent changes of direction are needed.

Upgrading existing stepper systems

Of the components in an integrated step motor system, the motor, power amplifier and communication costs generally will not increase when going from open-loop to closed loop. The control electronics may require a bit more central processing power or memory to servo control the motor, but these typically don’t have an impact on list prices. Much of the cost difference between open-loop and closed-loop stepper systems lies in the addition of a high resolution feedback device, but refinements in manufacturing have made these devices increasingly affordable. So now, closed-loop stepper systems maintain the cost benefits of open-loop stepper systems over other types of positioning systems — such as a traditional servo — but with greatly increased performance in nearly every way. Typically, the energy savings and increased throughput of a closed-loop system quickly pay for the slight increase in cost of the feedback device.

In addition to a minimal cost increase, upgrading from an open-loop stepper system to a closed-loop system is simplified with NEMA frame size offerings. A closed-loop NEMA 23 step motor has the same frame size, pilot diameter, bolt hole circle and bolt hole diameter as an open-loop NEMA 23 step motor, so mounting brackets stay the same. The greater torque available from the closed-loop system means the shaft diameter of the closed-loop step motor may be larger, but this can usually be solved quite easily with a simple change of the shaft coupling.

StepSERVO closed loop step motor technology is a product of Applied Motion Products. StepSERVO closed loop integrated motors were used in the quantitative examples in this feature. Get full results of the lab tests discussed in this article at www.applied-motion.com/stepservo-white-paper.

Reprint info >> || Applied Motion Products Inc. • www.applied-motion.com