Choosing a stepper drive over a servo has certain advantages, namely price. Where a stepper will do the job, consider using one — but not until you've read these expert tips and tactics for choosing wisely.
- Know the four most common microstepping myths.
Myth 1: A high microstep count will result in evenly spaced steps at that count.
Microstepping is good for smooth motion. However, having a 256 microstep/step driver does not automatically mean that you will get 256 evenly spaced increments of motion from the motor for those microsteps.
Myth 2: Half stepping yields more torque than microstepping.
Half stepping is a crude approximation to a sinusoidal current waveform, whereas microstepping yields a better approximation to the sinusoidal drive waveform; the area under the curve is not changed significantly, and thus the torque is the same.
Myth 3: Full stepping yields more torque than half stepping.
Full stepping with both motor phases fully on at the same time will yield more torque than running the same maximum current per phase in a half stepping or microstepping configuration. However, a stepper motor is limited by heat generation. Therefore, running a half or microstepping waveform at a higher peak current (times √2) gives smoother motion with the same torque, heating effect, and power consumption as the full stepping mode.
Myth 4: A servo drive is better than a stepper.
A servo drive will run smoother than a stepper. However, a stepper will follow a programmed trajectory exactly, whereas a servo will not necessarily do so, due to following error that may vary with disturbances.
- Determine load requirements.
First, determine the load type. Is it an inertial or frictional load? If purely inertial, use some type of transmission to match load torque to motor torque. Use a device to measure the force required to move the load at the desired speed and acceleration. For example, you can use a spring scale to measure the force on a linear axis or a torque watch to measure rotary force.
Second, determine the application supply voltage. Then select a stepper with at least twice as much torque as required at the target operating speed, and use a motor rated at about ¼ supply voltage. Next, look at the driver current required to get target motor torque. Select a stepper driver based on that number.
Tips 1 and 2 courtesy of David Goodin, AllMotion Inc.
- Be aware of torque issues.
Torque is often at the root of many stepper-related issues. Two relatively common occurrences include:
Not enough torque at speed: Most step motors are wound and built to perform at their optimal level at a certain speed range. If a high-speed motor is used in a low-speed application, you will more than likely be pumping in lots of power and yet the motor will still stall due to lack of torque. Using a low-speed motor for a high-speed application will yield similar results.
Inertia mismatch: Motors cannot accelerate quickly with an inertia mismatch between motor and load. The key is to select a motor size that closely matches load size. Otherwise, the motor will have a difficult time accelerating a heavy, unstable load and the system could easily stall if accelerated too quickly. On the other hand, the system may resonate and cause loud noise, jittery motion, or inaccurate steps, if connected to a motor with too much torque. When the load is not heavy enough to account for the high-torque motor, everything is amplified through the motor's resonance.
- Consider voltage and current needs.
A simple way to choose a stepper drive is to look for four things — voltage, current, microstepping, and maximum step pulse rate. Ensure that the drive can handle a wide range of current so that you can test the system at different voltage levels to fit your application. The driver should output at least 1.4 times the motor's rated current. Choose a driver that has several step resolutions to test different microstepping settings to get the smoothest motion. Finally, make sure the driver can receive enough step pulses to rotate your motor at the desired speed. Sometimes drivers are limited to something small like 10 kHz. If you're hoping to microstep even at 8× with a 1.8° stepper, your maximum revolutions per sec speed is 10,000/(8 × 200) = 6.25 rps.
Tips 3 and 4 courtesy of Kayvan Abbassian, Lin Engineering
- Apply correct voltage.
Microstepping can increase the resolution of a system, which smoothes rotation and prevents vibration and noise. However, problems will arise if incorrect voltage is applied to a PWM (pulse width modulation) or chopper drive. We receive many questions about these drivers. For example, if a motor is rated at 5 V, many users wonder why they need to apply larger voltages. They also wonder why they are not getting increased performance even after changing to a PWM/chopper drive. Engineers sometimes forget about motor fundamentals like back EMF and electrical time constants when they are using stepper motors and drives. This results in an incorrectly configured stepper motor drive or driver and motor, which are starved for power (voltage and/or current) in the application.
- Understand the goal of microstepping.
When an engineer does not understand the purpose of microstepping, a number of issues can arise. The main purpose is to increase smoothness of motor operation by leveling out the shocks of stepping, making operation more reliable. By misapplying microstepping, you can actually greatly decrease the available torque that the motor can produce. This usually requires a much larger motor than otherwise necessary. Those who don't understand the proper use of microstepping opt not to use it, instead turning to servo-based systems, which add unnecessary levels of complexity and cost. Engineers also sometimes complete mechanical designs and then attempt to hide or dampen system vibration. When an engineer chooses an incorrect stepper, the motor won't be able to move the load weight. Select the motor while considering not only load weight, but also the mechanism's frictional properties.
Tips 5 and 6 courtesy of Jeramé Chamberlain, Nippon Pulse America Inc.
- Correctly match motor and drive.
Don't believe that a motor will achieve the data sheet's rated speed and torque when it is matched to just any driver. Like a servo, the motor's stall torque, rated torque, and rated speed all depend as much on the drive and motor being correctly matched as they do on available voltage and current.
A performance curve (speed-torque curve) with a matched drive is the most reliable reference. Also, remember that a motor's stall torque is not an indication of the torque it can generate while moving — especially when accelerating and decelerating at higher torque.
Consider using a software program to properly size the motor and drive based on reliable speed-torque performance curves. Select a driver that matches the available bus voltage and has the desired features; then select a motor that offers the required performance curve — outside the machine's resonant frequency — using the performance curves of the matched motor and driver.
- Size right.
Improper sizing manifests itself in numerous ways. Undersizing a motor will at best cause excessive heat, unsatisfactory acceleration and deceleration, and poor performance. At worst, the motor will lose pulses, position improperly, or stall altogether under heavy loading or high acceleration or deceleration.
Over-sizing a motor will cause it to run louder and generate higher EMI/RFI. It may also cause users to pay more for a motor and driver in terms of money, as well as panel space or machine space, than is necessary.
Proper load-inertia to rotor-inertia matching is also critical, as the system is essentially open loop. Even after adding an encoder, the inertia mismatch cannot be much more than one order of magnitude. A larger mismatch will cause the motor to lose pulses, miss position, draw excessive current, or even stall.
Tips 7 and 8 courtesy of Dan Wolke and Lee Stephens of Kollmorgen
- Understand stall conditions.
Choosing the wrong stepper drive can lead to stall conditions, which are different than outright rotor stopping. The motors can actually fall behind by a few motor poles but continue to move the load, or in some cases, overshoot if commanded to stop too abruptly for the inertial load. An encoder used as a feedback device can report that condition and/or correct for it after the commanded move is done, but it cannot prevent it. Even with an encoder, a stepper inherently remains an open loop system.
- Use steppers to save money, but do it wisely.
Stepper drives always offer the cheapest solution, so use a stepper wherever appropriate. Remember these major considerations: First, does the system require position confirmation? Second: The wrong stepper drive can cause ringing, resonance, and poor low-speed performance. Third, during high speeds, stepper motors can whine. Because stepper drives have a high pole count, hysteresis and eddy current losses are also common at high speed; for these reasons, a stepper is not recommended for continuous operation above 2,000 rpm. Finally, because full current is needed to produce holding torque, step motors can get hot at a standstill.
Tips 9 and 10 courtesy of Gannon Holt and Ernest Hung of Parker Hannifin Corp.
Nippon Pulse America Inc.
Parker Hannifin Corp.