New Control Technique Combines Servo Performance With Step Motor Cost
| By: Chuck Lewin, Founder and CEO, Performance Motion Devices, Inc.
When it comes to step motors, a new drive technique called stepper servo is making everything old new again and winning back machine designers who may have relegated step motors to the category of low cost but low performance.
What makes this technique powerful is that it can use a generic non-custom step motor yet extract much more performance out of it. This is accomplished by adding an encoder and operating the motor as, effectively, a commutated two-phase Brushless DC motor.
The fact that an encoder is needed means that truly low cost applications will not be good candidates for stepper servo. But for applications that would otherwise have a Brushless DC motor, stepper servo is increasingly being considered as an alternate approach.
In addition to offering a lower cost solution than a comparable Brushless DC motor, stepper servo can actually outperform brushless motors in areas such as acceleration rate and torque output. This makes it a candidate for applications such as high speed point-to-point moves, textile equipment, coil winding, high-speed electronic cams, and more.
So how does stepper servo work? To answer this we will start with a quick review of traditional step motor control schemes and then dive into how stepper servos are different, and what they can do for today's machine designer.
You Had Me At Full Step
Step motors are popular first and foremost because they are easy to use. They do not require an encoder to maintain their position, and unlike DC Brush or Brushless DC motors when used for positioning, they do not require a servo control loop. Their advantages are low cost, high torque output, and brushless operation. Their main drawbacks are vibration, noise, and a limited speed range.
Figure 1: Traditional Step Motor Control Waveforms
Figure 1 shows traditional waveforms for driving a step motor. Step motors are a multi-phase device, meaning multiple motor coils are electrically excited to create motion. Most step motors have two phases but more exotic configurations such as 3-phase or 5-phase also exist.
In the world of step motors, the phasing techniques that the amplifiers employ are given special names such as full step, half step, and microstep control. These different techniques refer to the number of power levels that are applied to each motor coil during an electrical cycle. Whichever drive method is used, the motor moves forward or backward when the external controller alters the electrical phasing.
Step motors are usually constructed with 1.8 mechanical degrees per full step (90 electrical degrees). So this means a 1.8 degree stepper has 200 full steps per mechanical rotation. In addition to 1.8 degree step motors, other configurations exist such as .9 degree and 7.2 degrees.
In The Valley Of The B-Field
Now lets look at what's happening inside the motor, and understand in more detail how a traditional step motor is operated.
Figure 2: Spinning Bar Magnet Model for Step Motors
Figure 2 provides a simple magnetic model of a step motor. The rotor can be thought of as a spinning bar magnet that interacts with an externally controlled magnetic field (the stator). The rotational torque generated is zero when the N-S rotor field aligns with the stator N-S magnetic field (also called a B-Field), and maximum when the two fields are at an angle of 90 electrical degrees from each other. Its worth noting that the actual internal construction of a step motor looks nothing like this, but it's still a useful way to understand the motor operation.
When the stator coils are driven with current, a sinusoidal force 'valley' is created which drives the step motor to settle at a specific position. The more current that is driven through the coils, the greater the depth of the force valley. In this force profile of hills and valleys, wherever the curve is horizontal there is no mechanical torque generated, and wherever the curve is the steepest the generated torque is the largest.
Figure 3: Traditional Step Motor Drive Scheme
As the Figure above shows, in a traditional step motor drive scheme the motor settles to the 'bottom' of the force profile. At this point the net rotational motor torque generated is zero because the motor is at an equilibrium point. This explains why position can be maintained in a step motor without an encoder or servo loop.
To create motion, the controller moves this valley forward or backward by changing the stator phase via the external coil connections. The motor rotor than 'falls' forward or backward, maintaining itself at the bottom of the force valley in response. Think of a ball settling to the bottom of a trough.
Desperately Seeking Smoothness
As convenient and simple as this scheme is, it has a number of drawbacks. Here is a quick rundown:
- Indeterminate Accuracy. The actual position of the settled rotor is the sum of the internal equilibrium-restoring force and whatever external forces on the rotor may exist. Therefore in a given application, or for a given load, the exact actual profile path will vary on a small scale.
- Mid-Range Instability. Going back to the ball metaphor, when the phase angle is changed abruptly the rotor will advance but tend to ring around the equilibrium point and finally settle into the new phase angle. Normally this settling process, which happens very rapidly, is not a big problem, but when the natural ringing frequency equals the commanded step rate a phenomenon called mid-range instability can occur which can result in a dramatic reduction in available torque at that specific operating speed.
- Lost Steps. A sufficient external force can push the rotor away from its equilibrium position all the way up and over the force profile curve and into the next valley. This phenomenon is called losing steps and is often a run-away effect once it starts, meaning the rotor falls further and further behind the commanded profile and eventually comes to a halt.
- Excess Heat. To combat the phenomenon of lost steps the motor is operated at a torque level sufficient to handle the worst-case motion profile. This means at all other times the programmed torque is higher than is actually needed, generating excess heat.
- Noise. Step motors are noisy during motion for a few reasons. If a full step or half step drive scheme is used, the square edges of these coil drive signals excite resonances (read that noise) in the rotor. Another reason is the large number of electrical cycles per mechanical rotation. Just moving the rotor forwards or backwards requires the controller to constantly cycle the command voltages up and down for each phase, which induces noise in the coils and therefore the motor.
- Vibration. All of the factors listed above for noise can also generate vibration. But particularly with a microstepping drive there is a phenomenon that may generate little noise but can create a fair amount of vibration. Due to the geometry of the stator/rotor teeth (a feature of all step motors) and peculiarities of the resultant B-fields, the relationship of drive signals to translated motion is never perfect. In other words a plot of commanded position and actual measured position is not an exact straight line. This phenomenon results in rhythmic vibration during motion.
- Low Top Speed. To position accurately step motors move only a small amount for a corresponding advancement of the coil command waveform. A standard 1.8 degree step motor requires 50 complete electrical cycles per single mechanical rotation. By comparison a four-pole Brushless DC motor requires just two electrical cycles per mechanical rotation. Motor coil inductance limits how fast the phasing can be changed, and therefore step motors tend to have much lower maximum speeds then Brushless DC motors.
You Had Me At Stepper Servo
Now let's delve into the stepper servo technique, sometimes also called closed loop stepper operation (a term that is a bit ambiguous because it is also used to describe a traditional step motor control scheme that uses an encoder to verify the final position thereby 'closing' the loop).
Stepper servo operation is different in three key ways from regular step motor operation.