When a stepper motor makes a move from one step to the next, the rotor doesn't immediately stop. We call this "ringing" and it occurs every single step the motor takes. Similar to a bungee cord, the momentum carries the rotor past its stop point, it then "bounces" back and forth until finally coming to rest. In most cases, however, the motor is commanded to move to the next step before it comes to a rest.
The graphs below show the ringing under different load conditions. Unloaded, the motor exhibits a lot of ringing. A lot of ringing means a lot of vibration. The motor will often stall if it is unloaded or lightly loaded because the vibration is so high it will lose synchronism. When testing a stepper motor always be sure to add a load. The other two graphs show the motor with a load. Loading a motor properly will smooth out its performance.
For shorter, quicker moves, the ratio should be closer to to The motor will exhibit much wilder vibrations when the input pulse frequency matches the natural frequency of the motor. This is called resonance and usually occurs around Hz. In resonance, the overshooting and undershooting become much greater and the chance of missing steps is much higher. The resonance changes depending on the load inertia, but it is usually around Hz.
If you are missing steps in multiples of four, the vibration is causing a loss of synchronism, or the load is too great. If the missed steps are not a multiple of four, there's a good chance the wrong number of pulses or electrical noise is causing the problems.
There are a number of ways to get around resonance. The easiest way is to avoid that speed altogether. Most motors have a maximum starting speed around pps or so. So in most cases you can start the motor at a higher speed than the resonant speed. If you have to start at a speed below the resonant speed, accelerate through the resonant range quickly.
Another solution is to make the step angle smaller. The motor will always overshoot and undershoot more for bigger step angles.
If the motor doesn't have to travel far, it will not build up enough force torque to overshoot a large amount. Anytime the step angle is made smaller, the motor will not vibrate as much. This is why half-stepping and microstepping systems are so effective at reducing vibration. Make sure the motor is sized properly to the load. By choosing the proper motor you can improve performance. Dampers are also available. Dampers fit on the back shaft of a motor and absorb some of the vibrational energy.
They'll often smooth out a vibrating motor inexpensively. A relatively new technology in stepper motors is 5-phase. The most obvious difference between 2-phase and 5-phase see interactive diagram below is the number of stator poles. While 2-phase motors have 8 poles, 4 per phase, the 5-phase motor has 10 poles, 2 per phase. The rotor is the same as that of a 2-phase motor.
Since the pitch is still 7. Simply based on construction, the resolution of the 5-phase has steps per revolution versus the 2-phase with steps per revolution. The 5-phase offers a resolution 2. With a higher resolution you get a smaller step angle, which in turn reduces vibration. Since the step angle of the 5-phase is 2. In both 2-phase and 5-phase, the rotor must overshoot or undershoot more than 3. Because the step angle of the 5-phase is only 0. The chances of losing synchronism with a 5-phase stepper motor are very low.
In the diagram below, the wave drive method has been simplified to better illustrate the theory. In the wave drive method also called the 1-phase ON method , only one phase is turned on at a time. When we energize the A phase a a south pole, it attracts the north pole of the rotor. Each time only one phase is energized. If both phases A and B are energized as south poles, the north pole of the rotor will be equally attracted to both poles and line up directly in the middle.
In sequence as the phases are energized, the rotor will rotate to line up between the two energized poles. What advantage does the "2 phase on" method have over the "1 phase on" method?
The answer is torque. In the "1 phase on" method, only one phase is turned on at a time, so we have one unit of torque acting on the rotor. In the "2 phase on" method, we have two units of torque acting on the rotor, 1 at the 12 o'clock position and 1 at the 3 o'clock position. Five phase motors are a bit different. Rather than using the "two phase on" method, we use the "four phase on" method.
Each time we turn on 4 of the phases and the motor takes a step. The " phase on" method or half stepping combine the two previous methods. In this case, we energize the A phase. The rotor lines up. At this point we keep the A phase on and energize the B phase. Now the rotor is equally attracted to both an lines up in the middle. Now we turn off phase A but leave on phase B.
The motor makes another step. And so on and so forth. By alternating between one phase on and two phases on, we have cut the step angle in half. Remember that with a smaller step angle, the vibration is reduced.
The half step mode has an eight step electrical sequence. For five phase motor in " phase on" method the motor goes through a 20 step electrical sequence. Microstepping is a way to make small steps even smaller.
The smaller the step, the higher the resolution and the better the vibration characteristics. In microstepping, a phase is not fully on or fully off.
It is partially on. When the maximum power is in phase A, phase B is at zero. The rotor will line up with phase A. As the current to phase A decreases, it is increasing to phase B. The rotor will take tiny steps toward phase B until phase B is at its max and phase A is at zero.
The process continues around the other phases and we have microstepping. There are some problems associated with microstepping, mostly accuracy and torque. Also because the torque differential between steps is so small, the motor sometimes cannot overcome the load. In those cases the motor may be commanded to move 10 steps before it actually starts to move. In many cases it is necessary to close the loop with encoders which add to the price.
Stepper motors are designed as an open loop system. A pulse generator sends out pulses to the phase sequencing circuit. The phase sequencer determines which phases need to be turned off or on as described in the full step and half step information.
The sequencer controls the big power FETs which then turns the motor. With an open loop system, however, there is no position verification and no way to know if the motor made its commanded move. The most popular method of closing the loop is adding an encoder on the back shaft of a double shafted motor. United Arab Emirates. United Kingdom. Vatican City.
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It does not change their basic features. Stepper Motors Stepper motors are brushless DC motors that move in precise steps. Features of stepper motors Stepper motors move in precise, repeatable steps, which is why they are perfect for applications such as 3D printers, CNC, camera platforms and X, Y plotters.
Stepper motor types There are many different types of stepper motors on the market, some of which require highly specialized drivers.
Stepper motor step count Another thing to consider is the required positioning resolution. Stepper motor drive mechanism The way to achieve high positioning resolution is through gearing. Stepper Motor shafts Another thing to consider is how the motor will connect with the rest of the driveline.
The motors are available with different shaft types: Round or "D" Shaft : They are available in a variety of standard diameters and there are many pulleys, gears, and shaft couplings designed to fit. The "D" shafts have one flattened surface to prevent slippage. They are desirable for high torques. Shaft with gear : Some shafts have gear teeth milled right into them. They are typically designed to work with modular gear trains. Lead-Screw Shaft : Lead screw shaft motors are used in the construction of linear actuators.
Their miniature versions can be found as head positioners in many disk drives. Wiring for stepper motors There are many variations of stepper motor wiring. Unipolar and bipolar stepper motors Unipolar drivers always energize the phases in the same way. Additional components It must be remembered that the stepper motor is a component and does not work independently; therefore, both the motor and the controller should be considered when building the drive.
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The amount the stepper motor rotates is proportional to the number of pulse signals pulse number given to the driver. The relationship of the stepper motor's rotation rotation angle of the motor output shaft and pulse number is expressed as follows:. The speed of the stepper motor is proportional to the speed of pulse signals pulse frequency given to the driver. Stepper motors generate high torque with a compact body. These features give them excellent acceleration and response, which in turn makes these motors well-suited for torque-demanding applications where the motor must start and stop frequently.
To meet the need for greater torque at low speed, Oriental Motor also has geared motors combining compact design and high torque. Stepper motors continue to generate holding torque even at standstill. This means that the motor can be held at a stopped position without using a mechanical brake. Once the power is cut off, the self-holding torque of the motor is lost and the motor can no longer be held at the stopped position in vertical operations or when an external force is applied.
In lift and similar applications, use an electromagnetic brake type. The AlphaStep consists of stepper motor and driver products designed to draw out the maximum features of a stepper motor. These products normally operate synchronously with pulse commands, but when a sudden acceleration or load change occurs, a unique control mode maintains positioning operation.
AlphaStep models can also output positioning completion and alarm signals, which increase the reliability of the equipment which they operate. Each stepper motor and driver combines a stepper motor selected from various types, with a dedicated driver. Drivers that operate in the pulse input mode and built-in controller mode are available. You can select a desired combination according to the required operation system.
The motor can be controlled using a pulse generator provided by the user. Operation data is input to the pulse generator beforehand. The user then selects the operation data on the host programmable controller, then inputs the operation command. The built-in pulse generation function allows the motor to be driven via a directly connected personal computer or programmable controller.
Since no separate pulse generator is required, drivers of this type save space and simplify wiring. A stepper motor is driven by a DC voltage applied through a driver. In the VAC motor and driver systems, the input is rectified to DC and then approximately VDC is applied to the motor certain products are exceptions to this. This difference in voltage applied to the motors appears as a difference in torque characteristics at high speeds.
This is due to the fact that the higher the applied voltage is, the faster the current rise through the motor windings will be, facilitating the application of rated current at higher speeds. Thus, the AC input motor and driver system has superior torque characteristics over a wide speed range, from low to high speeds, offering a large speed ratio.
It is recommended that AC input motor and driver systems, which are compatible over a wider range of operating conditions than DC input systems, be considered for your application.
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