Month: August 2018

The Four Wire Stepper Motor

There is not much detailing here. The four wire stepper denotes a single possible configuration and that is of a bipolar hybrid stepper motor. We do not need to bore us with details such as whether this motor is variably reluctance, permanent magnet or hybrid as that only relates to construction. What we need to realize is that two wires are for PHASE A and the other two wires are for PHASE B. Which one is PHASE A and which one is PHASE B is kind of arbitrary.

If you have the motor datasheet then you know which wires represent which. But if you do not have this document, just do a quick continuity test and determine which two wires are connected together through an inductor. You can also use a simple BACK EMF test in which you short two leads together. If it is harder to move the rotor, then those two wires form one of the phases. If the rotor moves as easy as with no wires crossed over, then those two wires are not connected through a winding. Keep on going until you find both phases.

Once you have determined both phases, you can wire your motor as shown on the picture above. If you do not know which one of the phases is A and which one is B, just wire the motor until you get the direction you want.

If we want to take on the advantage of a parallel connected winding we will need the eight wire nema 24 step motor. This posting is already too long, but I may study it in a future release. In the mean time, I do hope you are wired!

It is important to know how to calculate the steps per Revolution for your stepper motor because only then you can program it effectively.

In Arduino we will be operating the stepping motor in 4-step sequence so the stride angle will be 11.25° since it is 5.625°(given in datasheet) for 8 step sequence it will be 11.25° (5.625*2=11.25).

Steps per revolution = 360/step angle

Here, 360/11.25 = 32 steps per revolution.

Why so we need Driver modules for Stepper motors?

Most stepper motors will operate only with the help of a driver module. This is because the controller module (In our case Arduino) will not be able to provide enough current from its I/O pins for the motor to operate. So we will use an external module like ULN2003 module as stepper motor driver. There are a many types of driver module and the rating of one will change based on the type of motor used. The primary principle for all driver modules will be to source/sink enough current for the motor to operate.

Arduino Stepper Motor Control Circuit Diagram and Explanation

The circuit Diagram for the arduino stepper motor control project is shown above. We have used the 28BYJ-48 Stepper motor and the ULN2003 Driver module. To energise the four coils of the stepper motor we are using the digital pins 8,9,10 and 11. The driver module is powered by the 5V pin of the Arduino Board.

But, power the stepping driver with External Power supply when you are connecting some load to the steppe motor. Since I am just using the motor for demonstration purpose I have used the +5V rail of the Arduino Board. Also remember to connect the Ground of the Arduino with the ground of the Diver module.

The voltage of your cnc power supply should be greater than or equal to the rated voltage of your stepper motor. Otherwise, the motor will not receive its full rated current and you will not get the full performance that the motor is capable of. It is OK for the power supply voltage to be higher than the rated voltage of the motor because the Tic has active current limiting. (It rapidly switches the power to the motor on and off while measuring the current to make sure it does not go too high.)

A higher power supply voltage is usually desirable since it allows higher speed and torque. However, if the power supply voltage is extremely high compared to the stepper motor’s rated voltage and you want to use microstepping, you might experience skipped steps.

The voltage of your power supply should be within the operating voltage range of the Tic. Otherwise, the Tic could malfunction or (in the case of high voltages) be damaged.

The continuous current per phase of the Tic should be greater than or equal to the rated current of the stepper motor. Otherwise, the Tic will not be able to deliver the full rated current to the motor and you will not get the full performance that your motor is capable of.

We generally recommend you choose a power supply with a current limit that is at least at least twice the current limit you are planning to use on the Tic as that amount of current should always be safely beyond what the Tic will draw. The current limit you configure on the Tic should generally not exceed the stepper motor’s rated current and should not exceed the continuous current per phase of the Tic.

It is worth noting again that since the Tic actively limits current through the motor coils, you can safely use power supplies with voltages above the rated voltage of the stepper motor as long as you set the current limit to not exceed the best stepper motor’s rated current.

What are the advantages and disadvantages of the different functionalities?

Solution
In general, 2-phase stepper motors can have 4, 6 or 8-wire leads (not including any optional encoder lines).
Some hybrid  step motors have a motor case ground that can be tied to the ground of the system. It is usually a black wire, and it will add one additional wire to the overall count (4 coil wires + 1 casing ground = 5 wires total).

The best solution is to obtain the pinout from the motor manufacturer. If you do not have access to the pinout, then the following procedure will help you in wiring the 2-phases.

If you have four coil wires from the stepper motor:
Approach 1 (using a multimeter)
Each of the two phases should have the same resistance when measured with a multimeter. When measuring the resistance across one wire from each of the two phases, the resistance should be infinite because the circuit is open. Locate the two pairs of wires that represent the two phases; both pairs of wires will have similar internal resistance.
Connect each phase to the amplifier and ignore the polarity (+ / -), for now. You have a 50 percent chance of guessing right.
Send a command to move the motor. If the motor rotates in the wrong direction, then switch either phase A and A- or B and B- (effectively reversing directions).

Approach 2 (without a multimeter)
Connect the four coil wires to the amplifier in any arbitrary pattern. Send a command to move the motor.
If the motor moves erratically or not at all, then switch one wire from phase A with one wire from phase B.
If the motor is rotating in the wrong direction, then switch either phase A and A- or B and B- (effectively reversing directions).

If you have six coil wires, then each phase has a center tap wire:
The center tap wire should have half the internal resistance of the full phase. The easiest option is to use a multimeter to find the two pairs of wires that have the maximum resistance.
Connect each phase to the amplifier, and ignore the polarity (+ / -) for now. You have a 50 percent chance of guessing right. The stepping motor should rotate, and if it is in the opposite direction, then switch either phase A and A- or B and B- (effectively reversing directions).
If you have eight coil wires, then it is highly recommended you find the exact pinout for the motor.
The eight wires represent four pairs of wires, and each pair has the same resistance. It is not easy to find what two pairs represent phase A and phase B without dismantling the motor.

How to identify four-wire stepper motor coil pairs with a multimeter

Over the years there have been discussions about the 1.8 degree step angle versus 0.9 degree stepper motor. Most stepper motors today have the standard step angle of 1.8 degrees, resulting in a 200 step per revolution. However, in the early days of stepper motors, before microstepping, low end resonance played a significant role in many applications. Most application engineers suggested either increasing the load, to lower the bandwidth frequency, or simply avoiding this low end resonance region altogether.

For some stepper motor designs the idea of a smaller step angle was created to lessen the ringing around each tooth. The result was a mechanical change, which reduced the step angle from 1.8 degree to 0.9 degree resulting in a 400 steps per revolution stepper motor.

This did reduce the ringing and in some cases allowed for smoother operation accelerating through the low end resonance region. However, the reduction to such a small step angle resulted in higher saturation of the lamination steel around the tooth. This resulted in lower torque due to the core losses. As the current increased, the losses became more significant and the anticipated gains lost due to saturation.

 
0.9 Degree Step Angle 1.8 Degree Step Angle

In the early years, most of the motors utilizing this 0.9 step angle were Nema 23 or single stack Nema 34 frame motors, which had lower currents. The lower currents produced lower saturation in the steel laminations and therefore were successful in some applications.

While there is still a place for 0.9 degree step angle steppers, the introduction of microstepping and the expanded higher end filtering of today’s stepper drives, low end resonance found in 1.8 degree steppers can be successfully accelerated through and perform within this region.

Closed loop-capable stepper motors merge the benefits of stepper and servo motor technology. They run more quietly and have a lower resonance than stepper motors, provide position feedback and control, feature short settling times, and exhibit no step loss at all. They are an alternative to stepper motors if energy efficiency, smooth running and a high load tolerance are required.Compared to servo motors, they have advantages due to their high torque at low speeds, short settling times, correct positioning without back swing and a lower price for sizes that are often smaller.

The closed-loop method is also referred to as a sinusoidal commutation via an encoder with a field-oriented control. The heart of closed-loop technology is power-adjusted current control and feedback of control signals. Through the encoder, the rotor position is recorded and sinusoidal phase currents are generated in the motor coils. Vector control of the magnetic field ensures that the magnetic field of the stator is always perpendicular to that of the rotor and that the field strength corresponds precisely to the required torque. The current level thus controlled in the windings provides a uniform motor force and results in an especially smooth-running motor that can be precisely regulated.

 

True/pseudo closed loop

There are stepping motors that dress themselves up as being closed loops and work with encoders but do not provide any field-oriented control with sinusoidally commutated current control. They only check the step position, and cannot correct step angle errors during operation. True closed loop with field-oriented control compensates step angle errors during a run and corrects load angle errors within a full step.