Wednesday, June 27, 2018

Advantages and Disadvantage of Hybrid Step Motor

The word Hybrid means combination or mixture. The Hybrid Stepper Motor is a combination of the features of the Variable Reluctance Stepper Motor and Permanent Magnet Stepper Motor. In the center of the rotor, an axial permanent magnet is provided. It is magnetized to produce a pair of poles as North (N) and South (S) as shown in the figure below.

Advantages of Hybrid Stepper Motor


At both the end of the axial magnet the end caps are provided, which contains an equal number of teeth which are magnetized by the magnet. The figure of the cross section of the two end caps of the rotor is shown below.



The stator has 8 poles, each of which has one coil and S number of teeth. There are 40 poles on the stator, and each end cap has 50 teeth. As the stator and rotor teeth are 40 and 50 respectively, the step angle is expressed as shown below.
hybrid stepper motor eq 1
The rotor teeth are perfectly aligned with the stator teeth. The teeth of the two end caps are displaced from each other by half of the pole pitch. As the magnet is axially magnetized, all the teeth on the left and right end cap acquire polarity as south and north pole respectively.
The coils on poles 1, 3, 5 and 7 are connected in series to form phase A. Similarly, the coils on the poles 2, 4, 6 and 8 are connected in series to form phase B.
When Phase is excited by supplying a positive current, the stator poles 1 and 5 becomes South poles and stator pole 3 and 7 becomes north poles.
Now, when the Phase A is de-energized, and phase B is excited, the rotor will turn by a full step angle of 1.8⁰ stepper motor in the anticlockwise direction. The phase A is now energized negatively; the rotor moves further by 1.8⁰ in the same anti-clockwise direction. Further rotation of the rotor requires phase B to be excited negatively.
Thus, to produce anticlockwise motion of the rotor the phases are energized in the following sequence +A, +B, -A, -B, +B, +A…….. For the clockwise rotation, the sequence is +A, -B, +B, +A……..
One of the main advantages of the Hybrid stepper motor is that, if the excitation of the motor is removed the rotor continues to remain locked in the same position as before the removal of the excitation. This is because of the Detent Torque produced by the permanent magnet.

Advantages of Hybrid Stepper Motor

The advantages of the Hybrid Stepper Motor are as follows:-
  • The length of the step is smaller.
  • It has greater torque.
  • Provides Detent Torque with the de-energized windings.
  • Higher efficiency at lower speed.
  • Lower stepping rate.

Disadvantages of Hybrid Stepper Motor

The Hybrid Stepper Motor has the following drawbacks.
  • Higher inertia.
  • The weight of the motor is more because of the presence of the rotor magnet.
  • If the magnetic strength is varied, the performance of the motor is effected.
  • The cost of the Hybrid motor is more as compared to the Variable Reluctance Motor. 



Thursday, June 21, 2018

MPCNC Motor Parts Sourcing in China


Build or buy? This was the decision I had to make for MPCNC Parts Sourcing here in Calgary, Alberta.
mpcnc parts sourcing
In addition to providing a detailed Bill of Materials (BOM), the originator of the Mostly Printed CNC machine also makes MPCNC parts sourcing easier by providing a complete hardware kit, shown above. It costs US$ 255, plus shipping to Canada for US$ 46. I wondered whether this was a reasonable deal compared to MPCNC parts sourcing individually. In part, this was because I already had a few of the components (Arduino Mega, Ramps 1.4 board, DRV8825 stepper drivers, power supply) which account for around 15% of the total cost.
For the rest of the parts, I listed them in a worksheet. All of the hardware (nuts, bolts, screws, washers, threaded rod) was available at Home Depot, Lowes or Princess Auto here in Calgary. Prices varied a bit between these stores. The “big ticket” hardware items turned out to be the #6-32 and 5/16 nylon insert stop nuts at around $6 and $10 total, respectively.
The package of 5 NEMA 17 high torque stepper motors costs around US$75 or CDN$99 on Amazon.com.  Unfortunately, the vendor, Stepper China, does not ship to Canada through its Amazon offering. However, with a bit of searching I was able to find the same offer from the same vendor on eBay.
There were additional savings to be achieved on eBay. The cost of the 608 2-SRS bearings (60), the M3-10mm screws for mounting the stepper motors (24) and the GT-2 timing belt and pulleys was an order of magnitude cheaper direct from China than local sources.
I did have trouble finding two of the parts locally. One was the 1.75 inch bolt. Hopefully I can substitute something as I go along. The other exclusion was the 7/8 inch long coupling nut for the anti-backlash assembly. I did find this coupling nut online at Canadian suppliers, but all had a minimum order to 50 – which is 49 more than I need.

I was able to find reasonably priced 5/16 threaded rod, but did not buy it yet. I might consider some re-design for the Z-axis using a lead screw or ACME screw for better durability and accuracy. That will no doubt the the final part of my build. (Threaded rod and coupling nut are shown on the far left side of the picture.)

MPCNC Parts Sourcing – Did I save money?

First, let me say that the hardware kit provided by http://www.vicious1.com/ is probably a good deal if you live in the United States (without currency exchange and with cheaper shipping) and if you don’t already have some of the more expensive parts. The total cost would be $US268.
However, for me in Canada, with exchange rates, extra shipping and the costs and potential hassles of custom tariff and brokering, my cost of would been at least CDN$ 425. By sourcing hardware locally and more specialized parts from offshore, I think my final cost will be under $CDN 300. I will do a full accounting when all is said and done. But I think the saving of CDN$125 for doing MPCNC parts sourcing myself was worth the effort. Of course, I am not accounting for my time, waiting for Chinese shipments, and driving around to the big box hardware stores.
Incidentally, I discovered that much of the hardware actually costs less in Canada, even after currency exchange. For example, both types of nylon insert stop nuts were well less than half the price in Canada. compared to the Home Depot US web site. I have no idea why.

Friday, June 15, 2018

Parameters to be considered for a stepper motor system

Parameters to be considered for a stepper motor system

a) Distance – The distance to be traversed to complete the motion.
b)Time – The maximum time allowed for a traverse.
c)Accuracy – The desired detent (static) accuracy.
d)Overshoot – The desired dynamic accuracy (overshoot).
e) Settling Time - The time allowed for dynamic accuracy to return to static accuracy specification (settling time).
f) Step Resolution – The required step resolution (combination of step size, gearing, and mechanical design).
g) Friction – The system friction: All mechanical systems exhibit some frictional force. When sizing the stepper motor, remember that the motor must provide torque to overcome any system friction. A small amount of friction is desirable since it can reduce settling time and improve performance.

Parameters to be considered for a stepper motor system

h)Inertia - System inertia: An object's inertia is a measure of its resistance to changes in velocity. The larger the inertial load, the longer it takes a stepper motor online to accelerate or decelerate that load. The speed at which the stepper motor rotates is independent of inertia. For rotary motion, inertia is proportional to the mass of the object being moved times the square of its distance from the axis of rotation.
i) Speed/Torque – There a specific speed/torque characteristics to each stepper motor, which will determine your overall system performance. Torque is defined as rotational force (in ounce-inches) defined as a linear force (in ounces) multiplied by a radius (in inches).
IMPORTANT NOTE: When selecting a stepper motor/driver, the capacity of the stepper motor must exceed the overall requirements of the load. The torque any stepper motor can provide varies with its speed. Individual speed/torque curves should be consulted by the designer for each application.
j) Torque-to-Inertia Ratio - This number (ratio) is defined as a stepper motor's rated torque divided by its rotor inertia. This ratio (measurement) determines how quickly a stepper motor can accelerate and decelerate its own mass.
Note: Stepper motors with similar torque ratings can have different torque-to-inertia ratios as a result of varying construction. Performance will vary among different manufacturers.
k) Torque Margin: Whenever possible, a digital stepper motor driver that can provide more torque than is absolutely necessary should be specified. Torque margin allows for mechanical wear, lubricant hardening, and other unexpected friction. Resonance effects can cause the motor's torque to be slightly lower at some speeds. Selecting a stepper motor drive that provides at least 50% margin above the minimum required torque is ideal. More than 100% may prove too costly.

Some Question about NEMA 17 motor

Friday, June 1, 2018

How to drive and operate the stepper motor driver


Micro stepping tells us how many micro steps should a stepper make to produce one full step. The 1/1 value tells us that the stepper must make one micro step to produce one full step (so there is no micro stepping). Value of 1/2 is called a half step and tells us that the stepper motor must make 2 micro steps for one full step. This means that the stepper motor should make 400 steps for one full revolution. A value of 1/8 will tell us that the motor should make 8 micro steps for one full step and 1600 steps for one full revolution. The same principle applies for all of the micro stepper motor for sale values. Most stepper motor drivers have a step/dir input. This means there are only two signals needed for each driver. The step signal is used for making steps and looks like a PWM signal. Each pulse means that the stepper will move for one step (or micro step). 
step motor drive
The dir signal means direction and is used to signal in which direction (CW or CCW) will the stepper turn. We have found out that the stepper motor driver is a must have if our design requires the use of a stepper motor since the controller can’t produce enough current and enough high voltage. There are different types but the chopper drivers offer the best performance. Also the micro stepping offers a great solution at first sight but produces a problem of decreased torque. It is still extremely useful but must be used properly. There are a lot of different ICs available for driving the stepper motor and many already made solutions like PoStep25-32 and PoStep60-256 which provide plug and play solution and are easy to use. 
stepper motor driver for sale
When driving stepper motors with full steps the output of the stepper motor driver for sale looks like a square signal and produces rough movements. The bigger the micro stepping the more the output signal looks like a sine wave and the stepper motor moves more smoothly. But there is a downside to this. This most often occurs when the torque produced by the micro step is insufficient to overcome the friction torque of the component that’s being driven (such as a leadscrew or ball screw). With increasing micro stepping value the torque drops a quite lot and if the value is too great it could happen that the motor can’t produce enough torque to even turn. Usually 1/4, 1/8 or even 1/16 can produce satisfactory smooth movements while still producing enough torque. 

Motion Controller and Driver Selection Tips You Should Know

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