Specific stepper motor parameters

1. Step angle – represents angular displacement of the rotor for one control impulse;

1. Maximum no load start frequency – represents the maximum control impulse frequency at which the unloaded 
2. Best stepper motor can start, stop or reverse without losing steps;
3. Limit start frequency – represents the maximum impulse frequency at which the motor can start without losing steps, when a given moment of inertia and torque load are presented at the shaft;
4. Pull-in torque – represents maximum torque load at the shaft, at which the motor can start without losing steps;
5. Maximum no load frequency – represents the maximum impulse frequency that the motor can follow without losing synchronization;
6. Maximum frequency – maximum frequency of impulses at which a motor keeps its timing for given torque load and inertia;
7. Pull-out torque – maximum torque that can be maintained by the motor at a certain speed, without losing steps;
8. Angular speed – is calculated as a product between the stepping angle and the control frequency;
9. Detent torque – represents the value of the holding torque presented by at the motor shaft when it is not electrically energized.

Also read about how to correctly implement different types of control sequences in our dedicated article about stepper motor control.

Geared stepper motor to main focuser shaft

This solution is most widely used in commercial focusers. It consists of relatively small stepper motor connected with proper gear box with ratio in the range 1:10 to 1:200. Shaft from the gearbox is connected to main focuser shaft. Backlash is present, as it is intrinsic quality of each mechanical gearbox, but it can be easily compensated with proper settings of focuser driver. This solution is also more expensive than other two, because there is additional cost of gearbox. 

Example stock solutions that are available for this solution:

• Nema 16 9V 0.6A motor with 1:14 gearbox
• Nema 17 12V 0.4A motor with 1:27 gearbox
• cheap 35BY412 motors with 1:42 gearbox available on amazon, ebay or aliexpress (I will test these motors soon)
• low power stepper motors available with gearbox at different retailers (like https://www.mclennan.co.uk/product/p535-series-geared-stepper-motor – can be pretty expensive)

Drawbacks of this solution are price and backlash. Advantages are: low power consumption, no power required when idle, compact size, high resolution and compatibility with many commercial solutions (Robofocus, Moonlite, USB Focus, Pegasus Astro, AstroLink and other). 

Custom ATM adaptation of stepper motor with planetary gearbox to main focuser shaft. Aluminium shaft coupler is visible in the picture between motor and focuser.

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Stepper motor to microfocuser shaft

This is quite easy solution that requires low power stepper motor, because torque to move microfocuser shaft is pretty low. All you need to do is to find suitable stepper motor for 9-12V. It can be both unipolar or bipolar motor, and it needs to provide torque at level 2-3 Ncm or more. Motor shaft needs to be connected to microfocusing shaft using a shaft coupler with proper diameter. 

Example motors that will work for this solution:

• Nema 8 geared Bipolar 1.8deg 0.3A 12V 3Ncm motor
• Nema 14 geared Bipolar 1.8deg 0.4A 12V 14Ncm motor
• Nema 14 geared Bipolar 1.8deg 0.4A 10V 5Ncm motor
• Round Nema 14 geared Bipolar 0.9deg 0.5A 8.5V 5Ncm motor
• Nema 16 geared Bipolar 1.8deg 0.4A 12V 21Ncm motor 
• Nema 17 geared  motor Bipolar 1.8deg 0.3A 12V 16Ncm

This kind of connections is quite simple and inexpensive. The drawbacks of this solutions are: microfocusing knob is not available anymore for manual focusing (however you can use shaft coupler to rotate it manually), and when microfocusing gear is not adjusted well, there can be some slip at its mechanism.

Stepper motor coupled with microfocusing shaft. No additional gearbox is required.

Advantages of Electric Linear Actuators

Reduced downtime. Electric linear actuators (whether screw- or belt-driven) are very low-maintenance. Regreasing may be the only regular maintenance necessary, and many screw-driven models are lubricated for the life of the actuator.

Electric actuators commonly use stepper motors or brushless DC (BLDC) servo motors to generate torque. Because both motors are brushless, there is no contact between the rotor and stator other than the load bearings. This eliminates motor maintenance, and allows the motor life to equal the life of the bearings.

A stepper motor may run at 100 percent of its rated current during a motion, and have a substantial steadystate holding current as well. This holding current can waste energy because it locks the rotor to a position regardless of whether any work is being performed. However, many stepper drivers can reduce the current to minimal levels.

Electric step motor linear actuators can meet these demands because they offer more control over position, speed, and force. High-resolution feedback devices are common today and easily allow for micrometer-scale precision. Advanced drive tuning can compensate for things like changing loads, inertia, and preventing mechanical resonance. Electric actuator manufacturers often provide sizing software to accurately size an electric actuator to the application, and select a motor with enough torque to reach the desired speeds.

THE STEPPER MOTOR CONTROLLER YOU CHOOSE MATTERS!

Equally the stepper motor controller that you use will have a major impact on the mechanical performance you are able to achieve using the motor. If the controller is not able to deliver more power than the motor can handle then it is unlikely that you will be able to achieve the maximum possible mechanical performance from the motor.

As an example of this, our stepper motors with integrated controllers have higher powered controllers the bigger the motors get.

See the table below for an overview of frame sizes.

WHAT NEXT?
If your interest in motor sizes was purely academic then we hope we have helped. If you have any questions about this please do not hesitate to get in touch and we will do our best to help.

Alternatively if you are looking for a motor and aren’t sure which is best for your application then you could start by having a quick look at our standard range.

We offer a range of stepper motors of different sizes which are available in geared or standard format.

As always, if you have any questions about choosing the right motor or the pros and cons of a long stack NEMA 17 versus a short stack NEMA 23 MOTOR(for example) then you can get in touch with us via online chat, phone or email.

A Small and Llight Gearbox for a Stepper Motor

I wanted to build a small and light gearbox for a stepper motor (used as excuse: for building a light extruder for a 3D printer - didn't trust the bowden setup, and the scavenged driver might not have enough torque for a direct drive - real cause: playing around with the lasercutter :-)

I decided to use POM (polyoxymethylen), since it's quite strong, self lubricant and you can cut it with a lasercutter... if you have a good ventilation: Burning POM produces formaldehyde, not the best product for your health. So using a CNC mill might be more healthy.

On the other hand, we have now a three stage filtering system especially for formaldhyde, and a just fine tuned laser cutter - perfect for fine teeth.

I used apart from the laser cutter:

- POM (i used three different thicknesses, 2,3 and 4mm to get a really small and light version, but using only one (4mm) thickness is also possible)

- 3* bearings 3mm inner diameter, 7mm outer diameter, 3mm height

- 1* bearing 5mm inner diameter, 10mm outer diameter, 4mm height

- 3* 3mm hexagon socket head screw 12mm long, with 12 washer and 3 nuts

- 1* 5mm hexagon socket head screw 30mm long (but trimmed down) and 3 nuts (small height)

- 2* 3mm hexgon socket head screw 40mm long with 2 washers

- NEMA17 stepper driver (5mm diameter axis)

- MK8 drivegear for 5mm axis