Stepper motor and driver selection

Thread Starter

Pongi

Joined Feb 9, 2020
8
Greetings experts

I would appreciate some advice on selecting a proper stepper motor and driver for my project.

I plan to create an autopilot for my small fishing boat using a Raspberry Pi controlling a stepper motor, that rotates the steering wheel via a chain.
I will be getting the current heading from my existing Garmin GPS and the rudder angle from a potmeter on the rudder.

Here's a sketch of the system I plan on building:

project sketch.jpg

Trying to figure out the minimum required torque to turn the wheel I took a good resolution kitchen scale
and pressed up on the wheel as shown below:
(Note: I can turn the wheel with my pinky with no effort at all)


torque calc.jpg
I measured numerous times, and ended up somewhere between 200 - 250g to move the wheel.
I didn't calculate speed or acceleration into the equation as the motor will only travel a few degrees at a time,
and only very slowly. The wheel can only move ~3.5 turns from full port side to full starboard.
My first question is: Are my torque calculations are correct?
(I realize that using different sized cogwheels on the steering wheel and the motor affects the calculations.
I plan to use the same size on both ends)

In case my calculations are correct I went ahead and found a suitable stepper motor from a local supplier:
https://minielektro.dk/nema-23-stepper-motor-bipolar-1-8deg-0-9nm-57x56mm.html
Datasheet:
https://ecksteinimg.de/Datasheet/Schrittmotor/JK57HS56-2804/JK57HS56-2804-01.pdf
I oversized it on purpose (1.26 Nm), so I wouldn't run into any issues in that regard.

Next I tried to find a suitable driver for the motor, from the same supplier:
https://minielektro.dk/tb6600-stepper-driver.html
Datasheet:
https://www.mcielectronics.cl/website_MCI/static/documents/TB6600_data_sheet.pdf

The driver should be big enough (5A) with the motor rated at 2.8A, but my concerns are regarding the voltages.
The motors nominal voltage is 2.5V but the driver takes 9 to 36V.
I have 12V onboard.
I haven't worked with stepper motors/drivers before so I'm uncertain how the driver outputs/limits the voltage,
so my second question is: Can this motor and driver work together with 12V supply?

I would also appreciate some advice if anybody sees some obvious flaws in my design.

Thanks in advance.
 

MrChips

Joined Oct 2, 2009
20,204
Welcome to AAC!

I am in the process of constructing a 6-axis robotic arm. I have chosen DC motors with reduction gears and position encoders. I am using the readily available L298 H-bridge and two-digital outputs from a MCU. I believe stepper motors do not provide the torque and speed for applications such as this.

That's my opinion. Hope this helps.
 

nerdegutta

Joined Dec 15, 2009
2,676
Hi.

When you're not using the autopilot, how will you free out the chain and gear. Have you consider a V-drive belt? Then you can have a fly-wheel to tighten the belt. (I'm not sure if belt is the correct term here).

I second MrChips opinion.

Hows the wheel connected to the rudder? Electro-hydraulic pump? If so, could you use that?
 

Thread Starter

Pongi

Joined Feb 9, 2020
8
Hi guys, thank you very much for the answers and opinions. Much appreciated.

I don't quite understand the concerns about torque requirements. As I said, I can turn the wheel without any effort at all.
My calculations (assuming they are correct) are just to give me an indication in numbers, so I have an idea what motor to get. The one i chose is 2.5x the required torque. Speed is not an issue at all.
Position encoders seems overkill to me as I'm not interested in the precise position of the wheel. My feedback is the actual heading I get from the GPS, which is not going to be that precise anyway as there will be a considerable delay between turning the wheel to
receiving the new heading. This is going to be a very slow regulation.

It will probably be V-type-belt as you mention. I haven't decided on that yet. Either way, my plan was to just cut the power to the motor when not using the autopilot. I don't think a motor of that size turning along with the wheel will be noticeable.

The rudder is hydraulic in the sense that when you turn the wheel one way it pushes oil to one side of the cylinder and when you turn it the other way it pushes oil to the other side of the cylinder.
Unfortunately there's no electric pump. If there was I could have controlled it with solenoid valves.

Any input on my calculations or the voltage concerns?
 
Last edited:

MrChips

Joined Oct 2, 2009
20,204
Without position encoders you wouldn't know the position of the rudder. Stepping motors will not always step. Hence you cannot rely on the number of steps to reach a desired position.

I depend on DC motors to give the right amount of torque.
 

Reloadron

Joined Jan 15, 2015
5,110
I don't quite understand the concerns about torque requirements. As I said, I can turn the wheel without any effort at all.
OK, been a long, long time since my boat days but I have a question. When you say you can turn the wheel effortlessly I believe you. I once had a '63 Chris Craft Cavalier Express and I could also turn the wheel (two screws and two rudders) without any effort but when moving and water passing over the rudders there was more effort needed which seems to make sense. Sitting still there is no rudder resistance, moving along there will be some rudder resistance. Rudders are also always placed aft of the prop(s) so when moving the prop(s) are pushing water against the rudder(s). I am sure there is math that applies but to make it simple. Visit a few local marine dealerships. or online retailers. Look at their drive units or drive motors for boats similar in size and displacement as your boat. That's my best suggestion in choosing a motor.

Several years ago my brother convinced me I needed to go muskie fishing and the guy who took us out had a nice 30' boat. I was amazed at the electronics on that boat. Tell boat where you want to go, from an on board memory and the GPS takes over. The guy had all the good spots plotted. Then once there the GPS just followed a pre-programmed route cris-crossing. It was really pretty cool. Not having been out on a boat, discounting US war ships and fun cruise ships, the stuff on this boat was really amazing to me.

Ron
 

atferrari

Joined Jan 6, 2004
3,772
What is the LOA of your craft?

WRT your "auto-pilot" function: is your software aware of yawing, sea condition and off-course tolerance? Maybe, nothing of that is a concern in your boat; if so, good for you!

The "orders" from the steering wheel, how do they arrive to the rudder? What is that "remote control"?
/EDIT I see you posted about an hydraulic system /EDIT

That pot in the rudder, should be for heavy duty. They are expensive for a reason. In one of our vessels it even failed; it was then when I learn about them. I would not dare to use a 1$ piece.
 

Sensacell

Joined Jun 19, 2012
2,564
I second the thought about step motors for this application - not the right choice here.

1) Steppers are power hogs- even when not moving - they require significant power
2) The motor and drive must be sized for the largest transient overload - if the motor stalls, you have lost position- game over.
A motor stall is difficult to detect, not easy to automate a graceful recovery.
You will need an overkill monster motor.

Use a DC motor/gearbox with an analog absolute position sensor.

a) Overloads will not cause loss of position- even if the motor stalls momentarily, nothing bad happens - this condition is also easy to detect.
b) Most of the time the motor will not be moving - power consumption would be close to zero under these conditions
c) The system could recover instantly from power loss without needing to 'home' the rudder.
 

Marley

Joined Apr 4, 2016
341
DC motor and gearbox with feedback is the way to go. Feedback with a potentiometer from the actual rudder would be best.
As the the rudder/wheel does not need to turn very fast, a simple CW-STOP-CCW control to the motor is all that is required. No motor speed control.
Presumably you will also have feedback from a compass or GPS so that you can automatically follow a course and compensate for wind and currents.
 

Thread Starter

Pongi

Joined Feb 9, 2020
8
Thanks for your valuable input guys, appreciate it

but when moving and water passing over the rudders there was more effort needed which seems to make sense.
That's an excellent point. I'll definitely take the boat out for a spin (once the current storm blows over) and do my measurements again!

Visit a few local marine dealerships. or online retailers. Look at their drive units or drive motors for boats similar in size and displacement as your boat. That's my best suggestion in choosing a motor.
I'm having a really hard time finding commercial systems that control the the steering wheel with an electric motor as I plan to do.
Most autopilots control the rudder by way of hydraulics or by wirepull/teleflex.
I found this, but that's a purposely build drive unit with gears an couplings:
https://www.pyiinc.com/jefa-autopilot/sprocketdrives.html

Several years ago my brother convinced me I needed to go muskie fishing and the guy who took us out had a nice 30' boat. I was amazed at the electronics on that boat. Tell boat where you want to go, from an on board memory and the GPS takes over.
That's what I plan to end up with, but first I would just like it to keep a heading :)
 

Thread Starter

Pongi

Joined Feb 9, 2020
8
What is the LOA of your craft?
It's 23'

WRT your "auto-pilot" function: is your software aware of yawing, sea condition and off-course tolerance? Maybe, nothing of that is a concern in your boat; if so, good for you!
I haven't written any software yet.
First step is to have it keep a heading. As I'll be continuously getting the heading for the next way-point from the GPS, it should not be difficult to implement an "auto-pilot" function.
First things first, I need to find a suitable motor.

That pot in the rudder, should be for heavy duty. They are expensive for a reason. In one of our vessels it even failed; it was then when I learn about them. I would not dare to use a 1$ piece.
I already have a spare "high power" potmeter the size of my fist lying around that I'll try.
Don't worry. I won't be putting a 1$ piece in :)
 

Thread Starter

Pongi

Joined Feb 9, 2020
8
I second the thought about step motors for this application - not the right choice here.

1) Steppers are power hogs- even when not moving - they require significant power
2) The motor and drive must be sized for the largest transient overload - if the motor stalls, you have lost position- game over.
A motor stall is difficult to detect, not easy to automate a graceful recovery.
You will need an overkill monster motor.

Use a DC motor/gearbox with an analog absolute position sensor.

a) Overloads will not cause loss of position- even if the motor stalls momentarily, nothing bad happens - this condition is also easy to detect.
b) Most of the time the motor will not be moving - power consumption would be close to zero under these conditions
c) The system could recover instantly from power loss without needing to 'home' the rudder.
I get what you guys a saying about the torque limitations of the stepper motor and all of the above pros and cons. It makes sense, but I'll be needing torque at 0 RPM to hold the rudder, and an ordinary DC motor won't be able to provide that. Please correct me if I'm wrong.

Could you guys please link a suitable DC motor w/ gearbox.

Thanks in advance
 
Last edited:

MaxHeadRoom

Joined Jul 18, 2013
19,986
I get what you guys a saying about the torque limitations of the stepper motor and all of the above pros and cons. It makes sense, but I'll be needing torque at 0 RPM to hold the rudder, and an ordinary DC motor won't be able to provide that. Please correct me if I'm wrong.
Could you guys please link a suitable DC motor w/ gearbox.
With a stepper motor, you would power the motor with the plate rated steady DC current. This will produce the rated torque of the stepper.
Although a DC motor can do the trick as any industrial servo is capable of.
Incidentally I believe the original reason for a PID loop (servo) was developed for automatic boat steering.
Max.
 
Last edited:

Thread Starter

Pongi

Joined Feb 9, 2020
8
With a stepper motor, you would power the motor with the plate rated steady DC current. This will produce the rated torque of the stepper.
Although a DC motor can do the trick as any industrial servo is capable of.
Yes, but I would really like to keep this as simple as possible, and rather not have a regulator work continuously just to keep the rudder steady. (KISS principle) ;)

Incidentally I believe the original reason for a PID loop (servo) was developed for automatic boat steering.
Max.
Heh, I didn't know that :)
 

Thread Starter

Pongi

Joined Feb 9, 2020
8
My first choice would be 12VDC windshield wiper motor. I can get these for under US$10 at surplus stores.
They will hold steady at zero power because they use a worm gear.

https://www.princessauto.com/en/search?Dy=1&Nty=1&Ntt=surplus+motor
That's a really good idea. Very simple and cheap. Brilliant!
I wouldn't be able to release it without some sort of clutch though.
The magnetic clutches I find are fairly expensive. I'll have to look around. Thanks :)
 

shortbus

Joined Sep 30, 2009
7,446
I wouldn't be able to release it without some sort of clutch though.
No clutch needed if the windless principle is employed on the wheel shaft. When the motor is off the wheel could turn free.
You said earlier there is no electric pump for the hydraulic system to the rudder, how do the hydraulics work then?
 

Sensacell

Joined Jun 19, 2012
2,564
Non-back drivable worm gears would make a quick manual intervention difficult.

A simple spur gearbox would be manually moveable when you cut the power.
The servo feedback system would hold the rudder against forces, no need for a locking gearbox.
 

Thread Starter

Pongi

Joined Feb 9, 2020
8
No clutch needed if the windless principle is employed on the wheel shaft. When the motor is off the wheel could turn free.
I'm not sure what you mean. Could you please elaborate?

You said earlier there is no electric pump for the hydraulic system to the rudder, how do the hydraulics work then?
When you turn the wheel in one direction you push oil down to a piston that moves the rudder to one side and vice versa.
In this system you are the "motor". There's clearly some internal gearing inside in the pump, which makes the wheel very easy to turn. This type of steering is very common on small boats like mine.
 
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