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Stepper motor and driver selection
- Thread starter Pongi
- Start date
I used to ,before my heart attacks, cut fire wood. I built a windlass or capstan, they go by both names, to bring logs up out of gullies to then cut them to length. It has a motor(old chainsaw) gear box and a drum. You put 5 wraps of rope around the drum and then down to the log. By then pulling on the free end of the rope it pulls the log, like a winch. When you stop pulling on the free end it slips on the drum. The pull is a fraction of the pull do to the capstan power, it just tightens the rope so it lets the drum do the work.
Now to how it would work in your situation. there would be a rope or cable wrapped 5 times around the steering wheel shaft and it would go to your motor pulley. That pulley would also have a pinch wheel that makes the rope move when the motor does. The rope (or cable) is an endless loop so that which ever direction the motor is turning, it tightens the grip on the steering shaft and turns it. With the motor off the wheel is free to move.
The 5 wraps on the capstan is a known amount I found when researching how they work, the literature said that 5 wraps is the optimum amount.
I see no particular problem with the stepper motor concept. As I understand Pongi's early statement his goal is not to maintain an ideal rudder setting - it is to maintain an approximate course. The rudder position will wander with the forces on the boat - wind, waves, people shifting around in the boat. Also, some course wandering will be of no consequence. Not being a servo control expert, I would expect that all he needs is a slight rudder angle change to achieve a desired course correction with no concern about the absolute rudder angle and therefore no need for angle feedback. Of course the actual course would wander about the desired course - pretty much as it does under manual control. I am undoubtedly naive about automatic course control but I would think that Pongi cares little about rudder position - what he cares about is average course as indicated by his GPS signal. Therefore I don't understand the discussion of rudder position feedback requirements.
To answer Pongi's early question: the voltage rating of the stepper motor is of little consequence for most stepper controllers. The current is the important variable and most controllers chop the source voltage to maintain the desired coil current, relying on coil inductance to average the chopped voltge applied to the motor. I am presently using a stepper as a traction motor on a 50 lb. carriage that measures the straightness of railroad rails while climbing/descending a 5% grade and maintaining a speed of 1 meter per second. The power source is 54 volts (three Ryobi 4 amp hour power tool batteries in series) but the motor is rated at 2.8 volts, 5.5 amps. I chose a stepper for this purpose to maintain constant velocity without the need for a sophisticated servo control loop with its stability issues and tuning requirement. Just tell it to go a certain speed and it does, as long as there is no slippage. If it slips, I increase the spring tension on the traction wheels. If it stalls I increase the set motor current. Yes, yes, I understand that my control requirements are quite different from Pongi's.
Stepper motors need not be power hogs when idle. The controller I use cuts idle motor current to 100%, 75%, 50%, 25%, or 0% of running current as programmed. I program 25% to provide a small amount of braking while not moving or sometimes 0% and just stick a rubber shim under the wheel.
Most, but not all, steppers have negligible holding torque at 0 current so Pongi could provide a BIG RED BUTTON near the steering wheel (or a capacitive hand sensor ON the steering wheel) to cut power if he had to take control, eliminating the need for a clutch mechanism to release the wheel.
Once Pongi sets the approximate course with whatever rudder position is required I would guess the actual angular rotation of the wheel is slight. For this reason I don't understand why he wouldn't use a small stepper motor with a large pulley ratio to use more of the readily available, zero cost motor rotation capability and less of the expensive torque.
I also wonder about the advantage of chain drive from the motor to the wheel. I understand the attraction of marine tradition and history, but my bias would be toward using a large ratio timing belt drive.
To answer Pongi's early question: the voltage rating of the stepper motor is of little consequence for most stepper controllers. The current is the important variable and most controllers chop the source voltage to maintain the desired coil current, relying on coil inductance to average the chopped voltge applied to the motor. I am presently using a stepper as a traction motor on a 50 lb. carriage that measures the straightness of railroad rails while climbing/descending a 5% grade and maintaining a speed of 1 meter per second. The power source is 54 volts (three Ryobi 4 amp hour power tool batteries in series) but the motor is rated at 2.8 volts, 5.5 amps. I chose a stepper for this purpose to maintain constant velocity without the need for a sophisticated servo control loop with its stability issues and tuning requirement. Just tell it to go a certain speed and it does, as long as there is no slippage. If it slips, I increase the spring tension on the traction wheels. If it stalls I increase the set motor current. Yes, yes, I understand that my control requirements are quite different from Pongi's.
Stepper motors need not be power hogs when idle. The controller I use cuts idle motor current to 100%, 75%, 50%, 25%, or 0% of running current as programmed. I program 25% to provide a small amount of braking while not moving or sometimes 0% and just stick a rubber shim under the wheel.
Most, but not all, steppers have negligible holding torque at 0 current so Pongi could provide a BIG RED BUTTON near the steering wheel (or a capacitive hand sensor ON the steering wheel) to cut power if he had to take control, eliminating the need for a clutch mechanism to release the wheel.
Once Pongi sets the approximate course with whatever rudder position is required I would guess the actual angular rotation of the wheel is slight. For this reason I don't understand why he wouldn't use a small stepper motor with a large pulley ratio to use more of the readily available, zero cost motor rotation capability and less of the expensive torque.
I also wonder about the advantage of chain drive from the motor to the wheel. I understand the attraction of marine tradition and history, but my bias would be toward using a large ratio timing belt drive.
