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June 8, 2012
The easy way to do this is to make it so that the limit switches, when pressed, cut off the flow of electricity to the motor. But, put diodes across each limit switch so that they will allow current to flow through them in a direction that allows the motor to reverse away from the limit switch.
But be sure the diodes are rated for a high enough currrent to handle the motor’s draw, and that they will not break down with reverse polarity. Consider using Schottky diodes for a low forward voltage drop. I might recommend 1N5822 diodes.
The open-loop way of working this would require you to manually change direction when a limit is reached (presumably by flicking a switch/button on your controller). But a microcontroller should be able to sense that the current or voltage drop through the motor has suddenly changed, and automatically reverse. But that will require an additional sense circuit and some care taken to protect the analogue input on the microcontroller from high voltage, flyback, etc.
I would not consider this slider suitable for heavier DSLRs unless you’re only using it as a starting point. It needs modification. As is, It will support point+shoot cameras, some lightweight mirrorless cameras with pancake lenses, a GoPro, a smartphone… but I wouldn’t trust it with much else. Whether it merely moves the camera is not the concern… as a good gearmotor will definitely handle the load. It’s the actual ruggedness of the slider itself that is the problem.
The first thing to do to make it heavier duty is to use the channel slider A. Then, install limit switches and diodes as necessary to prevent damage to your gearmotor when it hits the end. Next, I would use one of Actobotic’s round base plates as a mounting point for the camera. And, if desired, put a ballhead on that.
I’d also consider mounting the motor so that it’s in line with the channel, using bevel gears. With the tiny, flimsy plastic feet on this thing, a motor sticking out the side can easily cause it to tip over.
Then stick a few ¼-20 camera/tripod mount “hubs” on the bottom of the channel for attachment to a tripod.
Regarding the little plastic feet it comes with… pretty useless. I’m going to have to replace them with something much better.
Lots of work to do on this thing to make it suitable for my DSLR. But IMO it is a decent starting point… that said, I think if I were doing this again, I’d go with one of the IGUS slider kits from Actobotics.
The supply voltage doesn’t matter. Because the coil current is regulated by the EasyDriver, independently of the voltage. The voltage drop over the motor depends only on the current and coil resistance, and so too would the power dissipated. Brian explains something similar on the EasyDriver website.
See the first and second entry in the FAQ here: http://www.schmalzhaus.com/EasyDriver/
So just go ahead and use a 9V or 12V supply.
Does your extrude require 0.8A per phase, or 0.8A in total? The EasyDriver supports up to 0.7A per phase, or 1.4A peak for the entire two-phase motor.
Whether you can get away with this depends entirely on how much load the motor is driving. It is very common to apply less than the rated current to stepper motors, as the needed current really depends on how much torque you require.
There is also the Big Easy Driver, which isn’t a whole lot more expensive. Looks like a good board for your application.
about 4 years ago
Since the motor still magnetically holds position when turned off, I’m guessing PM.
Stepper motors draw a fairly constant current when the coils are energized, regardless of speed. This motor contains two-phases. Each phase is essentially a completely separate electromagnet with 34 ohms of resistance. At 12V, when both coils are energized simultaneously, the motor draws 706mA. When only one is energized, the motor draws 353mA.
Whether one or two phases are energized and how often is based on the drive mode. In two-phase on, full-step mode (probably the most common mode), both phases are always energized regardless of speed, and at 12V, this motor draws 353mA per phase - 706mA in total - constantly. Even when it’s not moving and just in a “holding” state.
Half-step mode uses a bit less current, because half of the time, only one phase is one. So in half-step mode, you can expect this motor to draw 530mA at 12V on average. The current it will draw in a holding state depends on at what stage of the drive mode the motor was held. If you are programming your own logic, you can actually control this - which might help prevent overheating.
Now, it’s possible that some motor drivers will switch off all phases in between steps, regardless of drive mode. I have no idea why they’d do that, and mine certainly doesn’t (it’s just an L293D that I control will a shift register), but I’m just covering my butt here. If your driver does this, then it’s possible for speed to have an effect.
For stepper motors, the speed doesn’t determine the current use, but rather, the drive mode does. The drive mode and speed together determine the torque. In general, more torque is correlated with more current.
I’ve got this motor working very nicely with an L293D quad half-H-bridge, a 600mA 12v supply (I might use a higher current supply, as this takes it to its limit), and an Arduino UNO. I’m running it at 120rpm. It also runs fine at 240rpm, although torque is lower, and if you accelerate the motor to 240rpm, there is at least one resonant frequency at which it will skip steps.
Both full and half stepping work well.
It gets comfortably warm, but not hot, after about 40 minutes of continual use on full-step mode (which has both coils energized all of the time, so uses the max current).
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