Friday, April 15, 2011

Friday, April 15

Today I was able to do some more construction on the XY table since we received the linear rack gears yesterday. I was also able to begin figuring out how to use the stepper motors and drivers.
The linear rack gears need to be glued in a perfectly straight line next to one of the X-axis tracks, and next to the Y-axis tracks. Since the tracks themselves are straight, it wasn't too difficult to mount the linear gears because I just had to push them flush against the metal lip of the tracks after applying glue. I was able to complete the bottom linear gear track as shown in the figure here, before and after gluing:

The stepper motors will have large circular gears attached to their axle that will come in contact with these linear gears. This will convert the circular motion of the stepper motors into linear motion.
The glue that we will use for the entire project is West Systems Epoxy. I had some left over from a different project. It is really good epoxy and I highly recommend it to anyone to use.

After working on this I went to look at the stepper motors. A picture of the driver, and actual motor is shown below:

The weaker motor that we received is a bipolar motor, and only has 4 leads. The more powerful one coming in the mail soon can be either unipolar or bipolar and has 6 leads. We will most likely use it as bipolar so that we don't have to use as many leads. Bipolar steppers have a single coil per phase and require more complicated control circuitry (typically an H-bridge for each phase). Here is an idea of how the wiring will look when interfaced with the HCS12:

If the logic circuits are pulled up to VDD, it is good practice to use a high value pull-up resistor in order to limit current to the logic inputs, should an overvoltage event occur. Logic inputs
include: MSx, SLEEP, DIR, ENABLE, RESET, and STEP. Here are a description of some of the inputs:

Step Input (STEP). A low-to-high transition on the STEP input sequences the translator and advances the motor one increment. The translator controls the input to the DACs and the direction of current flow in each winding. The size of the increment is determined by the combined state of inputs MS1, MS2, and MS3.

Microstep Select (MS1, MS2, and MS3). Selects the microstepping format, such as full step, half step, quarter step, eight step, and sixteenth step. MS2 and MS3 have a 100 kΩ pull-down resistance. Any changes made to these inputs do not take effect until the next STEP rising edge. If the MSx pins are pulled up to VDD, it is good practice to use a high value pull-up resistor in order to limit current to these pins, should an overvoltage event occur.   

Direction Input (DIR). This determines the direction of rotation of the motor. When low, the direction will be clockwise and when high, counterclockwise. Changes to this input do not take effect until the next STEP rising edge.

 The outputs to the bipolar stepper motor are pins 1A, 1B, 2A, and 2B. I still need to read more to figure out exactly what these do.

The driver datasheet makes it seem that it won't be too difficult to use. One issue that I saw was trying to figure it out was how to adjust the max / cutoff / output current to be 1.2A so that we can supply the larger motor with this much current.  There is a variable resistor potentiometer that must be tweaked to set the supplied current. However, what I am confused about is how to actually measure what the current output actually is, and I need to study the datasheet some more.
The second issue I saw was that the datasheet stated that when the current was above 1A, a heat sink or cooling method was recommended because the driver can easily overheat. We think that we will be alright when it comes to this because we won't need to have the two drivers confined in a small place or close to one another. Also, I have a small fan that I could put next to them just in case. I also have the heat sink from the Wii that I tore down...

Here is a link to the datasheet in case anyone is interested, or wants to help us out.

We are still waiting on the circular gears, the reed switches, and the strong motor.

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