Summing Points

[Javier ARANTEGUI]

In some process control textbooks, the mixer symbols is used as a Summing Point. I have put an example from K. Ogata’s Modern Control Engineering book (Ogata.png).

I have edited the Mixer class in dsp.py to make possible to create these summing points. Here is the code:

class Mixer(Circle):
    ''' Mixer

        Args:
            N: text in North sector
            S: text in South sector
            E: text in East sector
            W: text in West sector
            font: Font family/name
            fontsize: Point size of label font

        Anchors:
            * N
            * S
            * E
            * W
            * NW
            * NE
            * SW
            * SE
    '''

    def __init__(self, N: str=None, E: str=None, S: str=None, W: str=None,
                 font: str=None, fontsize: float=10, *d, **kwargs):
        super().__init__(*d, **kwargs)
        rad = .5
        k = rad*math.sqrt(2)/2  # Diagonal distance
        self.params['lblloc'] = 'top'
        self.params['lblofst'] = 0.2

        self.segments.append(Segment([(rad+k, k), (rad-k, -k)]))
        self.segments.append(Segment([(rad+k, -k), (rad-k, k)]))

        if N:
            self.segments.append(SegmentText(
                (rad, rad/2+0.065), N,
                font=font, fontsize=fontsize))
        if S:
            self.segments.append(SegmentText(
                (rad, -rad/2-0.065), S,
                font=font, fontsize=fontsize))
        if E:
            self.segments.append(SegmentText(
                (rad*1.5+0.05, 0), E,
                font=font, fontsize=fontsize))
        if W:
            self.segments.append(SegmentText(
                (rad/2-0.05, 0), W,
                font=font, fontsize=fontsize))

I think it doesn’t break anything.

The attached file ‘Mixer with in-labels.ipynb’ shows my recreation of the block diagram (d) of the image Ogata.png.

I would like to add a few final comments:

1. Do you think it’s a good idea to change the Mixer class or would it be better to have a new class? If you think it’s not a good idea to include this in Schemdraw, it’s OK.
2. I have tweaked the coordinates of the text to make the more aesthetically pleasant. I’m not sure it’s necessary.
3. To improve the readability of the summing point I would like to make the width of the lines inside the circle half of the width of the line of the circle, but I don’t know how to do it.

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[cdelker]

Good idea. I just checked in this code to the master branch. In response to your comments:

1. Changing the Mixer class is fine.. it won’t affect existing circuits if the new arguments are not provided.
2. Ok. I tweaked a bit more after changing the width of the inside “X” lines (see #3 below)
3. I agree. You can pass “lw” parameter into the Segment class to set the line width of individual segments. This will override any linewidth the end user specifies, so use sparingly, but I think it’s fine here.

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[Javier ARANTEGUI]

I forgot to say thanks!