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DNA Dan's discussion about secret drive

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DNA Dan:
Sure! The constant trigger on the switch poses it's own issues with having to have a magnet or something there to hold the rotation. Maybe this new relay method is better if it just involes tripping the sensor. However, you still need to "brake" the monitor in the correct position.

DNA Dan:
In all these scenarios, do you have to hold the button down to complete the circuit for the thing to rotate? Or is the SPDT a toggle that stays in that position? I mean what is the nature of that switch? Is it always in the center and you have to toggle it to either side, wait for the rotation, then let go. Or is it a flip the switch, it rotates, then gets killed by the limit switch? I guess what I don't understand is what happens if the switch is in the rotate position and the circuit is opened by a limit switch. Is the secret drive in brake mode like a servo? Is this bad to keep it like this while you play?

And dude, I'm not worthy! :notworthy:

DaOld Man:

--- Quote from: DNA Dan on March 11, 2011, 09:21:34 pm ---In all these scenarios, do you have to hold the button down to complete the circuit for the thing to rotate? Or is the SPDT a toggle that stays in that position? I mean what is the nature of that switch? Is it always in the center and you have to toggle it to either side, wait for the rotation, then let go. Or is it a flip the switch, it rotates, then gets killed by the limit switch? I guess what I don't understand is what happens if the switch is in the rotate position and the circuit is opened by a limit switch. Is the secret drive in brake mode like a servo? Is this bad to keep it like this while you play?

And dude, I'm not worthy! :notworthy:

--- End quote ---

Very good questions.
The first circuit with the SPDT switch is a type of switch that is maintained. This means it stays in the position until you flip it again.

When the limit switch opens, it is the same thing as flipping the SPDT switch to the off position. The drive turns off and stays off until you flip the switch to the other position. So even while the switch is in H position after monitor stops, the drive is still turned off by the limit switch being open.

For the latching circuits, the push buttons do not have to be held in. The circuit "latches" on until the limit switch resets it. You could substitute a "spring return to center" SPDT toggle switch for both push buttons. This type switch returns to center by itself when released.

From what I ascertain from the website, the secret drive goes into brake mode when both of its inputs are high (on) or low (off) AND its enable input is high (on).

It wont hurt the drive to be in braking mode all the time, unless something is trying to turn the motor. (By twisting the shaft of the motor).
Basically braking is done by "shorting" out the leads of the motor. When power is removed from the motor and the motor is coasting, it begins to generate current. By shorting this current, the motor gets very hard to turn, thus resulting in a braking effect.
There is no current produced when the motor is not turning.

DNA Dan:
I think I like the simplicity of the SPDT setup best. Would it be possible to have those limit switches close the circuit (instead of opening it) so that both signals are "high" and the motor would just brake in that position? Is this a possibility so as to not have something to "hold" the monitor there like a magnet? I have seen some servos programmed this way but I think it was a logic board. You can get very precise with logic servos but the cost goes up exponentially.

DaOld Man:
No, without adding anything else to the SPDT circuit, the limit switches need to be closed when the monitor is not in that position.
However,
According to the Solarbotics data sheet on the secret drive, as long as the enable input is tied to + VDC, both directions can be high or low to apply braking. So with the SPDT circuit, when the monitor reaches end of travel and the limit switch opens, both directions to the secret drive will be low. In my SPDT circuit, I have the drive's enable always tied to +vdc, so this circuit already does what i think you are wanting.

Here is a link to the data sheet:
http://www.hobbyengineering.com/specs/Solorbotics-kit10.pdf

Scroll down to page 9 and study the logic table.
With enable high, both inputs low or both inputs high produces braking.

Now about servos. A servo holds its position because it senses the position and constantly corrects for any drifting.
A DC motor does not do this. When current flows through the motor, it turns.
When current stops flowing, it coasts to a stop.
When power is removed, a DC motor will coast. However, while it is coasting it becomes a generator.
You can short the generator and it becomes harder to turn. But enough force can still turn it.
Unless you put a sophisticated tachometer or encoder on the dc motor and have a drive that can correct the motor for drift, you more or less just have to design your rig around the coasting of the motor.

Decreasing motor speed can cut way down on motor coast, but it also adds to overall time it takes for the monitor to rotate.
You can concentrate on the design of the flag (the thing that is attached to the disc and activates the switches).
A flag that is wide enough to still be activating the switch after the motor is done coasting is one way.
I think you need to design your rig so the monitor cant "back lash", or turn backwards slightly after stopping.
Motor braking may prevent this, provided that the monitor is not running really fast.

Hint:
My next MRotate version is going to have the option of adding an encoder like a mouse opto and wheel (could possible hack a mouse to get it.)
The encoder will count up or down a counter. You will be able to set slow down points along the counts which will allow the monitor to slow to a very smooth stop. Limit switches wont even be necessary to stop rotation, but rather be back ups in case something happens.
But, it will be fully automatic, controlled by the computer, which is something you are not looking for.
Hmmm, a DC brush motor acting like a stepper. I wonder if it will be as precise as the stepper?

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