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Tuesday, December 15, 2009

Saftey First

After what happened last time, I've become significantly more paranoid about damaging my robot.

Recently, while doing some more hacking of the wCK motion timing system, I decided to simply remove the element of risk altogether.

So far, separating the arms and legs from the body has been a good move (tm), and I encourage you to do the same if you start changing low level parts of the code.

You see, I am rapidly running out of those spanner shaped connectors.

I mentioned in a previous post how careless control over pointers can cause your robot to flip out like a hyperactive kid tripping over a beehive. The wCK servos have just over 11 kgFcm of torque, which is more than adequate to cause your robot to tear itself to pieces when a rogue pointer starts issuing random commands. I speak from experience.

Serial transmission of commands to the servos can be performed in two ways. The most common method is to use the "Transmission Buffer Empty" interrupt, to send a new character whenever there is nothing waiting to be sent. A transmission is initiated simply by unmasking the interrupt, and the interrupt is set to disable itself when there is nothing left to send. This is the preferred way of sending RS232 communications, as the interrupt will continually fire as long as as something remains to be sent.

The second method of transmitting the message is to use the "Transmission Complete" interrupt. If enabled, this interrupt fires whenever a character has finished being sent. This requires a single character to be manually loaded to "prime the pump", but transmission simply stops when no more characters are available. Since the next character is sent immediately, tighter control over transmission states can be achieved, such as when using an RS485 protocol.

The wCK servos use an RS485 protocol.

The reason that the robobuilder has a tendency to spaz out is due to two things. Firstly, the transmission buffer is circular. This means that messages can be added to the buffer at the same time as they are being sent. When the pointer gets to the end of the buffer, it simply returns to the beginning and keeps sending until it reaches the end marker.

Secondly, memory is never truly empty. A byte in memory simply contains the last thing that was stored in it. On reset, unallocated space usually contains junk data. Even if you set up your program to zero out your memory before you use it, it will still contain a zero. Therefore, even a small mistake which results in one of the transmission interrupts not being disabled correctly will cause a vast number of arbitrary commands being issued to the servos, causing the aforementioned spastic behavior.

Aaaaanyway, now that you know why it happens, you know how to avoid it. I still recommend detaching your arms and legs while you're testing though.

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