Robotic Arm- Process Synchronization

Watch the following video showing Tesla automated production line:

According to Tesla:
"Every Model 3 is being built on the Model 3 production line, which is fully installed, powered on, producing vehicles, and increasing in automation every day. However, every vehicle manufacturing line in the world has both manual and automated processes".

The following are examples of some of usegaes for the robotic arms in a veriety of fields:

Robofusion- robotic ice cream machine:

RoboCoaster- robotic roller coaster

Kuka- Film production (Making of Sherlock Holms)

Moley Robotics- Robotic kitchen

FarmBot- Robotic Garden

The following is an example of multiple robotic arms perfectly synchronized while working on the same car on a production line.
Volkswagen education using robotic arms:



Watch the following video of a robotic arm pick ups blocks baced on their location:

In this video we can see a robotic arm using a distance sensor to locate the coordinates of the following block moving on a conveyor belt.

The blocks are called critical resource.
Critical resource-is a resource shared among processes

The algorithm that is used to move the blocks in every process is called critical section.
Critical section- is a code that access a critical resource.

Knowing when the block is ready to be picked up and at what location is managing the synchroinzation between the processes.
The robotic arm is waiting for a signal that the previous process as finished it's job and the critical resource is available for a full access.
This behaviour is called mutual execution.
Mutual execution- only one process may be executing a critical section with respect to a particular critical resource at a time.

The answer to the question above describes a behavior that is called Spinlock.
The robotic arm is checking the data stored in the distance sensor in a loop.
Once the conveyor belt has finished it's task and the block is located in the correct position, the distance sensor can see the block.
The moment it is detected by the sensor, the robotic arm will be notified that the critical resource is available now by checking the distance sensor value in the next loop iteraction.
Spinlock- is a way to keep track on the availability of a critical resource.
A process that needs to get an access to the critical resource checks a variable that hold the information regarding the availability of the resource.
If the critical source is not yet available, the process will wait in a loop ("spin") and check again later.
Since the process remains active but is not performing a useful task, the use of such a lock is a kind of busy waiting.
Once acquired, spinlocks will usually be held until they are explicitly released.

There might be a need in your solution to know when more than one process has completed their jobs and procede only if those processes has finished.
This method of synchronization is called barriers.
Barrier- is a type of synchronization method. A barrier for a group of processes means any process must stop at this point and cannot proceed until all other processes reach this barrier.

In case of multiple robotic arms are allowed to manipulate the block at the same time, use semaphoes to limit the access to more than the maximum allowed.
Semaphores- similar to a spinlock for multiple processes if allowed to access to a critical resource at the same time with a limitation of a maximum processes at the same time.

The implementation for that can use multiple variables as a spinlocks if there is a need to keep track on each one of the processes, or to use a common accomulated variable to keep track of number of processes currently accessing the critical resource.

Watch the following video of a robotic arm sorting blocked based on their colors:

Let's code!

In order to create a color sorting algorithm, we need to break down the program into smaller sigments.
Each sigment is a function with a single job.
The following are some code samples as a reference.

Color reading:

Color sensor related blocks can be found under DobotAPI -> Additional menu.

In order to activate the  color sensor reading, use the SetColorSensor block.
Change the value to ON and the port to the port number the sensor is connected to (GP5):

The color sensor can recognize only the following colors: Red, Green and Blue.
Use the IdentifyColor mathod to query the sensor regarding the color of the object currently presented.

Change the value to r in order to read the red color, g for green and b for the blue color.
The mathod will return 1 if the requested color has been recognized, or 0 if a different color has been recognized.
0 will be also returned if the sensor cannot see the object. 

It is recommended to read the sensor once and store the reading inside a variable to be used multiple times later on if needed.

Since the sensor can only recognize 1 out of the 3 colors at any given point, only 1 variable (red, green or blue) can be equale to 1 if any.

Works only about 5mm to 1cm distance with no-reflection objects. If you put near it near than 5mm, can't tell the color

Distance reading:

When an object has been found on the conveyor belt, grab it and read it's color:

Setup initial coordinations and variables: