README.md

Robot arm

The robot arm is probably the most essential part of the Fischertechnik factory. It's the part that moves Tokens around from one part of the factory to another.

In order to move tokens, it uses a vacuum sucker to grab and move a token.

Modifications

As the robot arm uses a vacuum sucker to move tokens, it can happen, that the robot misses a token or looses it in transit. The original model isn't able to detect such situations.

In order to add this feature, I used a standard cylinder, a standard switch and a spring feather to construct something of detecting the presence of a vacuum.

The cylinder is simply added to the connection to the sucker via t-junction and as soon as there's under-pressure, is pulls back the cylinder, which presses the switch. As soon as the vacuum is lost, the spring feather pushes the cylinder back up and the switch is no longer pressed.

With this cheap little trick I can reliably detect if the token is still "hanging on" to the robot arm.

Hardware

For automating the robot arm modell I've been using a Fatek FBs-40MC PLC. In order to communicate with the device, I also added a FBs-CM55E which adds two RS-485/RS-232 ports. One of them (Port 4) seems to also be able to be accessible via Ethernet connection.

However instead of having 3 ports, I think the FBs-CM55E is much more two RS ports and one Ethernet-to-Serial Adapter connected to Port 4. As soon as I configured the Port 4 for serial, I was no longer able to communicate via Ethernet.

I am therefore programming the PLC via a serial cable connected to the on-board serial port (the one with the PS2-like connector).

I/O Connections

Here's how I connected all inputs and outputs to the PLC

Inputs

(The connections X0 to X11 are important that they are exactly as mentioned here. See why, in the configuration part)

• X0: Signal A of the optical encoder for vertical axis (Raise/lower the Arm)
• X1: Signal B of the optical encoder for vertical axis (Raise/lower the Arm)
• X3: Reference switch vertical axis
• X4: Signal A of the optical encoder for horizontal axis (Extend/retract the arm)
• X5: Signal B of the optical encoder for horizontal axis (Extend/retract the arm)
• X7: Reference switch horizontal axis
• X8: Signal A of the optical encoder for rotational axis (Turn the Arm clockwise/anticlockwise)
• X9: Signal B of the optical encoder for rotational axis (Turn the Arm clockwise/anticlockwise)
• X11: Reference switch rotational axis
• X12: Vacuum detection

Outputs

• Y0: Motor vertical up
• Y1: Motor vertical down
• Y2: Motor horizontal retract
• Y3: Motor horizontal extend
• Y4: Motor rotate clockwise
• Y5: Motor rotate counterclockwise
• Y6: Compressor
• Y7: Valve vacuum

Configuration of the PLC

Luckily Fatek has a "slightly" different attitude as most automation vendors. If you paid for a Fatek PLC you get the automation Software for free. It's called WinProLadder and can be downloaded from here: https://www.fatek.com/en/download.php?act=list&cid=141 after creating a free account. If you download it, it's currently something around 3.1MB in size (MB ... not GB as some other vendors).

Not quite sure, if it's a downside, however the software is anything but fancy. But it does the job.

I was actually able to use it quite intuitively, which I think is quite impressive in the automation sector. The only real downside, I would say, is that you can only programm it using ladder-logic. This is less programming for me, but more a puzzle game, of how you can trick the system to do what I want.

But if you ask me if I would rather live with that, instead of paying 4k€ for a TIA portal ... any time.

I/O Configuration

The robot arm uses 3 optical encoders. In order to process the signals, we need to use the built-in support for fast counters.

My FBs-40MC has 4 hardware counters included, however the inputs need to be connected accordingly. Each of them stores the current position in a pre-defined register. These are also readable via modbus.

A full description of the memory layout can be found here: https://www.esea.cz/support/fatek/doc/FBs-Manual_Web_English/FBs-Special-Relay-Register-List-EN.pdf

These are configured in the System Configuration / I/O Configuration tab, on the Timer/Counter tab. I configured them by selecting the sub-tabs: HSC0, HSC1 and HSC2 and setting them the following way.

MSC0 (Up/Down)

Counting Mode set to: A/B A-Phase set to: X0 B-Phase set to: X1 Mask(MSK) left unassigned Clear(CLR) set to: X3

The output register: R4096 (Hex 1000) PLC4X-compatible Modbus address: 404096

So the encoder signals are connected to the inputs X0 and X1, while the signal of the home-switch is connected to X3.

MSC1 (Retract/Extend)

Counting Mode set to: A/B A-Phase set to: X4 B-Phase set to: X5 Mask(MSK) left unassigned Clear(CLR) set to: X7

The output register: R4100 (Hex 1004) PLC4X-compatible Modbus address: 404100

MSC2 (Rotate clockwise/counterclockwise)

Counting Mode set to: A/B A-Phase set to: X8 B-Phase set to: X9 Mask(MSK) left unassigned Clear(CLR) set to: X11

The output register: R4104 (Hex 1008) PLC4X-compatible Modbus address: 404104

Enable the Hardware counters

After configuring the counters, we still need to provide computing-power to them. This is done by adding 3 function blocks 92.HSCRT to the ladder diagram and simply connecting them. Each of the function blocks will reference one counter: HSC0, HSC1 and HSC2.

Writing the PLC Programm

General notes on controlling the motors

As we'll have multiple ladder rows setting and resetting the inputs of the motors, directly setting them doesn't work. Therefore, I'm using multiple markers to decide if a motor should be enabled or disabled.

Markers

The 9x range is reserved for controlling things that aren't explicitly related to output.

M90: The initialization has been triggered M91: The initialization is in progress

To simplify reading and understanding the ladder program the markers M00 to M79 control the outputs. The last digit of the marker directly maps to the output number. So M34 controls output Y4.

• M{x}0: Raise the arm
• M{x}1: Lower the arm
• M{x}2: Retract the arm
• M{x}3: Extend the arm
• M{x}4: Rotate the arm clockwise
• M{x}5: Rotate the arm counterclockwise
• M{x}6: Turn on the compressor
• M{x}7: Open the valve

Sub Programms

Ladder programs can become quite messy, I therefore split it up into multiple Sub Programs. Each Sub Program taking care of one individual aspect of the program.

In WinProLadder, in order to create a Sub-Program, you need to first create a file in the Project tree.

Then in the ladder editor you need to define a Label by using the function block 65.LBL and giving the label a name. The last line in the editor needs to be a call to the function block 68.RTS which is the return-statement.

When calling the Sub Programm, you use the function block 67.CALL and reference the label you created.

In this ladder program you can also see, how the initialisation sub programm is only executed if the robot is not yet initialized or while it's still initializing and the positioning logic is only executed after initialisation is finished.

Initializing

Whenever the PLC is started, we need to ensure the arm is initialized, by moving it to the home position.

In order to trigger initialization, we're using one of the special markers M1927, which is only set to true on the first execution.

So when starting, all markers are set to 0, but on first execution of the ladder logic, M1927 is set to true. I'm using this signal to set M90 to true, wich I interpret as "the system is initializing.

In the Network N001 you can then see:

• As long as we're initializing (M90 is true) and we haven't finished (M91 is true) and we haven't reached the vertical top reference switch position, set M0 to true.
• As soon as the top reference switch is pressed, turn off M0
• Now we're in the row below, as X3 will stay true and here we repeat the same procedure: As long as the horizontal reference switch is not pressed (true ... this is when the arm is completely retracted), turn on the M2 motor and turn it off as soon as we reach the reference position.
• Last not least, turn the robot arm clockwise till we reach the rotational reference position (X11).
• As soon as we've also reached the rotational reference position, reset the initialization marker M91.
• The system is finished initializing.

NOTE: I have read that it's not good to use "set" and "reset" of coils, I would be happy for some feedback on how to do it without. WinProLadder is constantly giving out warnings that two ladders are referencing the same coil.

Positions

The following values are positions in the Fischertechnik factory.

Axis	vert	hor	rot
Register	R4096	R4100	R4104
Blue Bay	760	420	285
Red Bay	760	270	355
Green Bay	760	240	438
Oven Intake	420	840	880
Hig-Bay Ingest	40	20	1324
Hig-Bay Output	120	220	1324

However, I want the robot-arm to be as flexible as possible, so I'll be implementing this program in a way that on startup via Modbus a controller will be storing the position data into registers of the PLC.

Each one being a tripple of:

• vertical encoder postion
• horizontal encoder postion
• rotational encoder postion

Each of these tripples will simply be referred to by their index.

So a robot arm position will be referred to as position-number and contain the values of:

• Vertical postion = R5100 + ( 3 x {position-number} ) + 0
• Horizontal postion = R5100 + ( 3 x {position-number} ) + 1
• Rotational postion = R5100 + ( 3 x {position-number} ) + 2

In general the robot arm will be controlled by a controller, which generally can issue two types of commands:

• 1: Bring the arm into a position
• 2: Move a token from one postion to another
• 99: Reset to home position

In order to give the arm a command, the controller first writes the source-position number into the register R5000. If it's a token move command, it also writes the target-position number into the register R5001. After that's done, the controller tells the robot arm that it's finished and what type of command it's going to execute, by writing 1 or 2 into R5002.