How to use the Stern Magnet Processor Board

Related Config File Sections:

Stern uses a Magnet Processor Board (part number #520-6801-00) in Metallica for the coffin under the hammer. This board is special as it can detect the ball a ball near the magnet. In addition in can grab and hold the ball.

Connecting the Board

TODO: Add an image of the magnet processor board.

Supply Voltage

The itself magnet runs on 50VDC and the board needs a logic supply of 5VDC. Both of the ground rails seem do be isolated from each other and are not connected on the MPB itself. However, you need to connect those two GNDs to maintain common ground (see common ground for details).

Zero Crossing

The MPB has a 13VAC input on J1-8 which seems to be unused and is not needed for its operation.

Inputs and Outputs

There are 3 logic level (5V) inputs and 1 logic level output on the MPB. The inputs are called DATA6 (J1-5), DATA7 (J1-6) and STROBE (J1-7) on the MPB and are used to control the mode of operations of the MPB. The single output of the MPB is called Sw Drive (J1-4). This changes from low to high with ball detection. To enable the output to pass sufficient current to power an opto isolator (recommended) or to use it in a switch matrix, enable the output with +5V at Sw Return (J1-3).

Controller States

So let's go over the four states of the controller:

OFF:

Signals: D6 = low, D7 = low, pulse Strobe

The magnet does nothing.

GRAB:

Signals: D6 = high, D7 = low, pulse Strobe

The magnet activates for a second and then deactivates. This grabs the ball from centimeters away. GRAB is a one time event. The controller does not GRAB again unless it passes through another state or OFF- state first.

DETECT:

Signals: D6 = high, D7 = high, pulse Strobe

The magnet is pulsed with 4µs peaks to detect the ball. Ball detection requires a small distance of seperation between the magnet core and the ball. 3.5 mm of wood appears to be a compromize between detection and holding strength. The green LED on the MPB lights up when the ball is detected and the output pin 4 goes high.

HOLD(+DETECT):

Signals: D6 = low, D7 = high, pulse Strobe

The magnet is pulsed with three 4µs peaks (presumably) for detection and a longer pulse (presumably) for holding the ball on the magnet. The ball is held firmly, but the detection does not always work. Sometimes it was very reliable, sometimes there was no detection at all. This might require some more analysis.

Config

To use the magnet in MPF you can use the following config as starting point. Shows that pulse the input pins are run as a sequence of events, not as loops. The board will maintain its mode until instructed to change. Caution: if the board is in a hold mode and MPF crashes the board will remain in the hold mode until power is lost or MPF instructs it to enter the OFF-state.

digital_outputs:
  magnet_strobe:
    number: 1    # number depends on your platform
    type: driver
    enable_events: magnet_strobe_on
    disable_events: shutdown, magnet_strobe_off
  magnet_d6:
    number: 2    # number depends on your platform
    type: driver
    enable_events: magnet_d6_on
    disable_events: shutdown, magnet_d6_off
  magnet_d7:
    number: 3    # number depends on your platform
    type: driver
    enable_events: magnet_d7_on
    disable_events: shutdown, magnet_d7_off

switches:
  s_detect:
    number: 1     # number depends on your platform

shows:
  magnet_state_off:
    - time: 0
      events:
        - magnet_d6_off
        - magnet_d7_off
    - time: 20ms
      events:
        - magnet_strobe_on
    - time: 30ms
      events:
        - magnet_strobe_off
    - time: 50ms
      events:
        - magnet_d6_off
        - magnet_d7_off
  magnet_state_detect:
    - time: 0
      events:
        - magnet_d6_on
        - magnet_d7_on
    - time: 20ms
      events:
        - magnet_strobe_on
    - time: 30ms
      events:
        - magnet_strobe_off
    - time: 50ms
      events:
        - magnet_d6_off
        - magnet_d7_off
  magnet_state_grab:
    - time: 0
      events:
        - magnet_d6_on
        - magnet_d7_off
    - time: 10ms
      events:
        - magnet_strobe_on
    - time: 20ms
      events:
        - magnet_strobe_off
    - time: 50ms
      events:
        - magnet_d6_off
        - magnet_d7_off
  magnet_state_hold:
    - time: 0
      events:
        - magnet_d6_off
        - magnet_d7_on
    - time: 20ms
      events:
        - magnet_strobe_on
    - time: 50ms
      events:
        - magnet_strobe_off
    - time: 70ms
      events:
        - magnet_d6_off
        - magnet_d7_off

You can then turn the controller into detect in a mode by posting the magnet_state_detect event. Then add an event_player based on s_detect_active to turn the controller into the grab state. Finally, after a few seconds turn it into the hold state and check the state of s_detect to see if the grab succeeded.

TODO: Add some example config for this logic.

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