Further Hacking on the Shark

In my previous post, I described how the messages being passed between the joystick and motor driver of my wheelchair appeared to be a differential serial signal at 40,000 bits per second. The data appears to be call and response pairs. Messages from the joystick start with a ‘`’ character, messages from the motor controller start with an ‘a’.
Each message usually has 8 fields with a numerical value in them, and messages end with ’15’.

Tonight, I recorded the signal as I swept the joystick in a clockwise circle, starting at 12:00/full speed forward. Then I graphed the values of each of the fields.

Joystick messages

Fields one and two in messages from the joystick are the forward/backward and left/right axis of the joystick, respectively. The center position is around 128, full forward/left is 255, full backwards/right is 128, so dead center/off should be around 128 + (255-128)/2 or 191.

Field 4 is a very noisy signal between 191 and 128. It appears to peak with field 1 and bottom out with field 1, so it may be the raw magnetic joystick sensor value for that axis.

Field 8 is a very noisy signal between 128 and 255, with clear diagonal slopes at the maximum and minimum of field 1. It may also be related to the raw joystick signal.

Fields 3, 5, 6, and 7 are 191, 128, 132, and 128 all of the time. The joystick has four non-power-button buttons and a power button, and these fields are used to report their values.

Pressing the speed buttons raises and lowers field 3, from a maximum of 255 to a minimum of 128, in 4 steps. There exists a mode that changes the speed in finer steps, but as configured, this is the way my system works. When the speed is lower than half-speed, field 4 has the range of 191 to 128. When the speed is higher than half-speed, field 4’s range is 191 to 255.

Field 5 is 128 when the horn button is not pressed, and 130 when it is pressed.

Field 6 is 132 when the joystick is on and running, and raises to 140

Field 7 is 128 when the joystick is in motion mode, and 129 when it is in seat mode. In seat mode, forward and backward motion of the joystick results in up and down motion of the seat height actuator. The joystick does report left and right motion as well in chairlift mode, but it doesn’t have any effect.

Field 9 is 15, for end-of-message.

Motor driver messages

Fields 7 and 8 are mirror images of each other. Field 7’s minimum appears to be 128, and its maximum is 144. Field 8 maxes out at 236 and has a minimum of 220, so it appears to be 364-field 7. I’m not sure what this field’s value indicates, but it appears to vary with the joystick position, so it may be some sort of current monitor or motor speed monitoring signal.

Field 1 goes from 128 to 146 as the system powers on, and stays there unless the chairlift is used. It appears to transition sharply to 178 when the chairlift moves up, and back when the chairlift moves down, so it is probably connected to the magnetic reed switch that senses the chairlift position.

Field 2 drops from 192 to 128 as the system powers on, and stays there. Fields 3, 5, and 6 are 128 all the time.

Field 6 goes from 128 to 129 the message after the joystick enters chairlift mode, so it is probably an acknowledgement to the joystick that chairlift mode was entered.

Field 4 goes from 128 to 160 as the system powers on, and stays there most of the time. It returns to 128 whenever the chairlift is not moving, so it may be related to breaking or motor activation. Before field 4 goes to 160, the joystick does not send any position commands, so the fields from 1 to 6 may be state signals that the motor driver sends to the joystick.

Field 9 is usually the end-of-packet field, but there are regular messages from the motor driver that are of the form

a '146' '128' '128' '160' '128' '128' '135' '229' '26' '133' '167' '185' '15'

rather than the more usual

a '146' '128' '128' '160' '128' '128' '135' '229' '15'

The additional values do not appear to change. The longer messages occur every 49th message, regularly, and starting with the second message from the motor driver, so I do not think that they are glitches.