Joints

Dexter's hardware has 5 joints in the base model, and the v2 Tool Interface adds 2 more for a total of 7. Each FPGA measured joint has an encoder.

#	FPGA	Connector	Human / Description	Slots	Eye Rotation*
1	1	Base	Roll. Shoulder rotate (vertical)	200	Counter
2	3**	Pivot	Pitch. Shoulder lift	180	Counter
3	2	End	Pitch. Elbow. Pwr set R65/66	157	Counter
4	4	Angle	Pitch. DiffA1 Differential with 5	115	Counter
5	5	Rotate	Yaw. DIffA2 4 and 5 make the wrist	100	Clockwise
6	6	External	(not used, could be extrude)
6	n/a	J20 bottom pin	Rotate Dynamixel	n/a	n/a
7	n/a	J20 bottom pin	Gripper Dynamixel	n/a	n/a

* The direction of the rotation of the eye for Joint 5 is the opposite of the others.
** Development of the FPGA code started in the middle and so the numbering of the joints is not consistent. Specifically, when reading (or writing) the AdcCenters.txt file from the share folder, the 2nd and 4th, and 3rd and 5th, lines must be swapped

AxisCal.txt

Because joints may (do) have different gearing and drive systems, the conversion between one step of the motor and 1 degree of motion for the joint requires a factor. These also account for microstepping and the conversion from degrees to arc seconds. There is also an compensation factor required for joints 4 and 5 when joint 3 is moved (ANGLE_END_RATIO). All of this information is stored in the AxisCal.txt file on the share. Anytime an angle value is sent or recieved by the Firmware, it is multiplied by the conversion factor for that joint (JointsCal array). The following Javascript (for use in DDE) calculates the correct values given the gear ratios, stepper motor steps per revolution, and stepper driver microstepping setting.

var motor_steps = 400
var micro_step = 16
var gear_ratios = [
    52 / 1,  //Joint 1: Harmonic drive (changed for cycloidal, worm, etc...)
    52 / 1,  //Joint 2
    52 / 1,  //Joint 3
    90 / 16, //Joint 4: Pulley on Joint 3 / Pulley on motor shaft
    90 / 16  //Joint 5: Pulley on Joint 3 / Pulley on motor shaft
]

var AxisCal_string = ""
for(let i = 0; i < 5; i++){
	AxisCal_string += -(gear_ratios[i]*micro_step*motor_steps) / (3600*360) + "\n"
}
AxisCal_string += -Math.round(gear_ratios[3] / gear_ratios[2] * Math.pow(2, 24))
// gear_ratios is a zero index array so [3] is joint 4 and [2] is joint 3.

The axis 5 line in the file, second line from the end, may need it's sign swapped. If you do a move_to([x,y,z], [-1,0,-1]) the end effector should be pointing in the negative x direction. That is, if the [x,y,z] values are all positive, and you are standing behind the robot (robots point of view), then the end effector should be pointing to your left at 45' with a direction of [-1,0,-1]. If this is wrong, the position of the tip of the end effector will change when going to a point from different directions. E.g. move_to([0,0.5,0.075], [-1,0,-1]) with place the end effector at a different point than move_to([0,0.5,0.075], [1,0,-1])

Status

Status data for each of the joints is returned via the 'g' command oplet and automatically after most commands.