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Currents and temperatures

Motor currents

The control board sets each motor's running current in the firmware. The values are chosen for the motors of the BOM and for the printed parts around them; joints 4 and 5 run below their motors' rating because their motors are enclosed in the forearm.

Joint Maximum current (constants.h) Current while moving Holding current
Joint 1 2000 mA 85 % of the maximum: 1700 mA 70 % of that: 1190 mA
Joint 2 2000 mA 1700 mA 80 %: 1360 mA
Joint 3 1900 mA 1615 mA 80 %: 1292 mA
Joint 4 1700 mA 1445 mA 80 %: 1156 mA
Joint 5 1700 mA 1445 mA 80 %: 1156 mA
Joint 6 965 mA 820 mA 88 %: 722 mA

The firmware calls rms_current(MOTORx_MAX_CURRENT * 0.85, hold_multiplier); the hold multiplier is 0.7 for joint 1, 0.8 for joints 2 to 5 and 0.88 for joint 6 (motor_init.cpp; Firmware parameters). Lower currents give less torque, which means lower reachable speeds and accelerations and a higher chance of lost steps; higher currents heat the motors and the plastic around them.

Temperatures

Stepper motors are rated for 100 °C and more, but PAROL6 is plastic, and PETG softens around 80 °C. These are the motor case temperatures measured on a robot at the default currents, at room temperature:

Joint Holding position for 5 h Moving for 2 h
Joint 1 51 °C 64 °C
Joint 2 54 °C 64 °C
Joint 3 48 °C 52 °C
Joint 4 60 °C 73 °C
Joint 5 60 °C 72 °C
Joint 6 61 °C 65 °C

Through a thermal camera, a powered PAROL6 shows where the heat is: the motors of joints 2 and 3 beside the shoulder, the elbow (the hottest point in this picture, 50 °C) and the wrist motors in the forearm, and the control board at the back of the base. The scale runs from 29.6 to 50.0 °C.

A powered PAROL6 through a thermal camera: warm motors at the shoulder, the elbow and in the forearm, and the control board in the base
Figure 3.14: A powered PAROL6 through a thermal camera: warm motors at the shoulder, the elbow and in the forearm, and the control board in the base.

Long runs

Joints 4 and 5 come within 10 °C of the plastic's limit after two hours of continuous motion. For a robot that runs for hours, reduce the currents in the firmware (MOTORx_MAX_CURRENT in constants.h, or the hold_multiplier in motor_init.cpp), lower the duty cycle, or let it rest in a pose that does not load the motors. A deformed forearm cannot be straightened: it has to be printed again.

The stepper drivers protect themselves: a TMC5160 that gets too hot shuts its outputs down, and its joint goes limp. The fan in the base keeps them well below that (Control board technical data). Firmware 1 does not tell the PC when a driver is hot: the joints' temperature error bits in its packet are always 0. The driver temperature is not the motor temperature.

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