The emergency stop system acts as a final resort for stopping the robot incase anything goes wrong (robot runs away or arm acts unpredictably). The challenge with this project was to not simply cut off power to the robot, but to only stop the motors when the emergency stop is initiated. After the motors have been stopped, the robot’s main computer should still be in operation so we may access sensor data, logs, and perform debugging.
To initiate the emergency stop, users simply press the big red button (top picture and “S1“ in the bottom picture), to release the robot from its stop state, the button must be rotated in the direction of the arrows. When pressed, the button triggers relays which will cut off power to the motors connected to it. An illuminated red LED signals the emergency stop has been initiated, and motor power has been cut off. In contrast, an illuminated green LED signals voltage pass-through, and all motors in operation.
Completely cutting off motor power may not always be ideal (if the robot is stopped on a hill, or if it was holding onto something). Thus a signal line has been implemented into the circuit (“Signal flag in figure 2) which will be read as high (+5V) when the stop is initiated, and low (0V) when the motors should be in operation. We can then choose to hold a motor’s position when the emergency stop has been initiated, instead of cutting off power completely.
Each relay circuit will be responsible for a different voltage (with the exception of the one on the far left labelled 5V, which powers the LED's and relays). TE Connectivity T9AS5D22-5 relays have been chosen to act as the power relays on the circuit. These relays can handle 20A at the contacts, and require 5V to trigger the coil. A diode has been placed across the relay’s coil terminals to prevent possible back voltage from the motors.
P1-P8 are 2x4 Molex Mini Fit Jr connectors. P1-P4 will connect to the onboard DC-DC converters. and P5-P8 are the outputs which the motors will connect to.
P1-P8 in the PCB layout are unconnected because we plan to connect these to the circuit via external 16AWG wire instead of pcb traces because of the possible high currents (up to 15A) being delivered by the DC-DC converters.