Hbridge Movement Panel

How to use the Hbridge Movement Panel robot skill

Drives a robot with a dual channel H-Bridge motor controller, such as an L298N, L293D, TB6612FNG, BTS7960 or Sabertooth in analog mode.

This is a movement panel: once it is in the project, everything in ARCx that moves the robot - the Movement Joystick, the Wiimote, Movement.Forward() in a script, the Autonomous AI skill, a ROS /cmd_vel binding - drives these two motors. Only one Movement Panel can be in a project at a time.

Requirements

• A connected EZB.
• A dual channel H-Bridge, one channel per side of the robot.
• Four digital ports, two per motor.
• Two PWM-capable ports as well, if the board has speed (enable) inputs and you want speed control.

How an H-Bridge works

Each motor channel has a pair of inputs, usually labelled IN1/IN2 and IN3/IN4. The pair decides which way that motor turns:

Many boards have a third input per channel, labelled ENA/ENB or PWM. That one is not a direction, it is a throttle: the duty cycle on it sets how fast the motor turns. Boards without it, or with those pins jumpered to 5V, always run at full speed.

Adding it to a project

1. Connect to the EZB.
2. Add the skill (Movement Panels category).
3. Open the configuration and choose the four trigger ports - Trigger A and Trigger B for each motor.
4. If the board has enable inputs, tick H-Bridge has PWM speed inputs and choose the two PWM ports.
5. Save, then push the joystick in the skill's window.

Opening the configuration stops the robot first, so the ports are never left latched HIGH while they are being reassigned.

Wiring

A typical L298N wired to EZB digital ports D1 to D6:

Those are the defaults, so a robot wired this way needs no configuration at all beyond ticking the PWM box.

Give the H-Bridge and the EZB a common ground, and power the motors from the H-Bridge's own motor supply - never from the EZB's servo rail.

The four ports do not have to be on the same EZB. Each board is sent its own single packet, so a robot with one channel on each of two boards still drives both sides together.

The window

• Joystick - push forward or back to drive, sideways to turn. How far the thumb is pushed sets the speed, and pushing forward at an angle blends the two sides so the robot drives in an ARC instead of a spin turn. The thumb springs back to the centre when released, which stops the robot.
• Left and Right readout - the current speed for each side, 0-255. These are ARCx's global movement speeds, so they also move when another skill or a script commands a movement. They are only shown when PWM speed control is turned on, because without an enable input the speed has nowhere to go.

How direction works

Turning is done by driving one side forward and the other in reverse (a spin turn):

Speed control

With H-Bridge has PWM speed inputs turned on, every movement also writes a duty cycle to the two enable ports. ARCx's movement speed of 0-255 is scaled to a duty cycle of 0-100%, so speed 255 is full throttle and speed 128 is roughly half. Each side uses its own speed, so a script or a joystick can set the two differently to drive in an ARC instead of a spin turn.

Stopping writes 0% as well as dropping the triggers.

Small DC motors often will not turn below about 30% duty cycle. If the robot stalls and buzzes at low speeds, that is the motors, not the skill - raise the speed.

With the option turned off, the enable ports are never written and the motors always run at whatever speed the motor supply gives them. The speed values are then ignored by this skill, although other skills still see them.

Configuration

The Window tab sets the robot skill's window title, its size on the workspace and notes about what it does in this project.

Troubleshooting