Kart/DC motor controller

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DC motor module

DC motor

The DC motor is a Modelcraft RB350018-2A723R.

It is powered in 12 V and draws a maximal current of 0.7 A.

The control board hosts a dual full-bridge and a circuitry to create the switching dead-times and the direction of the current.

FPGA design

dc motor block

The DC motor controller has to control the Kart's propulsion both in forward and reverse.

For this, two signals are generated :

  • A pwm with a controllable frequency whose duty-cycle is modified to control the speed
  • A forwards signal to drive either forward or backward.

The speed is set in the DC motor speed register, ranging from -15 to 15.

The frequency is given by the prescaler in the DC motor prescaler register with the formula f = fclk / (PWMsteps * prescaler) = 10M / (16 * prescaler).

To begin with, the design environment has to be downloaded, which contains the registers and events manager and an empty dcMotorPwm block which is to be completed.

Functionality

The block receives a speed signed number and has to drive the DC motor with a pwm and a forwards signal.

These signals are internally converted to two driving signals to control an H-bridge (they take into account the dead-times to avoid short-circuiting the motor while switching).

The mean amplitude of the DC motor's voltage is controlled by a Pulse Width Modulation (PWM):

  • The forwards signal is derived from the sign of the speed control.
  • The pwm signal is derived from the absolute value of speed.


Dialog-warning.png

The PWM signal is implemented with the help of a free-running counter and a comparator. However:

  • the power transistors cannot switch at too high frequencies
  • there is a need for a dead time where all power transistors are open between the PWM transitions

Because of this, the PWM period is limited to a certain value. This is achieved with the help of an en signal generated by a counter dividing the clock frequency. The counter only increments when this signal is '1'.


The minimal value of the PWM signal is studied in another part of the project. The period of the en pulse train is set in the DC motor PWM period register and given by the formula f = fclk / (PWMsteps * prescaler) = 10M / (16 * prescaler).


View-pim-tasks.png

Draw the schematics of the dcMotorPwm block.

Hardware orientation

The mechanical design can either lead the Kart to drive forwards or backwards when a positive voltage is applied to the DC motor.

In order to cope with this, a setup signal, normalDirection, is provided to the block. normalDirection being '1' means that a positive voltage applied to the DC motor lets the kart drive forwards.


View-pim-tasks.png

Update the schematics of the dcMotorPwm block in order to cope for the different mechanical design possibilities.

The setup bit is configured in the hardware control register.

Bluetooth connection

When the Bluetooth connection is lost, the DC motor should not turn.

In order to cope with this, a control signal, btConnected, is provided to the block. When btConnected is '0', the DC motor must stop.

The BT connection bit is configured in the hardware control register.


View-pim-tasks.png

Update the schematics of the dcMotorPwm block in order to stop the motor on connection loss.

Tests

The DC motor functionality can be tested through the DCMotor_test -> dcMotorController_tb block.

DC motor tester

The corresponding simulation layout file for Modelsim is available under $SIMULATION_DIR/DCMotor/dcMotorController.do. It will automatically run the simulation if it has not been modified.

DC motor tester
  • The blue header shows which test is performed
  • The yellow signals are those generated by the tester
  • The purple signals are the one generated by your implementation
  • You can add and remove signals at will, they will not interfere with the tests

Testing

The following tests are performed :

  • The prescaler signal is set to 10
  • Positive speed of 33, 66 and 100% are tested, expecting correct PWM and forwards = '1'
  • Negative speed of 33, 66 and 100% are tested, expecting correct PWM and forwards = '0'
  • The normalDirection is inverted
  • Positive speed of 50 and 100% are tested, expecting correct PWM and forwards = '0'
  • Negative speed of 50 and 100% are tested, expecting correct PWM and forwards = '1'
  • Restart is held at '1' while normalDirection is inverted again, expecting PWM = '0' and forwards = '1'
  • Restart is released, expecting correct PWM and forwards = '1'
  • The BT Connection is lost, expecting PWM = '0' and forwards = '1'

The transcript window gives you details on if tests passed or not:

  • For the PWM signal, you get PWM OK or PWM Error.
  • For the forwards signal, you get Direction OK or Direction Error.
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