Kart/DC motor controller
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[[File:Kart_dc_module.jpg|center||DC motor module]]
== DC motor ==
== DC motor ==
Revision as of 10:41, 30 June 2022
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.
The DC motor controller has to generate a
pwm and a
forwards signals to control the kart's speed.
speed is set in the DC motor speed register, while 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.
The block receives a
speed signed number and has to drive the DC motor with a
pwm and a
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).
forwards signal is derived from the sign of the
pwm signal is derived from the absolute value of
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 kart 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).
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.
The setup bit is configured in the hardware control register.
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.
btConnected is '0', the DC motor must stop.
The BT connection bit is configured in the hardware control register.