SNVAA20 July   2021 DRV8833 , DRV8833 , LMR33630 , LMR33630

 

  1.   Trademarks
  2. 1Introduction
  3. 2Motorized Resistive Load Architecture
    1. 2.1 Controller Board
    2. 2.2 Resistor Plate
  4. 3Motorized Resistive Load Design
    1. 3.1 Controller Board Design
      1. 3.1.1 Power Management
      2. 3.1.2 Power Converter Selection
      3. 3.1.3 Interface and ADC Selection
    2. 3.2 Resistor Plate Design
      1. 3.2.1 Motor and Motor Driver Selection
      2. 3.2.2 Resistor Track
      3. 3.2.3 Mechanical Arm Assembly
      4. 3.2.4 Feedback Control
  5. 4Thermal Considerations
  6. 5Performance and Results
  7. 6Summary
  8. 7Appendix
    1. 7.1 Controller Board Main Schematic
    2. 7.2 Controller Board Sub-Schematics
    3. 7.3 Resistor Plate Schematics
    4. 7.4 Python Code

3.1.1 Power Management

Proper power management entails that the total power must be calculated, and divided appropriately between the different critical components. The two elements that require the most power are the Raspberry Pi and the motor; therefore, their power consumption is calculated first. The Raspberry Pi 4 model B is typically powered by a USB Type-C adapter, and so requires 5 V and a maximum current of 3 A. It should be noted that minimal processing power is required and so the Raspberry Pi consumes less than 1 A for this design. The motor requires a motor power supply voltage (VM) of 5 V and 1 A per phase. The total power consumption of the main system components is:

Equation 1. P o w e r = P = V o l t a g e × C u r r e n t P S y s t e m = P C o n t r o l l e r + P M o t o r P S y s t e m = ( V U S B _ C × I P r o c e s s o r ) + ( V M ×   ( I P h a s e 1 + I P h a s e 2 ) )   P S y s t e m = ( 5 × 1 ) + ( 5 × ( 1 + 1 ) ) = 15  W

To ensure that this power is provided to the critical components, a standard 12-V, 2-A adapter is used to power the system. Such an adapter ensures that sufficient power will be available to fully power the system:

Equation 2. P A d a p t e r = 12 × 2 = 24  W

The remaining components require minimal power, but their power consumption can be calculated for completeness. The LCD requires an input of 3.3 V but only consumes 440 µA which equates to:

Equation 3. P L C D = 3 . 3 × 440 × 10 - 6 ≈ 1 . 5  m W

The LCD backlight requires more power as it takes in an input voltage of 3.3 V and consumes 30 mA which also passes through a 10-Ω resistor resulting in:

Equation 4. P B a c k l i g h t   = ( 3 . 3 × 0 . 030 ) + ( 0 . 030 2 ) × ( 10 ) ≈ 110  m W

The data sheet for the ADCs indicates that each requires 0.26 mW with a 3-V rail, totaling 0.52 mW. To conserve power, the motor driver may be disabled when not in use. This also prevents the motor from locking into place when not in use. If the motor driver is disabled, the weight of the mechanical arm assembly is enough to keep the arm in place when the motor is not in motion, but the arm may move if acted upon by an external force.