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

3.2.4 Feedback Control

Stepper motors can be controlled with high accuracy, but do not inherently have feedback to indicate their position. An external means of feedback is therefore necessary to control the stepper motor effectively. The resistive track provides a means to do so by using an external reference voltage which connects to the resistive track via a fuse and series resistor, forming a variable resistive divider. The middle node voltage of this variable resistive divider changes depending on the position of the metal arm assembly and is tracked by the second ADC located on the controller module. The middle node voltage can vary from 0 V up to the divided voltage, VDiv, which is defined by the fixed series resistance chosen as per equation:

Equation 6. V D i v = V R e f × R D i v ( R D i v + R R e f )

Where RDiv is calculated according to:

Equation 7. R D i v = R T o t a l - R L o a d

This design uses the quiet 3.3-V rail from the controller board as a reference voltage and a 50-Ω series resistor, which limits VDiv to values between 0 V (load of 40.5 Ω) and approximately 1.5 V (load of 0.0 Ω). The feedback scheme is outlined in Figure 3-6.

Figure 3-6 Feedback Control

Care must be taken to ensure that the LDO is able to provide the maximum required current. The selected peak current output current of the LDO is typically 150 mA and is no less than 100 mA, making it a suitable selection. The maximum LDO current for this design is defined as:

Equation 8. M a x   L D O   C u r r e n t = I L D O _ M a x = V R e f / R R e f I L D O _ M a x = 3 . 3 / 50 ≈ 70  m A

If a single value of resistor was used for the entire resistive track, the voltage and resistance relationship would be linear, but since different sized resistors were used, the relationship is piece-wise. The relationship can be empirically determined between the resistance and position. Alternatively, the voltage VDiv, can be calculated for each resistor.