Digital Control of Motor Current

This page contains instructions for modifying a QLA board to enable digital current control on the FPGA, rather than via analog hardware.

DO NOT FOLLOW THESE INSTRUCTIONS UNLESS YOU REALLY KNOW WHAT YOU ARE DOING!

Incompatibilities between the hardware and firmware or incorrect hardware modifications can lead to destruction!

Note that digital current control requires an updated firmware, which is currently available in the feature-digital-control branch of the mechatronics firmware. There is a corresponding feature-digital-control branch of the mechatronics software repository.

Block diagrams

Block diagram of original circuit with analog motor current control

Block diagram of new circuit with digital motor current control

Step-by-step instructions

Breaking the analog feedback loop

1. Remove R46
2. Short R47
3. Do one of the following:

• Replace C74 (100 nF) by 191 kOhm (if SMD resistor available)
• Remove C74 (100 nF) and insert 191 kOhm between pins 1 and 3 of the op amp U38A (if THT resistor available)

4. Insert 191 kOhm from U38A pin4 (non-inverting terminal) to U39A pin 4 (non-inverting terminal), best done on the backside

Adapting motor feedback filter cut-off frequency to 40kHz:

1. Replace R54 by 39 Ohm
2. Replace R55 by 39 Ohm