Electronics:
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Electronics:
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ProtoBot High-Power Motor Driver:
Schematic:
Board Footprint:
The next piece in the new line of boards I have created for ProtoBot, this board is the original piece that inspired the development of this entire robot. Designed originally as a test application for the Sojourner robot (featured elsewhere in this site) this board represents the high powered motor drive and sense function. I needed a driver that could handle the 17A stall currents of the motors I am working with without burning up and even though for normal operation a lesser driver would have fit the bill, inevitably during the development and testing phase of this robot, full stall or over drive will take place. With heat sinks and fans in place the drivers will easily sustain short term operation at 50A and continuous operation at 20A.
Click to enlarge
Along with calculating the filter components at build time, I added on piece that is not included in the schematic. A pair of balancing resistors have been added in parallel with the two large capacitors on the power supply front end.
Key Board Level Technologies:
- Intersil HIP4081A
power switching H-Bridge controller.
- International Rectifier IRFZ44E
- National Semiconductor LM2904
- National Semiconductor LM311
- Texas Instruments DCP010512DBP-U
- Fairchild Semiconductor 2N7002
One of the key features in the separation of high current drive and analog sensing functions is the close coupling of resistors with Kelvin sense taps to the operational amplifiers to sense and buffer the signal along with high frequency rejection (low pass) filtering to rule out false sensing on characteristic transients due to brush commutation. Another key piece to be solved will be the routing of airflow over the heat sinks and traces. A quick verification across IPC trace width tables shows a ~30-deg-C temperature rise on the traces at their worst case sizing without any airflow. This can be easily managed with proper fan placement, heat sink sizing, ducting and cable routing.
In this application, the continuous power delivered to the drive system is a function of power distribution components on the ProtoBot's main power board, including fuses and trace widths. Even though this board has much higher design criteria, the system level design drives a hardware current limit of 10A. This value is set by an indicator flag via the on board comparator. This function feeds back to the CPLD on ProtoBot's main controller board where a time out and retry function is implemented. In the schematic presented, the discrete analog components (resistors and capacitors) that set the various trip points have not been solved and await calculation prior to final stuffing.
If you don't have access to the classic reference material and need to determine fast parameters for trace width based on thermal variance, current and thickness, use the link to the right to access one of the commonly found trace width calculators out there on the internet.
Trace Width
Calculator:
Current Progress
At some point later I'll go back and add a photo of this board mounted in the acrylic enclosure on the robot. In addition, there are still a good number of cable assemblies to install and of course some software to develop to drive it correctly.
The only significant error (other than a large number of component values that had to be significantly tweaked) during the assembly process is the footprint for the S3A surface mount diodes. I had used an SMB footprint and somewhere I missed the fact that these diodes are actually in an SMC footprinted package. I have done my best to pre-solder the pads, use lots of flux and then sweat the parts on. No, not aerospace quality but hopefully it will get me by.
The Board Footprint
And here is how it looks installed on the robot.
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