A couple of weeks ago I built the first prototype of our Synaptron Mega (the board with the red glow in the photo). This is our Synaptron Micro operating system with a larger H-bridge. The operating system is designed to run on Microchip’s dsPIC33FJ64MC802, and creates a very versatile and programmable motion controller. It is designed to control brushed DC motors.
I hadn’t had a chance to test the board until this week. By the time I did get around to testing it there were problems. Not “flame-on” kind of problems, but certainly things I wasn’t expecting. First, every time I ran the microcontroller debugger the code would freeze. This caused me to spend a couple of hours troubleshooting the firmware. It turns out I had missed changing a couple of settings related to the H-bridge drive signals, and was creating a shoot-through condition whenever the PWM output was turned on. Shoot-through is when both the upper and lower side of H-bridge leg are turned on. Once that was sorted out, things started to go pretty smoothly. The shoot-through actually turned out to be a blessing in disguise since it was essentially testing a short circuit. The H-bridge limited the load current to 11A, and after a second or two thermal shutdown reduced that to 1.5A. No traces or surrounding components were lost. It was kind of a nice upside to wasting time troubleshooting.
The bottom board in the photo, connected to the USB-RS232 converter, is a test board. It allows us to break-out the connections to the Synaptron Mega to make plugging in encoders, potentiometers, serial converters, hardware debugger, and whatever else, easier. It also has spring loaded test points behind the motor connection terminal block. When the Synaptron Mega is “stacked” onto the test board we’ve got a nice compact interface board. The reason I don’t have them stacked in the photo is because I’m basically troubleshooting both boards with these first tests.
After fixing the shoot-through problem in the firmware I was able to get the position controller running in open and closed-loop operating modes. In fact, using the Synaptron Micro test software it only took a few moments to configure the Synaptron Mega to run as an analog controlled system using a quadrature encoder for feedback. The software was also useful in allowing me to test the communication link and all other input signals.
I noted the following issues on the schematic for the next iteration of the board. I might not implement all of these, but they are things to consider during the pre-production phase of the product design.
- Tighten up the TSSOP footprint models (the pin landings are too long).
- Use a larger footprint linear regulator or consider a switching regulator. With this design I assumed the on-board 5V regulator would be used to run a motor’s quadrature encoder, and if the extra current load is causing some heating at 12V, at 24V it’ll be worse. Maybe even twice as bad
- Change the 0402 discrete components to 0603 to give them a little more thermal mass (there’s plenty of room).
- Consider shrinking the board. This design needs to remain on a 2-layer PCB, and the mounting holes need to match standard cooling fan mounting holes. Shrinking the board may be tough, but I’d like to see it about 20% smaller.
- See if a bigger terminal block will fit.
All-in-all preliminary testing looks pretty good though. After a little current testing and calibration of the current measurement circuit, we should be able to get some applications done using the prototype board. If All goes well I might begin the second layout of the board next week.
I suspect the Synaptron Mega will be rated to run from 6-24VDC with about 6-7A continuous current with no cooling. The H-bridge itself is rated for operation to 40V and a peak current of 42A. But you never can attain those ratings in real life. A 24VDC system with transient voltages is probably generating voltages near 40V unless protection is employed. And any current load over 9A will cause thermal shutdown of the H-bridge unless significant cooling is attempted. So I like to spec our designs in a way that gives the user a good idea of what they can easily employ in the field.