Motor Driver Circuit Design
Introduction: The motor driver circuit takes a PWM speed control signal from the Motor Control Unit and creates a drive current sufficient to power the motor/propeller assembly. The PWM signal is a 5KHZ 0-100% square wave from the MCU. This circuit converts the low voltage TTL level signals to the 11.1V Vbat level and drives a low on-resistance power FET. The FET includes an internal reverse bias protection diode, so virtually the only extra component needed is an energy storage capacitor to reduce the harmonic content of the drive signal to the motor.
Multi-Project PWB: Since it wasn't clear how complex the driver circuit needed to be, I designed a number of driver circuits ranging from a simple buffer-FET driver all the way up to a full H-bridge driver with current sense. There are a number of options along the way, including the design I think we will ultimately use, which is a buffered FET driver with current sense. Figure 1 shows the circuit board layout that we sent to the PWB shop. Notice that on the PWB some designs are replicated twice. This is because for the lowest cost order from the PWB shop you can get three copies of a fixed size PWB for about $80 including shipping, but we need four driver boards for the helicopter application. While space on the board was at a premium, I put two copies of the high probability designs on each board (noted in the list below with an asterisk * character), but only one copy of the designs I thought were more experimental. If the experimental design is best, we'll do a respin of the board to replicate the best design and probably put something else on the board that we are experimenting with at the moment.
The layout of the PWB is shown in the figure below.
Figure 1 - Multi-Project PWB design containing five different experimental motor driver circuits.
The PWB design includes five designs labeled motCtl v0..4. Here is a summary of what each design is set out to do.
- V0*: This is the most basic FET driver with current limit resistor for the gate drive of the FET. If it works OK, it is the simplest of all designs since it only uses a FET, a resistor and a capacitor. There is space for an external reverse bias diode in case the internal diode isn't sufficient.
- V1: This is a simple variant of V0 where a second FET is added in parallel to the first. When we were testing the hand wired version of the motor driver using relatively cheap power FETs (IRF501s), a single FET would get pretty hot when driving the motor with 12V at 2A, but putting a second one in parallel solved the heating problem, at the cost of an extra component. This design uses FETs that were picked for their very low on-resistance, so hopefully this design won't be needed.
- V2*: This design uses a two transistor buffer to add gain to the gate drive signal of the power FET, in case the gate capacitance load is too high for the MCU to drive directly. It also includes a LM389 op-amp to form a current sense circuit to provide feedback to the MCU of motor current. Hopefully we will be able to use this to sense motor stalls and then have the MCU do something like stop the motor for a few tenths of a second and then try to restart it (assuming that it bumped into something and by stopping the motor it can fall slightly and get clear of it). The op-amp needs to generate an output that is in the range of the A/D converters on the MCU, so an on-board 7805 power regulator was included.
- V3*: This one is a simple variant of V2 that includes a place to mount a smart LED module at the top of the board. The module uses IIC signals, plus 5V power and ground, so these signals are passed from the bottom connector to the top.
- V4: This version gets a little bit more aggressive and uses four FETs in a full-bridge circuit with one set of buffers driving the PWM signals to the bottom transistors and another set selecting the polarity of the power being driven to the motor to allow it to spin forwards or backwards. This would potentially enable the helicopter to do aerobatic rolls, etc. The circuit also includes a current sense circuit that has a reference bias set to 1/2 way between the 5V supply and ground. The output signal will be 2.5V for no current and then plus/minus depending on the current and direction of motor spin.
The PWB has been fabricated and is back from the PWB shop. Figure 2 below shows what we got back in the mail. Before building up and testing the circuits, the PWB will be cut and each design separated using a bench top break tool (Harbor Freight special).
Figure 2 - Multi-Project PWB board back from fabrication house.
Figure 3 below shows the V0 circuit being prototyped and tested.
Figure 4 below shows the V1 circuit prototype/test.
Figure 5 below shows the V2 circuit prototype/test.
Figure 6 below shows the V3 circuit prototype/test.
Figure 7 below shows the V4 circuit prototype/test.
Summary: [tenative] The helicopter drive circuits proved that with low on-resistance FETs the motor control circuits can be mounted directly on the Motor Control Unit.