OSMC Driver

In the search for a suitable commercial motor driver the Open Source Motor Controller [36] was found. This is an open source motor driver that is capable of delivering a large 160A

37

School of Engineering and Advanced Technology, Massey University

continuous current and up to 300A pulse current with a fan installed. This is achieved through the use of four parallel MOSFETs for each leg of the H-bridge arrangement.

The board is based around a HIP4081A H-bridge driver chip [37]. This provides all necessary signals to drive both the high and low side MOSFETs as well as the required voltage boosting circuitry for the high side.

Figure 3-11: HIP4081A block diagram [36]

The block diagram in Figure 3-11 shows how the HIP4081A is connected to an H-bridge. The AHS and BHS pins allow the driver to produce a voltage equal to the source voltage +10V that is required to turn the high side FETs on; in this case 90V. This is achieved through the use of an external diode and capacitor in a capacitor switched charge-pump system, shown in Figure 3-12. Other features of this chip include a disable pin, timing delay pins to ensure there is no shoot through when switching, and inbuilt logic that makes it impossible to have a shoot-through situation, by turning off the high side if the corresponding low side FET is turned on.

38

School of Engineering and Advanced Technology, Massey University

Figure 3-12: Charge pump circuit

In order to achieve the very high current rating of the OSMC, parallel MOSFETs had to be used. The circuit for these is detailed in Figure 3-13. The MOSFETS are arranged in parallel by connecting the drain pins together and the source pins together for each leg of the H- bridge. Although the gates are also connected together there is some additional circuitry present. Because of the ever present danger of shoot through and the tendency for MOSFETs to turn on faster than turn off, due to gate capacitance, additional protection against this is necessary. The resistor and diode circuit shown on each gate ensures that the FET can turn off fast, through the diode, and turn on a little slower, through the resistor. The resistors also aid in limiting the gate drive current, which, with the combined capacitance of four MOSFETs, can be large enough to damage the HIP4081A chip. Also two zener diodes are used to clip the high and low voltages of the gates to ensure they are not damaged by a voltage spike.

Figure 3-13: OSMC Gate Drive [36]

Other features of the OSMC include the use of Transient Voltage Suppressors (TVS) which handle any voltage spikes that come from inductive loads and can potentially exceed the

39

School of Engineering and Advanced Technology, Massey University

FETs drain to source breakdown limit. A RC-snubber is also used to filter any high frequency noise.

Although this driver meets most of the requirements for the desired motor controller, including direct PWM control, high current capabilities and a fan not required at the desired current, it is still missing some important features of most commercial controllers. Current sensing for over current protection is the most important missing aspect, with the ability to detect when the driver is drawing too much current so that it is possible to shut down and avoid any damage. Smarter control circuitry would also be useful; the current design involves having two PWM signals, one for forward and one for reverse. It would be better to have a single PWM input with a directional control line. Another negative aspect of the driver is the fact that it is over engineered for the required purposes. The OSMC can handle a continuous current of 160A, when only 20A is required; this can lower the overall cost and size of the driver board.

Because the OSMC is open source, the design can be modified to suit the desired application and any extra features can be added to enhance the functionality of the design. Because of this a single pre-fabricated unit was purchased to test and observe if it would be suitable for the application.

40

School of Engineering and Advanced Technology, Massey University

This motor driver, shown in Figure 3-14, was first tested with a smaller motor using a breadboard to provide the four signals to the driver from the PWM and directional outputs of the microcontroller. The board proved to work correctly and had no problem controlling the motor. A joystick was integrated into the system and the driver continued to perform with the variable speed input. This trend continued when one of the larger motors was used with a 24V battery source.

Because of the satisfactory performance of the driver OSM, it was decided that designing and building an in-house driver, based on the OSMC, would provide the best performance and functionality for the platform.