To drive and control current in these motors an all DMOS full H-bridge with clamp diodes is used, along with an amplifier for sensing the load current, a comparator, a monostable , and a digital-to-analog converter (DAC) for the digital control of the chopping threshold. Together, they implement a fixed off-time chopper amplifier (Figure 3). Also incorporated are logic level shifting and drive blocks for digital control of the direction of the load current and for braking.
Figure: The fixed off-time chopper amplifier used by the motor controllers.
The H-bridge (Figure 4) consists of four DMOS power switches and associated clamp diodes connected in an H configuration. Turning ON a source switch and a sink switch in opposite halves of the bridge forces the full supply voltage--less the switch drops--across the motor winding. While the bridge remains in this state, the winding current increases exponentially towards a limit dictated by the supply voltage and the switch drops. Subsequently, turning OFF the sink switch causes a voltage transient that biases the diode of the other source switch. The diode clamps the transient at one diode drop above the supply voltage and provides an alternative current path. While the bridge remains in this state, it essentially shorts the winding and the winding current recirculates and decays exponentially towards zero. The above sequence repeats to provide a current chopping action that limits the winding current to the threshold. Chopping only occurs if the winding current reaches the threshold. During a change in the direction of the winding current, both the switches and the diodes provide a decay path for the initial winding current.
Figure: The H-bridge as part of the motors' current control.
The current sense amplifier (CSA) (Figure 5), another component of the micro-controller, uses one of the 4000 transistor cells of both upper power switches to provide a means for sensing the load current. It forces the voltage at the source of the sense device to equal that at the source of the power device; thus, the devices share the total drain current in proportion to the 1:4000 cell ratio. Only the current flowing from drain to source, the forward current, is registered at the output of the CSA. In these conditions, the CSA will provide around 250 A per Ampere of total forward current conducted by the upper two switches of the power bridge, developing a potential across R13 that is proportional to the load current. In this way, modifying the R13 value will modify the gain of the chopper amplifier.
Figure: The current sense amplifier (CSA).
The DAC sets the threshold voltage for chopping at Vref D/16, where D is the decimal equivalent (0 - 15) of the binary number applied at the four digital inputs of the DAC. The 5 V voltage reference is supplied by a 7805 voltage regulator IC. As the voltage at the CSA output surpasses that at the output of the DAC, the comparator triggers the monostable, and the monostable, once triggered, provides a timing pulse to the control logic. During the timing pulse, the power bridge shorts the motor winding, causing current in the winding to recirculate and decay slowly to zero. A parallel resistor-capacitor network connected to ground sets the timing pulse (the off-time) at about 1.1RC seconds.
In order to implement this motor-controller circuit an LMD 18245 (National Semiconductor) integrated circuit is used for each motor. Because we use 8 bits of data to control a robot, only 2 of the DAC's 4 digital inputs are used to obtain 3 speeds and the STOP state. Another bit sets the direction (high-forward and low-backward) and the remaining bits (1 for each motor) are used for special purposes if needed.
Step changes in current drawn from the power supply occur repeatedly during normal operation and may cause large voltage spikes in the power supply line. The voltage spikes need to be limited to less than the absolute maximum rating supported by the circuit. On the other hand, the initial load current tends to raise the voltage at the power supply rail at a change in the direction of the load current and the current transients caused by the reverse recovery of the clamp diodes tend to pull down the voltage at the power supply rail. Therefore, bypassing the power supply line at Vcc is required to protect the device and minimize the adverse effects of normal operation of the DC motor. Using both a 1 F high frequency ceramic capacitor and a 330 F aluminum electrolytic capacitor eliminates the problem. They have to be placed within one half inch of Vcc and their leads have to be as short as possible.