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Inertial Solutions™
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The programmable Time Processing Unit (TPU) channels of the MC68332 CPU chip are a powerful subsystem used to produce the above features. 14 of 16 TPU channels are allocated above. TPU channel #15 is used by the GPS Time Mark, and TPU channel #8 is used by the operating system (thus accounting for all 16 TPU channels). Sensor Interface Cards provide the above 14 TPU customer interfaces as standard features. In the example of the 8 servo control outputs above, the first 8 TPU channels are programmed at initialization to be Pulse Width Modulation (PWM) output channels with fixed period and initial pulse width as required to center geared motor driven servos. Thereafter, the pulse width control register is modifiable, thus precisely controlling the motor driven position of a servo. Inertial Solutions provides two methods of servo position control to the customer. First, customers may choose to have their application imbedded with the IS firmware, thus allowing internal calls to the Applications Program Interface (API) that controls servo output. Second, serial messages may be received to set servo positions. The first method of embedding the customer application is superior for most feedback loops where loop delay must be minimized. Manual Override allows the customer to provide an alternate source of PWM output signals to the attached servos (connected through the Sensor Interface Card), such as from a Radio Control (RC) receiver. Upon detecting the activity of the alternate (on one channel), the override circuit switches the source of all 8 PWM outputs away from the 8 TPU inputs over to the 8 alternate overriding inputs. Therefore in this example of a RC receiver as the source of the overriding alternate, when the RC transmitter is turned on, thus activating the receiver servo PWM outputs, then servo control is switched over to the RC system so long as the transmitter and receiver are active. Likewise, the source of PWM servo output signals reverts back to the processor card TPU channels when the RC servo input signal becomes idle (as when the RC transmitter is turned off). A connection point is provided to bypass the above automatic selection circuit, thus allowing selection control to be provided externally.
Four TPU channels are initialized as Input Transition Counters (ITC). An amplifier circuit is provided on the Sensor Interface Card that interfaces directly to a photo transistor, thus providing the necessary current gain as required to drive the ITC input channel. Again, two methods are available to the customer for using these input counters. First, customers may imbed their application, thus allowing periodic internal calls to the API’s that read these free running counters. Second, serial messages may be sent containing the count and time tag. Feedback loop delays are minimized by the first method of embedding the customer application into the IS host processor.
Photo Transistor inputs are connected to TPU Inputs as Input Transition Counters (ITC) on pins 11, 12, 13, 14 at the Sensor Interface Card edge shown above. Servo motor power buss stops at the 8th servo signal output, thus isolating servo motor voltage from the digital power supply on the Sensor Interface Card. Ground is common. Small Servo motor batteries are connected as illustrated. Servo Battery Connection Recommend large servo motor power be supplied direct to the servo.
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