Board

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Block Diagram

A small piece of hardware, shown to the right, was built which interfaces to a host PC via the parallel board. The board has two functions:

Generate four independent output analog waveforms to be sent out to the motors.

Sample four independent
analog acceleration readings coming from the two 2-axis accelerometers attached to the shaker table.

The signal generation function is accomplished by two Microchip PIC16C73A microcontrollers, PIC-A and PIC-B. Each chip outputs PWM signals which are low pass filtered and then output as four independent analog signals. In the figure, these are shown input to a power amp and then fed to the motors.

The four accelerometer signals are buffered and pre-amplified by a quad OP-AMP chip prior to being sampled by an Analog Devices AD7861 11-bit A/D converter. This chip is clocked (at 5 KHz) and controlled by PIC-A. Samples are available to the PC via the parallel port.

PCB Design

A printed circuit board (PCB) was designed (left inset) with a CAD program from ExpressPCB, a mail order PCB manufacturing company. The final artwork was submitted as .pcb file.

A picture of the final interface board with all components soldered is shown on the right.

Firmware

Firmware was written for the PIC16c73 using the excellent PIC-C v7.81 ANSI-C compiler donated to us all the way from Alderly, Australia by Hi-Tech Software. The firmware is divided into three files, available for download:

* firmware.h -- constants and data structures
* firmware.c -- parallel port servicing routines
* firmware_isr.c -- interrupt service routine (ISR)

The complete set of firmware files (along with the .hex and .obj files produced by the compiler) are grouped into firmware.zip.

Device Driver

All of our PC-side software was written over Red-Hat Linux version 6.0 (linux kernel 2.2.x). The device driver required parallel port access, which is simplified (and enabled to non-superusers) by installing (/sbin/insmod) the parport, parport_pc, and ppuser kernel modules, in that precise order, prior to running the device driver.

We wrote an API -- parallel.h -- which abstracts the ppuser interface to a few simple calls. The sets of functions available thru the device driver are condensed in a single API file, board.h.

A GUI-driven program has been written to test the API. The set of files comprising this program have been grouped into board.zip. The main user interface window is shown below -- it allows the user to interactively set (and recover) all parameters global to the signal-generation function of both PIC-A and PIC-B.

Signal Generation

Essentially, the desired output signal is a sin(t)+cos(2t) waveform shaped by an attack-sustain-decay envelope. Actual output measured with an oscilloscope is shown below:

Data Acquisition

With the table in motion (e.g., driven by four independent signals as in the above), the 4-channel acceleration samples cab be visualized in real time by a special program, and fit to smooth curves, as shown below:

The parameters recovered from the curve fitting step (4 for each wave, for a total of 16) are then used in the real-time estimation of the shaker table's center of rotation; a snapshot of the sensor measurements along with the position of the COR appears below: