PWM Circuit Function Automatic Test System Based on LabVIEW

Automatic test system (ATS) is a general term for a type of system that can automatically complete measurement, data processing, and display (output) test results. In different technical fields, the test content, requirements, conditions, and automatic test systems are different, but they all use computers to replace human testing activities. A general automatic test system includes a controller, an excitation source, a measuring instrument, a switch system, a human-machine interface, and a unit-to-machine interface. The test object of this automatic test system is a PWM circuit board, as shown in Figure 1. PWM (pulse width modulation) is a circuit that changes the output voltage by changing the duty cycle. PWM technology is widely used in DC motor speed regulation and other occasions.

Its working principle is as follows: As shown in Figure 2, a triangle wave is generated at point F. The amplitude of the triangle wave can be adjusted by adjusting RP3, and the frequency of the triangle wave can be adjusted by adjusting RP2. U1D is a voltage comparator. The waveform at point F and the waveform at point B are compared to finally obtain the waveform at point C. Since the PWM waveform at point C is obtained by comparing the voltages at points F and B, the voltage value at point B can be adjusted by adjusting RP1, and the duty cycle of the waveform at point C can be adjusted to ensure that the duty cycle of the waveform at point C exceeds 50%. The waveform at point C is driven by the OTL circuit to obtain the final test waveform at point D. After the amplified square wave signal is obtained at point D, it is used as a drive output signal through the CMOS tube to drive a motor or a signal light.
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1 Test Task
The test task of this design is: take the PWM circuit board as the test object, use the digital oscilloscope Tek TBS1012B-SC with USB interface, matrix switch, and digital I/O card to build a test platform, and design an automatic test system for measuring the waveform of the key points of the PWM circuit board on the LabVIEW development platform.
Make it have the following functions:
(1) It can observe and adjust the triangular wave output frequency and waveform, and can adjust it in a human-computer interaction way, fo=1kHz±5%; Up=3V±10%.
(2) It can observe the waveform of the comparator output point C. And it can adjust the duty cycle of point C to 50% in a human-computer interaction way.
(3) It can observe the modulation waveform of point D.
(4) The measured waveforms, frequency, and amplitude of points F, C, and D can be displayed on the interface of the test system.

2 Hardware Platform
This system uses LabVIEW as a development platform to write test programs. Test the key points of the PWM circuit to determine whether the circuit components have achieved the intended functions. The system hardware platform mainly consists of Tek TDS1012B-SC, voltage regulator, matrix switch, matrix switch driver - NI6509 digital I/O card, etc. The hardware platform is shown in Figure 3.

2.1 Oscilloscope
Since the system is used to measure the waveforms of key points F, C, and D in the PWM circuit, the circuit needs to be adjusted before the system starts testing. The debugging requirements are: adjust the frequency and amplitude of the triangle wave to make fo=1kHz±5%; Up=3V±10%. The debugging process needs to be carried out by observing the oscilloscope. In addition, after measuring the waveform of a test point, the general oscilloscope needs to manually switch the test point. If it is a Tek digital oscilloscope, the waveform can be tested continuously. Its function is to measure the waveform and automatically transmit the waveform and data to the computer.

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2.2 Matrix switch
To measure the waveforms of the key points F, C, and D in the PWM circuit, there is only one oscilloscope. One oscilloscope is needed to measure the waveforms of the three test points, so a multi-point switch is needed for switching. We use a matrix switch to switch between the points.
This test system uses a 4×24 matrix switch to assign different test points (such as F, C, and D) to the input end of the oscilloscope to achieve time-sharing measurement of multi-point waveforms by the oscilloscope. The principle circuit diagram of the matrix switch is shown in Figure 4.

Generally, the measuring instruments are connected to H0, H1, H2, and H3, and V0, V1, V2, ..., and V23 are connected to the test points. As long as the switch that crosses the row and the column is turned on, the instrument connected to the row can be connected to the test point of the column. For example, if the oscilloscope is connected to H0, as long as K0 is turned on, the oscilloscope measures the waveform at point V0. If k1 is turned on, the oscilloscope measures the waveform at the V1 test point. For this system, H0 is connected to the oscilloscope, V0 is connected to point F of the circuit, V1 is connected to point C of the circuit, and V2 is connected to point D of the circuit. The actions of K0, K1, and K2 are controlled by the driving circuit of the matrix switch, that is, under the control of the driver, K0 is closed to measure the waveform at point F, K1 is closed to measure the waveform at point C, and K2 is closed to measure the waveform at point D.
2. 3 Matrix switch drive component - NI6509 digital I/O card
The NI PCI-6509 industrial 96-channel digital I/O card for PCI has 96 bidirectional digital I/O lines, capable of high current drive (24mA) and without the need for jumpers. Using the PCI-6509, you can input and output at 5VDC digital levels and directly drive external digital devices such as solid-state relays (SSRs) at up to 24mA per channel. Each port (8 lines) can be configured for input or output, and no external power is required for output. With the programmable power-on state enabled, the initial output state can be configured in software to ensure safe and trouble-free operation when connected to industrial actuators (pumps, gates, motors, relays).
For applications that require onboard pull-up resistors, consider using the NIPCI-DIO-96 parallel digital I/O card.
In the event of a computer or application failure, the PCI-6509 uses a digital I/O watchdog to switch to a configurable safe output state, ensuring that once it is connected to the industrial actuator, it can detect the fault condition and recover safely. With change detection, when the digital state changes (no polling is required), the digital I/O board can notify and trigger your software. The programmable input filter can be used to eliminate faults/spikes and debounce digital switches/relays through the optional software digital filter.
The 4-row 24-column array switch selected in this system requires 96 I/O ports to drive, and the 6509 digital I/O card happens to have 12 8-bit digital ports, which can fully meet the needs. So the 6509 digital I/O card was selected for operation.