Inverter system with variable input gain apparatus and method for the same

An inverter system includes a comparator unit receiving an analog input voltage signal and compared with at least one threshold voltage value to determine a voltage range of the analog input voltage signal. An amplifier unit receives and amplifies the analog input voltage signal. A feedback-controlling gain unit adjusts a voltage gain of the amplifier unit according to the voltage range of the analog input voltage signal. An analog-to-digital converter unit converts an analog signal outputted from the amplifier unit into a digital count. A microcontroller unit is electrically connected with the analog-to-digital converter unit and the comparator unit. The microcontroller unit receives an indicating signal outputted from the comparator unit to know the voltage range of the analog input voltage signal. The microcontroller unit receives a digital count outputted from the analog-to-digital converter unit to correctly calculate an original value of the analog input voltage signal.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an inverter system, and more particularly to the inverter system with a variable input gain apparatus and a method for the same.

2. Description of the Prior Art

An inverter is used to convert the direct current (DC) power to the alternating current (AC) power. Therefore, the inverters have been widely applied in solar electricity generating system, motor scooter, uninterruptible power supply, and so on. In an inverter, a voltage/frequency (V/f) control is used to provide variable speed control for motors and consequently inverters can be advantageously applied in refrigeration and air conditioning system, high speed rail system, mass rapid transit system, and so on to promote use efficiency of motors so as to increase energy efficiency.

Reference is made inFIG. 1. It is a block diagram of an inverter system of the prior art. A host10A delivers an analog control signal to an inverter30A through an analog input/output module20A. The inverter30A includes an analog to digital converter (not shown) to convert the analog control signal into a digital signal, which is processed by a microcontroller unit300A so as to control the inverter30A.

At present, the inverters provide control on the frequency of output voltage according to an analog input voltage (between 0 to 10 volts).

However, the analog input voltage has to be processed through a proportional amplifier and then entered into analog input pins of the microcontroller unit300A. Generally speaking, an operating voltage of the microcontroller unit300A is +5 volts or +3.3 volts, so the proportional gain is usually a fixed scale-down gain value. Reference is made inFIG. 2. It is a prior art circuit diagram for processing an analog input signal of an analog input/output module. The analog input voltage signal VAI(between 0 to 10 volts) is proportionally transformed into a voltage between 0 to 5 volts through a non-inverting amplifier22A with a fixed proportional gain P (P=0.5) and the transformed voltage is sent into analog input pins of the microcontroller unit300A and converted by the analog-to-digital converter unit24A.

However, in the above-mentioned circuit, accuracy rate of the microcontroller unit300A is not high when the analog input voltage is lower than +1 volts.FIG. 3shows a conversion result performed by the circuit inFIG. 2, wherein an analog input voltage signal is converted into a digital count value with 10-bit resolution. A voltage variation is 5 millivolts (mV) (see formula 1) for a least significant bit (LSB) of the analog-to-digital converter unit24A when a reference voltage VREFthereof is 5.12 volts. If an error of the analog-to-digital converter unit24A is ±3 LSB, a digital count value N acquired by the microcontroller unit300A is 100±3 (see formula 2) when the analog input voltage signal is 1.00 volt.

A width of the LSB is 5 mV:

The digital count value N is calculated:

Hence, the analog input voltage signal VAIcan be inversely calculated from the digital count value N:

The analog input voltage signal is 1±0.035 volts when the digital count value N acquired by the microcontroller unit300A is 100. The error of the microcontroller unit300A is amplified due to the fixed proportional gain P (P=0.5<1), therefore, a maximum error is 0.035 volts. The error rate increases when the analog input voltage is lower; in another word, the accuracy rate decreases when the analog input voltage is lower so that microcontroller unit300A can't be accurately controlled.

Table 1 obviously shows that the accuracy rate of the microcontroller unit300A gradually decreases when the analog input voltage decreases.

TABLE 1Error rate and accuracy rate of the microcontroller for estimating ananalog input voltage signalAnalog input voltage signalError rate of theAccuracy rate of the(VAI)microcontrollermicrocontroller1.003.5%96.5%0.804.4%95.6%0.605.8%94.2%0.408.8%91.3%0.2017.5%82.5%0.1035.0%65.0%

Hence, in order to increase accuracy rate of the microcontroller unit300A when the analog input voltage signal is low, a switch circuit with a variable input gain is desirable. The switch circuit with variable input gain selectively increases the analog input voltage to the analog-to-digital converter so as to decrease error rate of the microcontroller unit300A.

SUMMARY OF THE INVENTION

It is a primary object of the present invention to provide an inverter system with a variable input gain apparatus and a method for the same.

To achieve the primary objective stated above, the present invention provides an inverter system with a variable input gain apparatus. The inverter system comprises a comparator unit, an amplifier unit, a feedback-controlling gain unit, an analog-to-digital converter unit, and a microcontroller unit. The comparator unit receives an analog input voltage signal and compared with at least one threshold voltage value to determine a voltage range of the analog input voltage signal. The amplifier unit receives and amplifies the analog input voltage signal. The feedback-controlling gain unit is electrically connected with the comparator unit and the amplifier unit for adjusting a voltage gain of the amplifier unit according to the voltage range of the analog input voltage signal. The analog-to-digital converter unit is electrically connected with the amplifier unit for converting an analog signal outputted from the amplifier unit into a digital count. The microcontroller unit is electrically connected with the analog-to-digital converter unit and the comparator unit, and the microcontroller unit receives an indicating signal outputted from the comparator unit to acquire the voltage range of the analog input voltage signal and receives a digital count outputted from the analog-to-digital converter unit to correctly calculate an original value of the analog input voltage signal.

It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed. Other advantages and features of the invention will be apparent from the following description, drawings and claims.

The drawings will be described further in connection with the following detailed description of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference is made inFIG. 4. It is a block diagram of a preferred embodiment of an inverter system with a variable input gain apparatus according to the present invention. An analog input voltage signal VAIis amplified by a variable gain unit and processed by an analog-to-digital converter. A microcontroller unit300is used to receive a digital count value and an indicating signal Xin to accurately determine an original value of an analog input voltage signal VAI. The inverter system mainly comprises a comparator unit40, a feedback-controlling gain unit42, and an indicating signal output unit44, and further comprises an amplifier unit22and an analog-to-digital converter unit24.

The comparator unit40compares the analog input voltage signal VAIwith at least one threshold voltage value to determine a voltage range for the voltage signal VAI. The feedback-controlling gain unit42is electrically connected with the comparator unit40for adjusting a voltage gain of the amplifier unit22according to the voltage range of the analog input voltage signal VAI. Specifically, the voltage gain of the amplifier unit22is increased by the feedback-controlling gain unit42when the voltage range of the analog input voltage signal VAIis smaller, and the voltage gain is decreased when the voltage range of the analog input voltage signal VAIis larger. The signal amplified by the amplifier unit22is converted into a digital count value N through an analog-to-digital converter unit24, and the digital count value N is processed by the microcontroller unit300. In order to process the digital count value N to correctly calculate input voltage signal VAI(the digital count value N is varied according to a variable input gain P), the indicating signal Xin is also processed by the microcontroller unit300.

Reference is made inFIG. 5. It is a result graph of converting analog input voltage signal into a digital count value of theFIG. 4according to the present invention. A full swing voltage of 10V is separated into five voltage intervals: namely, 0-2, 2-4, 4-6, 6-8, and 8-10 (unit in volt) by four threshold voltage values: namely, 2, 4, 6, and 8 (unit in volt). The digital count value N is ranged from 0 to 1023 for the analog-to-digital converter unit24with 10-bit resolution. The comparator unit40is used to determine a voltage range of the input voltage level so as to acquire the variable input gain P of the amplifier unit22. The five input gains P are 2.5, 1.25, 0.83, 0.625, and 0.5 separately to correspond to the five voltage intervals, respectively. That is to say, a larger input gain P, such as P=2.5, is provided to decrease the error rate of the microcontroller unit300when the voltage range of the analog input voltage signal VAIis small. On the other hand, a smaller input gain P, such as P=0.5, is provided to exceed a rated input value of the microcontroller unit300. Furthermore, the indicating signal Xin can be processed by the microcontroller unit300to acquire the exactly correct analog input voltage signal VAI. For example, when the digital count value N received by the microcontroller unit300is850, the analog input voltage signal VAIcorresponded can be 1.75, 3.4, 5, 6.75, and 8.5 (unit in volt). However, the analog input voltage signal VAIis certainly 3.4, when the indicating signal Xin indicates that the analog input voltage signal VAIbelongs to the second voltage interval, namely, 2-4 (unit in volt).

Reference is made inFIG. 6. It is a circuit diagram for implementing the inverter system with the variable input gain apparatus of theFIG. 4according to the present invention. The comparator unit40can be a comparator circuit comprised of four comparators U2A, U2B, U2C, and U2D, and a non-inverting input end of each of the four comparators U2A, U2B, U2C, and U2D is connected with different voltage levels, which correspond to the threshold voltage values, such as 2 volts, 4 volts, 6 volts, and 8 volts. The feedback-controlling gain unit42comprises four non-inverting drivers U3A, U3B, U3C, U3D and four resistors R8, R9, R10, R11electrically connected with the four comparators U2A, U2B, U2C, and U2D separately. The amplifier unit22includes an amplifier U1A which is a non-inverting amplifier with a fixed gain. When an input voltage level of the analog input voltage signal VAIis larger than a specific threshold voltage value, one of the corresponding comparators U2A, U2B, U2C, and U2D produces a low voltage level signal. The microcontroller unit300acquires a range of the input voltage level of the analog input voltage signal VAIthrough the indicating signal output unit44that is composed of outputs of the comparators U2A, U2B, U2C, and U2D. In addition, the corresponding comparators U2A, U2B, U2C, and U2D drives the corresponding non-inverting driver U3A, U3B, U3C, U3D to increase an input current of the analog input voltage signal VAIthrough resistors R3and R4and thus causes a larger voltage drop so as to decrease a input voltage of the non-inverting input end of the amplifier unit22. Therefore, it is to be equal to decrease the gain value P so that the input voltage level of the analog input voltage signal VAIis inverse proportion to the gain value P.

As shown inFIG. 6, a 5-section voltage gain can be switched by using an amplifier unit wherein four amplifiers are included, four non-inverting drivers and appropriate matching resistors. That is to say, a n-section voltage gain can be designed by using an amplifier unit wherein n−1 amplifiers are included, n−1 non-inverting drivers and appropriate matching resistors. The circuit shown inFIG. 6has been described with reference to the preferred embodiment thereof, it will be understood that the invention is not limited to the details thereof. Table 2 shows an operating result of the inverter system with the variable input gain apparatus according to the present invention and explains an excellent accuracy rate of the microcontroller for estimating a wide range of the analog input voltage signal.

TABLE 2Error rate and accuracy rate of the microcontroller for estimating ananalog input voltage signalanalog input voltage signalError rate of theAccuracy rate of the(VAI)microcontrollermicrocontroller1.000.7%99.3%0.800.9%99.1%0.601.2%98.8%0.401.8%98.2%0.203.5%96.5%0.107.0%93.0%

It follows from what has been said that the present invention has the following advantages:

1. Precise control of the inverter is achieved when the input voltage level of the analog input voltage signal is low.

2. Accuracy rate of estimating the analog input voltage signal for the microcontroller is raised.

3. Efficiency of the firmware filtering is promoted.

4. The circuit structure is simple to easily implement.

5. All these components used are less to bring lower cost.

6. Flexibility of the circuit design for determining a range of the input voltage level is excellent.

7. A wide scope of application is provided.