Angle of rotation detector

Abstract: An angle of rotation detector has a drive unit which supplies 90 degree phase shifted signals to a pair of stator coils and measures an analog output from a rotor coil in a resolver. A calculating device calculates the rotational angle of the resolver using the output of an analog to digital converter receiving the analog output of the rotor. In one embodiment, the calculating device performs a Fourier transform to compute the phase of a fixed frequency component contained in the output of the rotor coil. In another embodiment, the drive unit supplies signals wherein f(T/2 + t) =-f(t) and the calculating device performs a Fourier transform using data from successive half periods of the drive waveform. In another embodiment, the output of the rotor is sampled at 1/4 period of an excitation signal from the drive unit and the calculating device determines the phase difference between the excitation signal and the rotor output. In still another embodiment, the calculating device compares the outputs at two or more points having a fixed time relationship and compares the result to stored reference data to arrive at the angle of rotation.

Description: ANGLE OF ROTATION DETECTOR Background of the Invention 1. Field of The Invention This invention relates to an angle of rotation detector which detects an angle of rotation using a resolver with one rotor and two stators. 2. Related Art Heretofore, resolvers used to detect an angle of rotation were equipped with two stators, shown as 13 and 14 in Figure 5. Two sine waves, phase-shifted relative to each other at an electrical angle of 90°, as shown in Figure 6, are supplied to stators 13 and 14. This causes an excitation current to flow. The phase difference Θ between the output induced in a single rotor 15 and the excitation signal is obtained. This phase angle  is considered to be the angle of rotation. A conventional device for detecting angle of rotation using sine wave excitation is shown in Figure 7. This device consists of: oscillator 21; dividing circuit 22 to accept pulses from oscillator 21 and divide them; circuit 23, which generates a signal to start the timing; sine wave generator circuit 24 and cosine generator circuit 25, which generate, respectively, a sine wave and a cosine wave synchronously with the signal from circuit 23; driver circuits 26 and 27; resolver 28; output circuit 29, which derives the rotor output of resolver 28; low pass filter 30, which eliminates harmonics components from the output; comparator 31, which detects the zero-crossing point; counter 32, which counts the pulse signals from divider circuit 22 from the time the start signal is received until the detection of the zero-crossing point causes the stop signal to be input; and CPU 33. The conventional device just described to detect angle of rotation employs a sinusoidal excitation signal. It thus requires both a circuit to generate a sine wave and a circuit to generate a cosine wave. However, a circuit which can generate two such phase-shifted sine waves with good accuracy requires a large number of components, resulting in a relatively expensive device. Employing an easier to generate non-sinusoidal excitation current source producing a non-sinusoidal wave, such as a rectangular wave, results in a large measurement error due to the effect of harmonics, which are contained within the rectangular wave. A low pass filter is generally used to eliminate such errors. However, cutting the tertiary harmonics sufficiently (by a voltage ratio of 40dB), while allowing the basic or fundamental wave to pass, requires a multi-stage active filter or LC filter having a steep cutoff characteristic to effectively break the circuit. This complicates the design. Temperature dependency considerations of the phase shift at the frequency of the fundamental wave necessitate a design which reduces the measurement accuracy of the angle (which is perhaps 0.5°) within the applicable range of temperature (perhaps 0°C to 50°C). Thus, the filter must use elements whose response to temperature variations is extremely small, driving up the cost of the device. The "PATENT ABSTRACTS OF JAPAN" , Vol. 10, no. 9, (P-420) [2066] & JP-A-60 165 512 which represent the most relevant prior art disclose an angle of rotation detector comprising a resolver having a rotor and two stators. The stators are driven by sinusoidal excitation signals which are phase-shifted. However, a circuit which can generate such phase-shifted sine waves with good accuracy requires relatively expensive components. Furthermore, EP-A-0 137 730 discloses a discrete Fourier transformation apparatus provided with a circuit for effecting discrete transformation of a square wave and an input signal by a digital process, and a digital filter arranged after the circuit. Discrete Fourier transformation components are obtained as an amplitude and a phase of a sine wave which is outputted from the digital filter without using a digital multiplier. However, the Fourier transformation apparatus requires a digital comb filter, which represents a relative expensive complex component, for removing high odd number harmonics. EP-A-0 285 878 discloses an apparatus for detecting revolution using a synchro (resolver) comprising a rotor and two stators. Both stators are driven by a.c. excitation signals produced by a function circuit constituted by a ROM into which sine-wave and cosine-wave functions are written and a digital to analog converter. In order to calculate the rotational angle of the resolver, a phase difference produced between an a.c. excitation signal for driving one of the stators and a voltage signal induced at the rotor is detected. That means, the resolver is also driven by sine-wave signals, wherein the output from the resolver has to pass through a filter circuit to eliminate noises or distortion components. Summary of the Invention In view of the above limitations of conventional angle of rotation detectors, an object of the present invention is to provide a rotational angle detector which achieves a high degree of accuracy at a low cost. This object is solved by the features described in claim 1. In particular, the rotational angle detector of the invention detects angle of rotation using a resolver with one rotor or rotor coil and two stators or stator coils. It is equipped with a drive unit which supplies excitation signals phase-shifted by 90° from each other to the two stators. The rotational angle detector is further equipped with means to convert the output of the rotor from an analog to a digital signal and means to calculate the rotational angle of the resolver. The calculation device accepts the A/D signal and, by means of a Fourier transform computes the phase of a fixed or constant frequency component contained in the output of the rotor. From the fundamental frequency component of the transfer function between the excitation wave and the rotor output, which contains high frequency components, in particular, the harmonic components, the phase shift, which is the angle of rotation can be obtained. A particular embodiment of the invention is set out in the dependent claim 2. In this embodiment, the drive circuit is configured to generate excitation signal waveforms such that f(T/2 +t) = -f(t), where t is time and T is period. The device which performs the calculation accepts the signal from the A/D conversion device and performs a Fourier transform using data from alternate half-cycles of the drive waveform. In this way it computes the phase of the fixed frequency component contained in the rotor output waveform, thereby calculating the rotational angle of the resolver. The angle of rotation detector of the invention obviates the necessity of an expensive sine wave generator circuit (with a D/A converter and so on) such as was required by previous angle of rotation detectors. It also eliminates the need for an expensive low pass filter to remove harmonics. Because a calculation device built into an A/D converter is easy to obtain, this device is easier to build than its predecessors. Further, in the case where the angle calculation performed in the calculation device is completed during each half-cycle of the drive waveform, processing time is made available so that operations other than angle detection (data transmission, data display, comparison of data with a reference file, etc.) can be performed by the calculation device. This yields a high-capability angle of rotation detector. Additionally, the device has a low cost, the design can be simplified, and the angle of rotation can be detected with great accuracy. Brief Description of the Drawings The above objects of the invention are met by the invention described herein with particular reference to the drawings in which: Figure 1 is a flowchart illustrating the operation of a first embodiment of an angle of rotation detector according to the invention; Figure 2 is a block diagram of the angle of rotation detector in Figure 1; Figure 3 shows waveforms which illustrate the operation of the angle of rotation detector of the first embodiment in Figure 1; Figure 4 illustrates the relationship between frequency and phase differences for the fundamental frequency and higher frequencies; Figure 5 shows the basic principle underlying the design of a resolver; Figure 6 shows waveforms of excitation signals and output signals produced by a conventional angle of rotation detector; Figure 7 is a block diagram showing the organization of the circuitry in a conventional angle of rotation detector; Figure 8 is a flowchart illustrating operations performed by a second embodiment of the invention; Figure 9 shows waveforms illustrating operation of the second embodiment; Detailed Description of the Preferred Embodiments The Fourier transform is the method generally used to extract specified frequency components of a periodic function. This transform allows us to consider a function g (t) of time t to be a function of frequency f, in accordance with the calculation given below. By using this transform, we can obtain the fundamental frequency component of the transfer function between the excitation wave and the rotor output, which contains harmonic components. We can thus obtain the phase shift, which, in other words, is the angle of rotation. The rotational angle detector of this invention employs just such a method. We shall next discuss a concrete example of this calculation. If u (t) is the excitation wave and y (t) is the rotor output, a Fourier transform performed on u (t) and y (t) will yield the fundamental frequency component f of the excitation waveform. Thus the transfer function can be expressed as follows. G ( f ) = G 1 ( f ) + jG 2 ( f ) = y f u f = y 1 f + jy 2 f u 1 f + ju 2 f = y 1 f · u 1 f + y 2 f · u 2 f u 2 1 f + u 2 2 f + j { y 2 f · u 1 f - y 1 f · u 2 f } u 2 1 f + u 2 2 f The phase shift Θ is expressed as Θ = tan -1 G 2 f G 1 f = tan -1 y 2 f · u 1 f - y 1 f · u 2 f y 1 f · u 1 f - y 2 f · u 2 f Thus by solving Equation (1) we obtain the angle of rotation Θ Figures 2 and 8 are block diagrams illustrating sample rotational angle detectors according to a first embodiment and a second embodiment, respectively. With reference to these diagrams, signals of rectangular waveform, phase-shifted by 90° relative to each other, are output by timing generator 2 in calculation device 1, which consists of a CPU or some similar device. These signals ,e.g., signals a and b, are supplied to stators A and B in resolver 5 by way of driver circuits 3 and 4 respectively. The signals (a and b) which are supplied to stators A and B are shown in Figure 3. The induced signal c, which originated in the rotor of resolver 5 and is output by way of output circuit 6 is sampled at specified sampling times, for example, in one embodiment, each 1/4 period in A/D converter 7. In the second embodiment, the induced signal can be sampled at each 1/8 period in A/D converter 7. Signal c is converted to digital signal d (See Figure 2) and supplied to calculation device 1, where it serves as the basis for the calculation of angle Θ. Also according to the second embodiment, the angle of the rotor can be obtained by determining the phase difference between the excitation waveform and that of the rotor output. Thus, the angle of the rotor can be discovered by solving Equation (1) given above. Figure 8 shows the relationship between frequency and phase differences for the fundamental and higher frequencies. The excitation waveform u(t) is symmetric about the time axis at t = T/4 {u(t) = u (T/2 - t)}. If we select the symmetric waveform {u(t) = -u (t + T/2)} corresponding to the point T/2, (generally, rectangular, triangular, and trapezoidal waveforms, which are easy to generate, will fulfill this condition), then with regard to the definition it can be said that cosω ( T 2 - t ) = -cosω t and cosω ( t + T 2 ) = -cosω t It follows, then, that If we simplify Equation (1), we can obtain Θ = tan -1 - y 1 f y 2 f The rotor output including the harmonics components is expressed as E = A 1 sin(ω t + Θ) + A 3 sin(3ω t + Θ + 3 ) + A 5 sin(5ω t + Θ +  5 ) + ... + N where Θ is the angle of rotation;  3 ,  5 , ... are the phase differences for the higher frequencies relative to the fundamental waveform when Θ = 0; and N is the noise component due to even-order harmonic components and so forth. Thus if the signal is digitized after sampling (It is sampled at periods of T/m), we get Ed(n) = A 1 sin( n · 2π m + Θ) + A 3 sin( n · 6π m - Θ +  3 ) + A 5 sin( n · 10π m - Θ +  5 ) + ... + N(n) If we perform a Fourier transform on these data, the real part of the fundamental frequency component is and the imaginary part is It thus becomes helpful to solve the following equation. Figure 4 shows the relationship between frequency and phase difference of the fundamental and higher frequencies for the first embodiment. After the rotor output is sampled (let f s be the sampling frequency), it is converted from an analog to a digital signal. At this time, because of the folding phenomenon, data for components at frequencies larger than f s /2 are received as if they represented frequency components below f s /2. That is, after the signal is digitized, the data for the fundamental frequency component E' (f B ) can be expressed by the formula E' (f B ) = E(f B ) + E(f S - f B ) + E(F S + f B ) + ... (f S = 4f B , 6f B , 8f B ...) E'(f B ) = E(f B ) + E (3 f B ) + ... ( f S = 2 f B ) In this use of a Fourier transform, it is necessary to discover the true fundamental frequency component E(f B ) . In f s = 2f B , we must filter the rotor output through a low pass filter which will cut out the frequency components above 3f B before sampling. If f S ≥ 4f B , we must filter the rotor output through a low pass filter which will cut out frequency components above (f S - f B ) before sampling. If we use conventional methods, we would need a low pass filter to intercept the frequency component above 2f B . Use of the method described herein will allow the filter to be markedly simplified. We shall next explain, with reference to the flow chart in Figure 1, the operations involved in calculating the angle through the use of the aforementioned Equation (2) in the embodiment of a rotational angle detector. First, let the variable n, the real part of the fundamental frequency Y r , and the imaginary part of the fundamental frequency Y i all be 0 (Step 1-1). Next, the output of A/D converter 7 is input as Ed(0) (Step 1-2). Ed(n) x cos (2πn/m) is calculated and stored as Y r , and Ed(n) x -sin (2πn/m) is calculated and stored as y i (Step 1-3). A judgment is made as to whether n ≥ 8 (Step 1-4), and if the answer is "no," n is incremented by 1 (Step 1-5) and we return to Step 1-2. The real and imaginary parts of the fundamental frequency are calculated when the output of A/D converter 7 is input at each sampling time. When n = 8 in Step 1-4, the equation Θ = tan( - Y r Y i ) is calculated (Step 1-6), and the angle value Θ is output (Step 1-7). For the second embodiment, it is possible to select the excitation waveform so that it does not contain even-order harmonics. Such a waveform is expressed by the relationship f(t + T/2) = -f(t). The even-order harmonic components in the rotor output have a strong tendency to influence the measurement of the angle. If we call the period of the fundamental frequency T (T = 2π/ω), then from Equation (2) above we can produce: E ( t + T 2 ) = A 1 sin(ω t + π + Θ) + A 3 sin(3ω t + 3π - Θ + 3 ) + A 5 sin(5ω t + 5π - Θ +  5 ) + ... = - A 1 sin(ω t + Θ) - A 3 sin(3ω t - Θ +  3 ) - A 5 sin(5ω t - Θ +  5 ) + ... = - E ( t ) Because f(t + T/2) = -f(t) for both sinωt and cosωt, the Fourier transform will yield the same result for both 0∼T/2 and T/2∼T. Thus if we solve for 0∼T/2, we will obtain angle Θ. The equation to be solved is as follows. Normally, an angle of rotation detector does not simply detect an angle. It must also transmit its result to the exterior as data, process displays and input, and perform operations to compare the detected value with a reference file. In this embodiment, data are read in, the angle is determined, and calculations are performed during the half-cycle in which the excitation wave is generated. Other processing can be done during the remaining half-cycle. This allows the angle of rotation device to perform other processing simultaneously with angle detection. The above descriptions of the preferred embodiments are provided to enable those of ordinary skill in the art to make and use the invention. It will be known that various modifications to these embodiments can be made without departing from the scope of the invention as claimed.

Claims: An angle of rotation detector comprising: a resolver (5) having a rotor and two stators (A, B) ; a drive unit (3, 4) connected to said two stators (A, B) and adapted to supply excitation signals phase-shifted by 90° from each other to said two stators (A, B); an analog to digital converter (7); and means (1) for calculating a rotational angle of said resolver (5); characterized in that the excitation signals are of non-sinusoidal nature; that the analog to digital converter (7) is connected to receive at an input thereof an analog output of said rotor of the resolver (5); and that the calculating means (1) is connected to receive the digital signal of the analog to digital converter (7) and adapted to perform a Fourier transform to compute the phase of a fixed frequency component outputted by said rotor. An angle of rotation detector according to claim 1, wherein said excitation signals have a form where f (T/2 + t) = -f(t), where t is time and T is period, and said Fourier transform is performed by using data from successive half-periods of the waveform of one of said excitation signals to compute the phase of a fixed frequency component outputted by said rotor.

Inventors: FUJISAKI AKIYOSHI, KAKIMOTO TATSUMI, KUBO TAKESHI, NISHINA TERUYA, TAKAHASHI KUNIYOSHI, FUJISAKI, AKIYOSHI, KAKIMOTO, TATSUMI, KUBO, TAKESHI, NISHINA, TERUYA, TAKAHASHI, KUNIYOSHI, Fujisaki, Akiyoshi, c/o OMRON CORPORATION, Kubo, Takeshi, c/o OMRON CORPORATION, Takahashi, Kuniyoshi, c/o OMRON CORPORATION
Applicants: OMRON TATEISI ELECTRONICS CO, OMRON CORPORATION
Application year: 1991
Main classification: G01D5
IPC codes: G01D5