Motor load rate detection method based on heat

Abstract

The invention relates to the field of motor load rate detection, in particular to a motor load rate detection method based on heat, which comprises the following specific steps: S1, determining a heat time constant Er of a motor; s2, calculating a coefficient MCA; s3, calculating the load rate increment of the motor; and S4, calculating the load rate of the motor. Compared with a temperature sensor arranged in the motor, the motor load rate detection method based on heat accumulation provided by the invention has the advantages that the implementation process is simple, corresponding hardware does not need to be additionally purchased, and the load rate of the motor can be detected without increasing hardware cost; meanwhile, the invention provides a method for estimating the thermal time constant of the motor, the thermal time constant of the motor can be evaluated according to the overload curve, so that the load rate of the motor heat is evaluated by using a software algorithm, the method is simple and convenient to use, the load rate is easy to adjust, and the result precision of the load rate is relatively high.

Description

Technical Field

[0001] The invention relates to the field of motor load rate detection, and in particular to a motor load rate detection method based on heat. Background Art

[0002] With the continuous development of the robotics industry, the industry it undertakes is increasing. The motors of robots often need to work under heavier loads or higher speeds. If the motor works at full load for a long time, it is likely to damage the motor components, thereby reducing the service life of the robot. Therefore, accurate detection of the motor load condition is of great significance to maintaining the working ability of the motor.

[0003] At present, in practical applications, the bus load current of the motor is usually obtained first, and then the ratio of the bus load current to the rated current of the motor is used to calculate the motor load rate. However, since the motor load rate is affected by factors such as speed and torque, the bus load current cannot accurately reflect the load state of the motor. In some cases, the motor is required to operate under short-term overload to quickly respond to related functions, but the calculation delay of the bus load current is relatively large, which cannot better protect the motor. Summary of the invention

[0004] In order to solve the above problem, the present invention proposes a motor load rate detection method based on heat.

[0005] A method for detecting a motor load rate based on heat, the specific steps of which are as follows:

[0006] S1. Determine the thermal time constant Er of the motor: Obtain a more accurate value of the thermal time constant from the motor manufacturer or by self-testing, or obtain an approximate value by evaluating the overload point;

[0007] S2, calculation coefficient MCA: Calculate the coefficient MCA based on the two overload points P1 (I1, t1) and P2 (I2, t2), I1 and I2 are the currents of the motor under the overload state, t1 and t2 are the time the motor maintains under the overload current respectively;

[0008] S3, calculating the load rate increment of the motor: after calculating the thermal time constant Er in steps S1 and S2, the load rate increment ΔL of the motor is calculated by the formula during the operation of the motor;

[0009] S4. Calculate the motor load rate: The calculation formula for the motor load rate at time k is L(k)=L(k-1)+ΔL(k).

[0010] The calculation formula of step S2 is:

[0011] [(I1^2-Ir^2) / (I1^2-In^2)]^(t2 / t1)=[(I2^2-Ir^2) / (I2^2-In^2)],

[0012] MCA=t1 / ln[(I1^2-Ir^2) / (I1^2-In^2)].

[0013] The Ir is the rated current of the motor, ln(x) is the logarithmic function of the exponent e, and the thermal time constant Er can be selected to be 5 to 15 times of MCA. The specific value needs to be selected based on the actual situation.

[0014] The calculation formula of step S3 is:

[0015] ΔL(k)=[I^2 / Ir^2-L(k-1)] / Er*Δt.

[0016] The L(k) is the current load rate of the motor, I is the current of the motor, L(k-1) is the load rate of the motor at the previous moment, and Δt is the interval time from moment k-1 to moment k.

[0017] In the step S4, when the motor runs stably at the rated current, the calculation result of the load rate will be displayed as 100%.

[0018] The beneficial effects of the present invention are as follows: compared with setting a temperature sensor in the motor, the motor load rate detection method based on heat accumulation proposed in the present invention has a simple implementation process, does not require the purchase of additional corresponding hardware, and can detect the motor load rate without increasing hardware costs; at the same time, a motor thermal time constant estimation method is provided, which can evaluate the motor's thermal time constant based on the overload curve, thereby using a software algorithm to evaluate the motor's heat load rate. The method is simple and convenient to use, easy to adjust the load rate, and the load rate result has a high accuracy. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] The present invention is further described below in conjunction with the accompanying drawings and embodiments.

[0020] Figure 1 It is a schematic diagram of the process structure of the present invention;

[0021] Figure 2 It is a schematic diagram of heat accumulation of the motor of the present invention;

[0022] Figure 3 It is a schematic diagram of the change of the motor load rate of the present invention. DETAILED DESCRIPTION

[0023] In order to make the technical means, creative features, objectives and effects achieved by the present invention easy to understand, the present invention is further described below.

[0024] like Figures 1 to 3 As shown, a method for detecting a motor load rate based on heat, the specific steps are as follows:

[0025] S1. Determine the thermal time constant Er of the motor: Obtain a more accurate value of the thermal time constant from the motor manufacturer or by self-testing, or obtain an approximate value by evaluating the overload point;

[0026] S2, calculation coefficient MCA: Calculate the coefficient MCA based on the two overload points P1 (I1, t1) and P2 (I2, t2), I1 and I2 are the currents of the motor under the overload state, t1 and t2 are the time the motor maintains under the overload current respectively;

[0027] exist Figure 2 In the figure, let the mass of the motor be M, the specific heat capacity be C, the heat dissipation coefficient be A, the ambient temperature be T0, and the initial temperature of the motor be T1. When the current I flows through the motor coil, part of the heat generated by the current is absorbed by the motor, causing its own temperature to rise, and the other part of the heat is dissipated into the environment.

[0028] In a short period of time Δt, the heat generated by the current in the motor coil is: Δh1 = I^2*R*Δt, and the heat dissipated by the motor to the environment is: Δh2 = A*(T(k)-T0)*Δt, so the temperature rise of the motor can be calculated as: ΔT(k) = (Δh1-Δh2) / (M*C) = (I^2*R*Δt-A*(T(k)-T0)*Δt) / (M*C). The temperature of the motor at time k is: T(k) = T1+∑ΔT(k-1). Let the total temperature rise of the motor before time k be y = ∑ΔT(k-1), then the equation for temperature rise can be described as: dy / dt+A*y / (M*C) = [A*(T0-T1)+I^2*R] / (M*C). Substituting the initial conditions of the motor into the general solution of temperature change can be obtained as:

[0029] y(t)=[(T0-T1)+I^2*R / A]*[1-e^(-A*t / (M*C)]

[0030] The final temperature equation of the motor is:

[0031] T(t)=T1+[(T0-T1)+I^2*R / A]*[1-e^(-A*t / (M*C)]

[0032] It can be seen from the above formula that when the initial temperature of the motor is the same as the ambient temperature, that is, T0=T1, the value when the motor's operating temperature reaches a stable value under the rated current Ir is: Tr=T0+Ir^2*R / A.

[0033] S3, calculating the load rate increment of the motor: after calculating the thermal time constant Er in steps S1 and S2, the load rate increment ΔL of the motor is calculated by the formula during the operation of the motor;

[0034] S4. Calculate the motor load rate: The motor load rate calculation formula at time k is L(k)=L(k-1)+ΔL(k);

[0035] The load rate is defined as: when the ambient temperature is T0 and the initial temperature of the motor is the same as the ambient temperature, that is, T0 = T1, the motor load rate L is the ratio of the temperature rise under current I to the temperature rise reached by the motor under rated current. Therefore:

[0036] L=[T(t)-T0] / [Tr-T0]*100%

[0037] Simplifying the above formula:

[0038] L=(I^2 / Ir^2)*[1-e^(-A*t / (M*C)]

[0039] After transforming the above formula into the calculation formula for each moment k, we have:

[0040] ΔL(k)=[I^2 / Ir^2-L(k-1)] / (M*C / A)*Δt

[0041] L(k)=L(k-1)+ΔL(k)

[0042] From the above formula, we can see that M*C / A represents the temperature rise coefficient of the motor, and therefore has a correlation with the thermal time constant of the motor. If Er=M*C / A, the above formula can be expressed as:

[0043] ΔL(k)=[I^2 / Ir^2-L(k-1)] / Er*Δt

[0044] L(k)=L(k-1)+ΔL(k)

[0045] Next, the thermal time constant value Er of the motor is evaluated from the value of M*C / A. Considering that the motor overload curve contains the value of M*C / A, the overload curve is used to calculate the value of M*C / A. The formula for the motor overload curve is:

[0046] t=M*C / A*ln[(I^2-In^2) / (I^2-Ir^2)]

[0047] Taking M*C / A as a coefficient and synthesizing it into MCA, the coefficients MCA and In can be calculated based on the two overload points P1 (I1, t1) and P2 (I2, t2). Substituting into the above formula, we have the equation group:

[0048] t1=MCA*ln[(I1^2-In^2) / (I1^2-Ir^2)]

[0049] t2=MCA*ln[(I2^2-In^2) / (I2^2-Ir^2)]

[0050] After eliminating MCA, the formula including In is:

[0051] [(I1^2-Ir^2) / (I1^2-In^2)]^(t2 / t1)=[(I2^2-Ir^2) / (I2^2-In^2)]

[0052] Solving the above equation gives the value of In. Then the value of MCA is:

[0053] MCA=t1 / ln[(I1^2-Ir^2) / (I1^2-In^2)]

[0054] At this time, according to the actual situation, the thermal time constant of the motor Er = MCA*Kt, Kt is a coefficient selected according to the actual environment of the motor, and the general Kt value is between 5 and 15.

[0055] Compared with setting a temperature sensor in the motor, the motor load rate detection method based on heat accumulation proposed in the present invention has a simple implementation process, does not require the purchase of corresponding hardware, and can detect the motor load rate without increasing hardware costs. At the same time, a motor thermal time constant estimation method is provided, which can evaluate the motor's thermal time constant according to the overload curve, and thus use a software algorithm to evaluate the motor's heat load rate. The method is simple and convenient to use, easy to adjust the load rate, and the load rate result has a high accuracy.

[0056] The calculation formula of step S2 is:

[0057] [(I1^2-Ir^2) / (I1^2-In^2)]^(t2 / t1)=[(I2^2-Ir^2) / (I2^2-In^2)],

[0058] MCA=t1 / ln[(I1^2-Ir^2) / (I1^2-In^2)].

[0059] The Ir is the rated current of the motor, ln(x) is the logarithmic function of the exponent e, and the thermal time constant Er can be selected as 5 to 15 times of MCA. The specific value needs to be selected according to the actual situation. After analyzing the motor thermal model, it is found that there is a certain correlation between the motor overload curve and the motor thermal time constant. The motor thermal time constant can be evaluated by the overload coefficient calculated by two overload points. Therefore, the present invention provides a possibility for calculating the motor thermal load rate.

[0060] The calculation formula of step S3 is as follows:

[0061] ΔL(k) = [I^2 / Ir^2 - L(k - 1)] / Er * Δt.

[0062] The L(k) mentioned above is the load rate of the motor at the current moment, I is the current of the motor at the current moment, L(k - 1) is the load rate of the motor at the previous moment, and Δt is the time interval from the (k - 1)th moment to the kth moment.

[0063] In step S4, when the motor runs stably at the rated current, the calculation result of the load rate will be shown as 100%. Correspondingly, after calculating the thermal time constant, the present invention proposes a new calculation method for detecting the load rate of the motor based on heat accumulation. Using this scheme, it is not necessary to set a temperature sensor in the motor, and only corresponding calculations need to be performed according to the motor current to calculate the load rate of the motor in real time, which has the characteristics of being simple and easy to use.

[0064] The final motor load rates at different currents are as Figure 3 shown. Among them, the current I1 < I2 < I3. It can be seen from the figure that the finally calculated motor load rate increases with the increase of the current, which is in line with the actual working conditions of the motor. According to the above two formulas, the heat load rate of the motor at any moment can be calculated, so as to evaluate the load capacity state of the motor and reasonably design the working tasks of the motor.

[0065] The above shows and describes the basic principles, main features and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited by the above embodiments. What is described in the above embodiments and the specification is only the principle of the present invention. Without departing from the spirit and scope of the present invention, the present invention will have various changes and improvements, and these changes and improvements all fall within the scope of the present invention claimed. The scope of protection claimed by the present invention is defined by the appended claims and their equivalents.

Claims

1. A method for detecting a motor load rate based on heat, characterized in that: The specific steps are as follows: S1. Determine the thermal time constant Er of the motor: obtain the thermal time constant value from the motor manufacturer or by self-testing, or obtain an approximate value by overload point evaluation; S2, calculation coefficient MCA: Calculate the coefficient MCA based on the two overload points P1 (I1, t1) and P2 (I2, t2), I1 and I2 are the currents of the motor under the overload state, t1 and t2 are the time the motor maintains under the overload current respectively; S3, calculating the load rate increment of the motor: after calculating the thermal time constant Er in steps S1 and S2, the load rate increment ΔL of the motor is calculated by the formula during the operation of the motor; S4. Calculate the motor load rate: The calculation formula for the motor load rate at time k is L(k)=L(k-1)+ΔL(k).

2. The method for detecting a motor load rate based on heat according to claim 1, characterized in that: The calculation formula of step S2 is: [(I1^2-Ir^2) / (I1^2-In^2)]^(t2 / t1)=[(I2^2-Ir^2) / (I2^2-In^2)], MCA=t1 / ln[(I1^2-Ir^2) / (I1^2-In^2)].

3. The method for detecting a motor load rate based on heat according to claim 2, characterized in that: The Ir is the rated current of the motor, ln(x) is the logarithmic function of the exponent e, and the thermal time constant Er can be selected to be 5 to 15 times of MCA. The specific value needs to be selected based on the actual situation.

4. The method for detecting a motor load rate based on heat according to claim 1, characterized in that: The calculation formula of step S3 is: ΔL(k)=[I^2 / Ir^2-L(k-1)] / Er*Δt.

5. The method for detecting a motor load rate based on heat according to claim 4, characterized in that: The L(k) is the current load rate of the motor, I is the current of the motor, L(k-1) is the load rate of the motor at the previous moment, and Δt is the interval time from moment k-1 to moment k.

6. The method for detecting a motor load rate based on heat according to claim 1, characterized in that: In the step S4, when the motor runs stably at the rated current, the calculation result of the load rate will be displayed as 100%.

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