Motor position detection system
The motor position detection system addresses noise-induced unreliability in stepper motors by using trinary value comparisons, ensuring precise position detection and sequence adherence without physical access.
Patent Information
- Authority / Receiving Office
- GB · GB
- Patent Type
- Applications
- Current Assignee / Owner
- ULTRA PMES LTD
- Filing Date
- 2025-03-18
- Publication Date
- 2026-05-27
AI Technical Summary
Existing motor position detection systems for stepper motors are unreliable due to noise interference, leading to erroneous conclusions about the motor's position and adherence to the predetermined sequence, especially in inaccessible environments.
A motor position detection system that measures and compares three-phase voltages against reference values to determine trinary values, reducing noise effects and enabling reliable position detection without physical access to the motor output.
The system provides accurate motor position detection and sequence confirmation, effectively mitigating noise interference and ensuring correct motor operation without requiring direct access to the motor.
Smart Images
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Abstract
Description
The present invention relates to a motor position detection system to detect the position of a three-phase stepper motor. Stepper motors are motors whose position, speed and torque can be controlled precisely, and are typically used in applications requiring such precision control, for example, where the motor controls a positioning actuator, for example, but not limited to positioning rods in a nuclear reactor. Stepper motors are powered by, for example, a three-phase supply, and the output position controlled according to a sequence of pre-determined steps. It is desirable to know the position of the stepper motor output to confirm that the predetermined sequence is being followed. This is particularly difficult in environments where the output is not accessible. Known motor position detection systems determine the position of the stepper motor by measuring the three-phase voltages applied to the motor. However, noise in the measurement can cause unreliable and erroneous conclusions to de drawn, and therefore an inability to firstly determine the motor position and secondly confirm the pre-determined output sequence is being followed. An object of the present invention is to provide a more reliable motor position detection system. Thus, according to one aspect of the present invention there is provided a motor position detection system to detect the position of a three-phase motor (M) powered by a supply comprising a first phase, a second phase and a third phase, the system configured to: a. measure the voltage of the first phase, the second phase and the third phase to obtain a measured first phase voltage (Va), a measured second phase voltage (Vb) and a measured third phase voltage (Vc) at pre-determined times, b. compare the measured first phase voltage (Va), the measured second phase voltage (Vb) and the measured third phase voltage (Vc) to a first reference value (+Vo) and second reference value (-Vo), c. determine a trinary value (Vat,Vbt,Vct) for each of the measured first phase voltage (Va), the measured second phase voltage (Vb) and the measured third phase voltage (Vc), the trinary value (Vat,Vbt,Vct) equal to: 1 if the measured voltage (Va, Vb, Vc) is greater or equal to the first reference value (+Vo), 0 if the measured voltage (Va, Vb, Vc) is less than the first reference value (+Vo) and greater than the second reference value (-Vo), -1 if the measured voltage (Va, Vb, Vc) is less or equal to the second reference value (-Vo), d. compare the determined trinary value (Vat,Vbt,Vct) for each of the measured first phase voltage (Va), the measured second phase voltage (Vb) and the measured third phase voltage (Vc) with a plurality of unique reference trinary values (Vi,V2V3) for each phase, each of the plurality of unique reference trinary values (Vi,V2,Vs) corresponding to one of a plurality of reference motor positions, or an error condition, or a motor off condition, and e. determine a current motor position is one of the plurality of reference motor positions if the determined trinary value (Vat,Vbt,Vct) is the reference trinary value (Vi,V2,Vs) corresponding to one of a plurality of reference motor positions. Advantageously, the trinary values use a first and second reference value to reduce the effect of noise associated with the motor system, and therefore enable the motor position to be detected more reliably, and without requiring access to the motor output itself. Preferably, the motor position detection system is further configured to report the motor off condition if the determined trinary values (Vat,Vbt,Vct) for each phase is zero. Preferably, the motor position detection system is further configured to report the error condition if the determined trinary value (Vat,Vbt,Vct) is not a reference trinary value (Vi,V2,Vs) corresponding to one of a plurality of reference motor positions or the motor off condition. Preferably, the motor position detection system is further configured to compare the current motor position with a demanded motor position to determine if the current motor position is a correct motor position. Preferably, one of a plurality of reference motor positions comprises a sequence of pre-determined motor positions, the demanded position corresponds to a motor position of the sequence of pre-determined motor positions. Preferably, the pre-determined times are selected from a time between successive motor positions of the sequence of pre-determined motor positions. Preferably, the time between successive motor positions is selected when the supply has a stable waveform, preferably half-way between successive pre-determined motor positions. Preferably, the motor position detection system is further configured to compare the current motor position with the sequence of pre-determined motor positions to determine if the current motor position is a correct motor position. Preferably, the current motor position is compared to a last determined motor position to determine if the current motor position is a next position in the sequence of predetermined motor positions and if the current motor position is a correct motor position. Preferably, the current motor position is the correct position if the current motor position is the next position in the sequence of pre-determined motor positions, the current motor position is unchanged if the current motor position is the last determined motor position, or the current motor position is out of sequence if the current motor position is not the last determined or next motor position. Preferably, the plurality of reference motor positions is twelve reference motor positions. Preferably, the first reference value (+Vo) is a negative of the second reference value (-Vo), Preferably, a range defined between the first reference value (+Vo) and the second reference value (-Vo) is greater than a predicted noise value and less than half of a minimum operating voltage. Preferably, the minimum operating voltage is equal to a peak line-line voltage x Sqrt(3) / 2. According to another aspect of the present invention, there is provided a motor system comprising a motor (M) and including the motor position detection system. The invention will now be described by way of example only with reference to the accompanying drawings, in which Figure 1 is a schematic diagram showing a motor position detection system according to the present invention incorporated into a motor system. Figure 2 is a graph showing the relationship between the three-phase voltage supply, the trinary values and the stepper motion position according to the motor position detection system of Figure 1, Figure 3 is a flow diagram showing operation of the motor position detection system of Figure 1, and Figure 4 is a graph showing the relationship between reference trinary values Vi,V2,Vs and reference motor positions, error conditions, or a motor off condition. In Figure 1, a motor assembly 10 comprises a motor position detection system 12, a controller 14, and a motor M powered by a three-phase power supply (not shown). The three-phase power supply comprises a first phase a, a second phase b, and a third phase c. The motor M has twelve pre-determined motor positions or steps which are controlled as a function of time T as can be seen in Figures 2 and 4. The sequence of pre-determined motor positions of the motor M is controlled by a controller 14 which moves the motor M between the motor positions. With reference to Figures 1 to 4, the motor position is determined as follows: Firstly, the controller 14 starts a timer at To which measures the time T and moves the motor from one motor position in the pre-determined sequence to the next motor position. Figures 2 and 4 show the motor positions of one cycle, starting at position 0 and finishing at position 11, before the cycle returns to position 0 and repeats the sequence until the motor is turned off. The motor position is changed after a time period Ts from the last motor position (Figure 2). When the motor M is being powered by the three-phase supply, the voltage for each of the three phases a,b,c is measured at the same time T. For example, at time To, the voltage of the first phase, the second phase and the third phase is measured to obtain a measured first phase voltage Va, a measured second phase voltage Vb and a measured third phase voltage Vc. The voltages are measured when the three-phase supply is expected to have a stable waveform, in this embodiment, half-way between successive pre-determined motor positions, that is, half of the time period Ts between position 6 and position 7. The measured first phase voltage Va, second phase voltage Vb and third phase voltage Vc are then compared to a first reference value +Vo and second reference value -Vo which is equal and the negative of the first reference value +Vo. The first reference value +Vo and the second reference value -Vo are chosen to be greater than a predicted noise value and less than half of a minimum operating voltage. The minimum operating voltage is equal to a peak line-line voltage x Sqrt(3) / 2. Vo is the maximum acceptable amplitude of noise that can be superimposed on the voltage rail without causing performance degradation or malfunction. The minimum operating voltage is the lowest level voltage at which the motor can reliably operate. The peak line-line voltage is the maximum voltage measured between any two phases of the three-phase electrical system, i.e. the highest voltage difference between the phases. A trinary value Vat,Vbt,Vct is determined for each of the measured first phase voltage Va, second phase voltage Vb and third phase voltage Vc according to the criteria below: If the measured voltage Va,Vb,Vc is greater or equal to the first reference value +Vo then the trinary value Vat,Vbt,Vct is equal to 1. If the measured voltage Va,Vb,Vc is less than the first reference value +Vo and greater than the second reference value -Vo then the trinary value Vat,Vbt,Vct is equal to 0. If the measured voltage Va,Vb,Vc is less or equal to the second reference value -Vo then the trinary value Vat,Vbt,Vct is equal to -1. The determined trinary value Vat,Vbt,Vct comprises three values with combinations of 0, 1 or-1. The determined trinary value Vat,Vbt,Vct is then compared to a set of twenty-seven unique reference trinary values Vi,V2,Vs (Figure 4). In Figure 4, the reference values 1 to 12 correspond to a plurality of reference motor positions Oto 11, reference value 13 corresponds to a motor off position, and reference values 14 to 27 correspond to an error value. The motor positions and the motor off position are considered legal values in that the associated trinary values indicate a valid voltage combination (off or motor position). The error values are considered illegal values in that the associated trinary values are not the legal trinary values. The illegal values could indicate a physical issue is evolving, such as if a phase is short external to the inverter between phases, to a rail or to another external sink or source. At time Te, if the determined trinary value Vat,Vbt,Vct is 1 ,-1,0, then a comparison with reference trinary values Vi,V2,Vs confirms that the current motor position is a legal motor position, and that the current motor position corresponds to the correct or demanded motor position 6 at time Te. At motor position 6, the timer is re-started, and after a given period of time, half-way past time T?, the time at which the voltage is expected to stable at position 7, the voltage of the first phase, the second phase and the third phase is measured, and trinary values Vat,Vbt,Vct are determined based on the measured voltages at time T? in the same as described above in relation to time T?. No voltages are measured until the timer passes half-way past time T?. The determined trinary value Vat,Vbt,Vct at time T? is then compared to the reference trinary values Vi,V2,Vs (Figure 4). If the determined trinary value Vat,Vbt,Vct is 1 ,-1,1, then a comparison with reference trinary values Vi,V2,Vs confirms that the current motor position is a legal motor position, and that the current motor position corresponds to the motor position 7 at time T?. If at motor position 6, the last position, the demand from the controller was a right rotation, then the next expected or demanded motor position is position 7 and the current motor position at time T? is the correct position and the motor has undergone a correct movement. If the determined trinary value Vat,Vbt,Vct is 1 ,-1,0 then a comparison with reference trinary values Vi, V2,Vs confirms that the current motor position is a legal motor position, and that the current motor position corresponds to the motor position 6 at time T?. The current motor position at time T? is therefore not the correct position and the motor has not moved or is unchanged (stuck) from the last position. If the determined trinary value Vat,Vbt,Vct is 1 ,-1,0 then a comparison with reference trinary values Vi, V2,Vs confirms that the current motor position is a legal motor position, and that the current motor position corresponds to any motor position other than motor position 7, then the current motor position at time T? is not the correct or expected position and the motor has moved out of sequence. It will be appreciated if the motor position is detected as unchanged, then that could indicate a stuck position, or the motor has moved to the same position. In either case, the motor position is not the correct position. If the determined trinary value Vat,Vbt,Vct is 0,0,0, that is, all three measured voltages Va,Vb,Vc are within a range defined by the first reference value +Vo and the second reference value -Vo, then the motor is in the off position as indicated at time T12 and motor position 12 in Figures 2 and 4. Finally, if the determined trinary value Vat,Vbt,Vct is anything other than the off position or any of the motor positions 0 to 11, then the voltage is an illegal voltage as indicated in Figure 4 I reference values 14 to 27 and at time T13 in Figure 2. The above determination of voltages of each of the phases, determination of trinary values, and comparison with reference values continues when the motor is running through its pre-determined sequence. It will be understood that the motor position detection system of the present invention enables the motor position to be detected, an error condition, or if the motor is off, without requiring physical or visual access to the motor output.
Claims
1. A motor position detection system (10) to detect the position of a three-phase motor (M) powered by a supply comprising a first phase (a), a second phase (b) and a third phase (c), the system configured to:a. measure the voltage of the first phase, the second phase and the third phase to obtain a measured first phase voltage (Va), a measured second phase voltage (Vb) and a measured third phase voltage (Vc) at predetermined times,b. compare the measured first phase voltage (Va), the measured second phase voltage (Vb) and the measured third phase voltage (Vc) to a first reference value (+Vo) and second reference value (-Vo),c. determine a trinary value (Vat,Vbt,Vct), for each of the measured first phase voltage (Va), the measured second phase voltage (Vb) and the measured third phase voltage (Vc), the trinary value (Vat,Vbt,Vct) equal to:1, if the measured voltage (Va,Vb,Vc) is greater or equal to the first reference value (+Vo),0, if the measured voltage (Va,Vb,Vc) is less than the first reference value (+Vo) and greater than the second reference value (-Vo), or-1, if the measured voltage (Va,Vb,Vc) is less or equal to the second reference value (-Vo),d. compare the determined trinary value (Vat,Vbt,Vct) for each of the measured first phase voltage (Va), the measured second phase voltage (Vb) and the measured third phase voltage (Vc) with a plurality of unique reference trinary values (Vi,V2,Vs) for each phase, each of the plurality of unique reference trinary values (Vi,V2,Vs) corresponding to one of a plurality of reference motor positions, or an error condition, or a motor off condition, ande. determine a current motor position is one of the plurality the reference motor positions if the determined trinary value (Vat,Vbt,Vct) is the reference trinary value (Vi,V2,Vs) corresponding to one of a plurality of reference motor positions.
2. A motor position detection system according to claim 1 further configured to report the motor off condition if the determined trinary values (Vat,Vbt,Vct) for each phase is zero.
3. A motor position detection system according to claim 2 further configured to report the error condition if the determined trinary value (Vat,Vbt,Vct) is not a reference trinary value (Vi,V2,Vs) corresponding to one of a plurality of reference motor positions or the motor off condition.
4. A motor position detection system according to any preceding claim further configured to compare the current motor position with a demanded motor position to determine if the current motor position is a correct motor position.
5. A motor position detection system according to claim 4 in which one of a plurality of reference motor positions comprises a sequence of pre-determined motor positions, the demanded position corresponds to a motor position of the sequence of pre-determined motor positions.
6. A motor position detection system according to claim 5 in which the pre-determined times are selected from a time between successive motor positions of the sequence of pre-determined motor positions.
7. A motor position detection system according to claim 6 in which the time between successive motor positions is selected when the supply has a stable waveform, preferably half-way between successive pre-determined motor positions.
8. A motor position detection system according to any one of claims 5 to 7 further configured to compare the current motor position with the sequence of predetermined motor positions to determine if the current motor position is a correct motor position.
9. A motor position detection system according to claim 8 in which the current motor position is compared to a last determined motor position to determine if the current motor position is a next position in the sequence of pre-determined motor positions and if the current motor position is a correct motor position.
10. A motor position detection system according to claim 9 in which the current motor position is the correct position if the current motor position is the next position in the sequence of pre-determined motor positions, the current motor position is unchanged if the current motor position is the last determined motor position, or the current motor position is out of sequence if the current motor position is not the last determined or next motor position.11.A motor position detection system according to any preceding claim in which the plurality of reference motor positions is twelve reference motor positions.
12. A motor position detection system according to any preceding claim in which the first reference value (+Vo) is a negative of the second reference value (-Vo),13. A motor position detection system according to any preceding claim in which a range defined between the first reference value (+Vo) and the second reference value (-Vo) is greater than a predicted noise value and less than half of a minimum operating voltage.
14. A motor position detection system according to claim 13 in which the minimum operating voltage is equal to a peak line-line voltage x Sqrt(3) / 2.
15. A motor system (12) comprising a motor (M) and motor position detection system (10) according to any preceding claim.