Electricity: motive power systems – Positional servo systems – 'reset' systems
Reexamination Certificate
2001-05-03
2004-06-29
Ro, Bentsu (Department: 2837)
Electricity: motive power systems
Positional servo systems
'reset' systems
C318S594000, C318S596000, C318S615000
Reexamination Certificate
active
06756762
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a method for stopping an electrical drive, by means of closed-loop position control, in a predetermined nominal position, and to corresponding drive control.
When drives with closed-loop position control and a conventional cascaded closed-loop control structure, which are also used for rotation speed control, are stationary by means of closed-loop position control, its rotation speed regulator may experience undesirable oscillations.
As a rule, this oscillation is associated with low position resolution and position sampling at discrete times which leads to “minimum quantization” of the rotation speed, which energizes the rotation speed regulator when new position information occurs. This results in position deflection, which the position regulator has to counteract.
FIG. 1
shows a block diagram of such a known conventional cascade closed-loop control structure, in which this problem can occur. In this structure, PI current control
3
forms the innermost control loop, by supplying a control voltage u
s
for control equipment
4
in order to produce actuation signals for the electric motor (for example for the active power devices in a three-phase bridge inverter for a three-phase machine). The actual current value i which occurs in the armature circuit
5
whose input side is actuated by the motor voltage u is fed back negatively to the input of the current regulator
3
.
At a higher level, there is a PI rotation speed regulator
2
, whose associated actual rotation speed value n, which is likewise fed back negatively to the input, is formed by differentiation of the actual position value &phgr;. The output of the rotation speed regulator
2
with the nominal current value is is limited by current limiting
8
to the maximum permissible current i
max
.
The outer control loop is formed by a P-position regulator
1
whose nominal position value &phgr;
s
is predetermined by higher-level path control (not shown). An actual position value &phgr; is likewise fed back negatively to the input of the position regulator
1
.
Drives having a positioning capability are nowadays normally provided by using DC motors with or without brushes, or synchronous motors with permanent-magnet excitation. As a rule, the actual position value is formed via an incremental sensor with a downstream step-up/down counter
7
, whose input is actuated by a mechanical integrator
6
. The mechanical integrator
6
has actual current values i applied to it, and describes the physical relationship between the rotation speed n and the torque M in accordance with
ω
=
1
J
·
∫
M
·
ⅆ
t
where
&ohgr;=2
·&pgr;·n, M≈i,
which results in the following relationship:
n=K·∫i·dt.
The overall open-loop sequence control and closed-loop control are preferably carried out digitally in a microprocessor, with the individual control loops
1
,
2
and
3
being calculated cyclically at fixed time intervals, and with the cycle time of a respective low-level control loop having to be less than or equal to that of the higher-level control loop.
In the past, this undesirable state has been solved and good stationary response with closed-loop position control has been achieved either by the incremental sensor having very high position resolution or by low gains in the position and rotation speed control loops.
Increased position resolution in the incremental sensor reduces the minimum rotation speed, which is governed by the quantization. Implementation with high position resolution used to minimize the rotation speed quantization results, however, results in expensive sensor systems with small quantization intervals. The small quantization intervals also have to be evaluated “in a complex manner” by interpolation, which cannot be done cost-effectively in all applications.
On the other hand, reducing the gains in the position and rotation speed control loops reduces the control quality, particularly when load torques are present. If the motor has magnetic cogging torques, undesirable compensation movements can occur.
The object of the present invention is thus to provide closed-loop control in which this undesirable oscillation of the rotation speed regulator when the motor is stationary is avoided, despite low position resolution.
According to the present invention, this object is achieved by a method for stopping an electrical drive, by means of closed-loop position control, in a predetermined nominal position by switching a closed-loop control system for movement operation and after reaching the predetermined nominal position to position-maintenance closed-loop control, having rotation speed control with a proportional element and higher-level position control with a proportional element and an integrating element.
According to a first advantageous refinement of the method according to the present invention, current limiting is carried out after reaching a predetermined nominal position and until the next movement instruction, in that, if a predetermined current limit value is exceeded in the armature circuit of the drive, the control voltage for the rotation speed control is limited.
For implementation of the method according to the invention, it has been found to be advantageous for the proportional gain of rotation speed control to be optimized to the mass moment of inertia of the electric motor being used.
For the proportional gain and the integrating-action time of the integrating element of the closed-loop position control, it is recommended that these items be permanently set to the controlled system of the position-maintenance closed-loop control.
For integration of the method according to the invention in an existing control system of an electrical drive with closed-loop position control, this drive is moved to a respective nominal position, according to a further advantageous refinement, with cascade closed-loop control by means of current control, higher-level rotation speed control and even higher-level position control, in which case, after reaching this predetermined nominal position and until the next movement instruction, the drive is controlled by modified cascade closed-loop control without current regulation as described above.
In order to avoid discontinuities when switching between movement operation and position-maintenance closed-loop control, the integrating elements of the respective control structure which are not required may be set to zero.
Furthermore, the object of the invention is also achieved by drive control for stopping an electrical drive, by means of closed-loop position control, in a predetermined nominal position, with a cascade closed-loop control structure comprising a P-regulator for rotation speed control which is used to produce a control voltage for control equipment for the drive, and having a higher-level PI regulator for closed-loop position control.
Such drive control according to the invention is particularly effective if the actual position values can be obtained by means of an incremental sensor with a downstream counter, and actual rotation speed values can be determined from this by a means for differentiation.
A further advantageous refinement of this drive control according to the invention additionally comprises a means for limiting the control voltage on the output side of the P-regulator by an integrator when a predetermined current limit value is exceeded in the armature circuit of the drive.
The method according to the invention and the drive control according to the invention thus reduce the oscillation of the rotation speed regulator when stationary—despite low position resolution and a high minimum rotation speed resulting from this—by switching the closed-loop control structure when the drive has moved to the nominal position.
REFERENCES:
patent: 3721882 (1973-03-01), Helms
patent: 4647826 (1987-03-01), Ota
patent: 5079490 (1992-01-01), Kita et al.
patent: 5090002 (1992-02-01), Chow et al.
patent: 57-009277 (1982-01-01), None
patent: 62-
Baker & Botts LLP
Ro Bentsu
Siemens Aktiengesellschaft
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