The invention relates to a method for determining the optimum
operation control parameter values for a screw joint tightening process carried
out by means of a power wrench controlled by a programmable control system in accordance
with such determined optimum operation control parameter values.
Power wrenches controlled by programmable control systems
are usually used in production work, assembly line operations in particular, where
the accessibility of the power wrench and production efficiency have high priority.
Programming of the control system and determining the optimum power wrench operation
characteristics usually takes some time. The method usually practised today means
that parameter values believed to be suitable for the intended tightening process
are programmed into the control system, and a number of tightening operations are
performed. The results are evaluated, the parameter values are adjusted, and another
series of tightening operations is performed. In order not to loose too much time,
this is usually done during the regular assembly work, mostly by slowing down or
stopping the assembly line.
However, this type of iterating process to find the most
favourable operation control parameter values is not only time consuming per se,
but often results in readjustments and in some cases even replacement of damaged
screw joints tightened during the programming procedure. The probability to find
the optimum tightening parameter values by the very first more or less randomly
chosen values is very small, which means that a number of "test" tightenings have
to be performed.
Alternatively, the power wrench could be lifted out of
production for programming and test tightening. However, this means a highly undesirable
reduction of the power wrench accessibility and production efficiency.
In order to speed up the programming procedure and substantially
avoid readjustments of the screw joints tightened during the programming procedure,
the invention provides a method for calculation of the optimum operation control
parameter values for obtaining a tightening process having desired characteristics,
including an acceptable tightening target accuracy. The method comprises a mathematical
simulation of the tightening, including the use of parameter values characteristic
for the actual type of screw joint as well as operation parameter values characteristic
for the power wrench actually being used.
In WO 98/22263 there is described a screw joint tightening
method where the characteristics of the screw joint are used for calculating a desired
final tightening condition of the screw joint, and where the actually obtained tightening
parameter values are compared to the calculated parameter values at the desired
final tightening condition for interruption of the tightening process. This known
method does not involve any tightening simulation sequence where the optimum operation
control parameter values are determined.
Further objects and advantages of the method according
to the invention will appear from the following specification.
The invention is described below in further detail with
reference to the accompanying drawings.
On the drawings:
- Fig. 1 shows a flow chart illustrating the method according to the invention.
- Figs. 2 and 3 show diagrams illustrating two two-step tightening processes with
- Fig. 4 shows a diagram illustrating a one-step tightening process.
The method according to the invention is suitably carried
out by means of an electric power wrench which is connected to a control system
having a drive means for supplying a suitable drive voltage to the power wrench.
The voltage output of this drive means should have a variable frequency and voltage
magnitude for varying the speed and torque output of the power wrench. The control
system should also comprise a programmable data processing and storage means. A
control system suitable for this purpose is marketed by Atlas Copco Tools AB under
the name: POWER FOCUS 2000.
The method according to the invention is illustrated in
the flow chart in Fig. 1, wherein a program unit is supplied with parameter values
characteristic for the screw joint to be tightened and stored in a data base 11,
operation parameter values characteristic for the power wrench being used and stored
in a data base 12, as well as operation control parameter values chosen from a great
number of values stored in a data base 13.
In the simulation step, the tightening process is performed
mathematically in a data processing unit 14 on the basis of the screw joint and
power wrench characteristic parameter values provided from the data bases 11 and
12, as well as on the basis of operation control parameter values provided from
the data base 13. The method for determining the optimum tightening process control
parameter values is carried out in that such control parameter values which are
estimated to give an acceptable result of the tightening process are pre-chosen
from the data base 13 and used for a first simulation.
The result of the simulation is evaluated by a comparator
16 in which data representing desired criteria values chosen for the actual tightening
process and stored in a memory 17 are compared with the simulation result. Such
evaluation criteria may be: the tightening accuracy as regards the tightening target
value, tightening process duration, ergonomically favourable reaction forces etc.
If the result of the first simulation is accepted in view
of the desired evaluation criteria values, the pre-chosen process control parameter
values are kept in the memory of the control unit and used for production tightening.
If, on the other hand, the result of this first simulation
is rejected the simulation process is restarted with changed control parameter values
chosen from the data base 13. Such a restart of the simulation procedure will be
repeated one or more times with successively changed control parameter values until
the simulated tightening result is accepted. Then, the last used control parameter
values are kept in the control system memory and used for production tightening.
As described above, the method according to the invention
means that a number of operation parameter values are optimised via a mathematical
procedure wherein the tightening process to be performed on a certain type of screw
joint is simulated using basic parameter values that are characteristic for the
actual screw joint and parameter values characteristic for the power wrench actually
being used. The most important parameter related to the screw joint is the so called
joint rate, i.e. the growth in axial load in relation to screw joint rotation.
Important characteristic parameters related to the power wrench and its drive means
are torque and speed capacities as well as dynamic characteristics like acceleration
and retardation abilities, including the motor and drive train inertia.
As mentioned above, the main objectives of the method according
to the invention are to obtain a fast and safe programming of the control unit and
to optimise the tightening process in respect to process duration, tightening target
accuracy and ergonomy. The optimum operation data have to be related to the priority
given to these three aspects. If tightening target accuracy is given the highest
priority, the tightening speed, at least during the final stage, has to be low enough
not to let the inertia of the rotating power wrench parts cause any undesirable
torque overshoot. The speed could be successively reduced towards the end of the
process according to a predetermined deceleration ramp.
If the process duration is given the highest priority,
the power wrench speed has to be increased which might result in a somewhat reduced
tightening target accuracy.
Ergonomic aspects on the tightening process may affect
the process time in so far that an adaptation of the reaction torque characteristics
to the operator's ability to counteract the reaction torque may require a somewhat
slower process. However, an extended tightening process may likewise be straining
to the operator. An optimum of duration and magnitude of the reaction torque is
to be found to provide an ergonomically favourable process.
The method according to the invention may also comprise
a sensitivity analysis wherein some parameters are changed at repeated tightening
simulations to see how much these parameters may vary in relation to other parameters
without causing the tightening process result fall outside the predetermined evaluation
In order to illustrate how the tightening process may be
changed by varying some of the tightening parameters, there are illustrated three
different ways of tightening a screw joint to a predetermined target torque level
In Fig. 2, there is illustrated a two-step process in which
the screw joint is first rotated at a relatively low speed up to a speed shift level
TS . At this point the power wrench is stopped for a short interval and
then restarted and rotated at a relatively low speed up to the target torque level
TT. Since the target torque level is reached at a relatively low speed,
the torque overshoot To is very small. In other words, the final tightening
accuracy is high. The process time, though, is rather long.
In Fig. 3, there is also illustrated a two-step tightening
process wherein the screw joint is first tightened up to a speed shift level TS
where the power wrench is stopped. After a short intermission, the second step is
commenced. Compared to the process illustrated in Fig. 2, this process is performed
at a higher speed. This means that the speed shift level TS is reached
much quicker. Also the second step is performed at a higher speed, which means that
the target torque level TT is reached much quicker as well.
However, the relatively high rotation speed during the
second step and the subsequent high kinetic energy of the rotating parts of the
power wrench means that the target torque level TT is passed to a higher
degree than in the previously described example. The torque overshoot To may become
high enough not to be accepted under the accuracy requirements. If, however, the
torque overshoot is acceptable there is obtained a much less time consuming tightening
Another way of changing the process in order to comply
with set-up conditions is to choose a one-step process as illustrated in Fig. 4.
Like the process illustrated in Fig. 3, this process is performed at a relatively
high speed. This means that the speed shift level TS is reached quickly.
At the speed shift level TS , however, the rotation is not interrupted
just the speed is down shifted so as to approach the target torque level TT
at a speed low enough not to cause any unacceptable torque overshoot To.
Since the intermission at the speed shift level TS is omitted, this one-step
process is still shorter than the process illustrated in Fig. 3.
Apart from altering the power wrench rotation speed and
determining whether the tightening should be performed in one or two steps, there
are other parameters to consider when optimising the process. For instance, there
may be used any suitable speed reduction ramp at the end of the process in order
to avoid or at least reduce the torque overshoot To.