This invention concerns a method for self-programming of
a computerized control system for a power nutrunner as defined in the claims.
A method according to the preamble of independent claims
1 and 4 is disclosed in
US 5 637 968
The object of the invention is to provide a self-programming
method for a power nutrunner control system, whereby the efficiency of the nutrunner
operation is increased by decreasing the cycle time without increasing the risk
for torque overshoot at tightening of so called stiff screw joints, i.e. screw joints
having a steep torque growth per angle unit of rotation.
The invention relates primarily to the two-step type of
tightening process which comprises a high speed first step and a low speed second
step. The two steps are divided either by just a speed change or by an intermediate
standstill. The first step is always the most time consuming part of the process,
because it contains the relatively long running down or nut setting phase. Therefore,
to bring down the time spent on the first tightening step, the rotation speed of
the fist step has always been kept at a high level in prior art methods.
To safely avoid overtightening of stiff screw joints, however,
the speed change point, i.e. where the first high speed step is succeeded by the
second low speed step, has been set at a low torque level. The reason is that the
kinetic energy of the rotating parts of the nutrunner should be prevented from having
any influence on the end result, even in cases of a very steep torque growth characteristic
of the screw joint.
This means on the other hand that when tightening screw
joints having a very slow torque growth characteristic, this low level down shift
point results in an unnecessary long and time consuming low speed second step.
The above described type of two-step tightening is commonly
used, and by shifting down to the low speed second step at an early low torque point
the method is safely applicable on all types of screw joints. In order to speed
up the process, the shift down point as well as the individual speed levels may
be adjusted in relation to the characteristics of one specific type of screw joint.
If, however, the system with this specific setting is used on another type of screw
joint having a steeper torque growth characteristic, there is a great risk for getting
an undesirable torque overshoot.
, there is described a two-step screw joint tightening method by which
the risk for overtightening is avoided by stopping the first high speed step at
a very low torque level and carrying out the second step at a successively increasing
speed. Thereby, the rotation speed in the second step is still low when reaching
the intended final pretension level at a stiff screw joint, and this low kinetic
energy in the nutrunner parts does not cause any dynamic torque overshoot. A drawback
with this prior art method, however, is an undesirably slow and time consuming second
step when tightening weak screw joints.
, there is described a self-adjustable nutrunner control system by which
the shut-off point for the nutrunner is automatically adjusted in view of the result
of preceding tightening processes. This method, however, does not comprise any detection
and calculation of the actual screw joint characteristics during tightening, and
the described process is performed in a single step only. Neither is there anything
disclosed in this reference about how to decrease the cycle time by adjusting successively
one or more nutrunner operation parameters in response to empirically determined
and calculated screw joint characteristics during a number of initial tightening
EP 0 753 377
there is described a single-step bolt tightening method where a number
of speed-torque curves for the actual tightening equipment are produced, one of
which is selected for a specific screw joint and for obtaining an optimum bolt tightening
as regards process time and tolerable error of the end result. This known method,
however, is disadvantageous in that the single-step-process is less adaptive to
occurring variatons in screw joint characteristics.
there is described a screw joint tightening method wherein the torque
rate is calculated for each joint during an initial stage of the tightening procedure,
and in case the torque rate is above a certain first level the rotation speed is
reduced to a medium speed level, and in case the torque rate of the actual screw
joint is above a second higher level, i.e. the joint is a so called hard joint,
the speed is reduced to a low speed level. In case the calculated torque rate is
below the first level the joint is classified as a soft joint and the speed is not
reduced at all. The purpose of this speed reducing technique is to avoid overtightening
of hard and medium hard screw joints, but still keep down the process time for softer
joints. According to this method the torque rate is calculated for each screw joint
which means that it is not adapted to tightening a number of equal screw joints
at an optimized low process time at a guaranteed safety against overtightening.
The main object of the invention is to provide a method
for self-programming of a nutrunner control system during one or more initial complete
tightening processes, for obtaining automatically and without any programming expertise
an optimum setting of the nutrunner operating parameters for the most time efficient
tightening process, irrespective of the torque growth characteristics of the actual
A further object of the invention is to provide a method
for self-programming a nutrunner control system by detecting and calculating during
one or more initial complete tightening process one or more screw joint characteristics,
and by adjusting successively during a succeeding number of tightening processes
one or more operating parameters of the nutrunner until the tightening process is
performed at a satisfactory time efficiency.
A still further object of the invention is to provide a
self-programming method for a nutrunner control system intended for performing a
two-step tightening process, wherein during one or more initial tightening processes
one or more screw joint characteristics are detected and calculated, and during
a succeeding number of tightening processes one or more nutrunner operating parameters
are successively adjusted in order to adapt the speed shift point between the high
speed of the first step and the low speed of the second step, thereby extending
the first step to a certain point which differs from the point corresponding to
the predetermined final pretension level of the screw joint by a certain amount.
Further objects, characteristics and advantages of the
method according to the invention will appear from the following specification and
The invention is below described in further detail with
reference to the accompanying drawings on which:
- Fig. 1 shows a diagram illustrating the torque/angle characteristic of a relatively
soft type of screw joint, as well as the rotation speed at different programming
- Fig. 2 shows a diagram illustrating the torque/angle characteristic of a relatively
stiff type of screw joint, as well as the rotation speed at different programming
The relatively soft type of screw joint illustrated in
Fig. 1 is intended to be pretensioned to a final torque level MF. This
torque level is the used as the end target for the tightening processes to be performed
at this type of screw joint.
The self-programming tightening process comprises a few
initial tightening processes each of which is a complete tightening process pretensioning
the actual screw joint to the intended pretension level. This means that the method
according to the invention may be carried out in a regular production work, i.e.
no special programming operations have to be carried out.
As stated above, the main object of the invention is to
accomplish a screw joint tightening process by which the joints are tightened to
a desired final pretension level in an optimum time interval, i.e. as quick as possible,
without risking overtightening of the joints. This means that the process has to
be carefully adapted to the specific characteristics of the actual type of screw
joint. This means that a few initial tightening processes are carried out during
which the system is "learning" the characteristics of the joint. In particular,
the system is programmed with the torque rate of the joint, i.e. the torque growth
per angle of rotation. Also variations in the frictional resistance of the joint
is registered. All this information is of great importance for avoiding overtightening
when trying to speed up the process.
As mentioned in the first part of this specification, the
only way to speed up the process is to extend the first high speed step as long
as possible and to complete the process with a short low speed second step. Preferably,
the first tightening step should be extended to point in which the angular position
of the joint is 50 - 80 % of the angular position when reaching the final pretension
level. In order to accomplish this, the preliminary target for interrupting the
first tightening step and the rotation speed during the first step is initially
set at a very low level.
For obtaining the relevant information of the screw joint,
the very first tightening process is performed at a low speed, during the first
step as well as the second step.
In order to find a suitable preliminary target torque by
which the rotation during the first step should be interrupted, there are at least
two possible ways to proceed. One way is to step up the speed during the first tightening
step while aiming at the same preliminary target torque MPT1 and to indicate
what the installed preliminary torque level will be, and, thereafter, when the maximum
speed Vmax of the system is reached the target torque level MPT
could be stepped up as well.
However, these two ways of adapting the down shift point
to the characteristic of the actual type joint could be combined. This means that
after the very first screw joint analysing low speed process, the speed of the first
step is step-wise increased simultaneously with a step-wise increase of the preliminary
target torque level MPT. This combined adapting process is quicker and
is initiated automatically by the control system when the screw joint characteristic
is very soft.
As illustrated in Fig. 1, the programming of the process
control system is carried out in two subsequent steps, starting with a first running
down step at a speed v1. This step is interrupted at a preliminary target
torque level MPT1. Due to the kinetic energy stored in the rotating parts
of the power tool used for this operation, the preliminary target torque is superseeded
by a certain amount and the resultant installed torque is a preliminary torque MP1.
The process is continued by a second step carried out at
a very low speed v21 until the final desired torque level MF
is reached. At that point the angular position of the screw joint is &PHgr;F.
It is clearly illustrated, that this first tightening process is extremely slow,
because not only the speed levels are low per se, but the shift down point from
the first step to the even slower second step takes place very early, which means
that the very slow second step becomes very long, from a point &PHgr;P1
to the final angular position &PHgr;F.
After this first process, the speed of the first tightening
step is increased to v2 whereby at the same time, the target torque level
is increased from MPT1 to MPT2. The resultant installed torque
then becomes MP2 and the angular position of the joint at the end of
the first tightening step is &PHgr;P2. The angular distance to the aimed
final position &PHgr;F is still far too long. The following slow second
step is still too long and the cycle time for the entire process is too long. The
rotation speed during the second step is increased to v22, though, which
reduces the cycle time to some extent.
Accordingly, the first step speed is increased to v3
and the preliminary target torque level is increased to MPT3. As the
joint stops after power interruption at MPT3, the installed torque is
MP3 and the resultant angular position of the joint is &PHgr;P3.
Still, the second step is too long.
In the next programming step, the rotation speed during
the first tightening step is increased to the maximum capacity of the system vmax
and the preliminary target is increased to MPT4. The angular distance
from the resultant position &PHgr;P4 after the first tightening step
to the aimed end position &PHgr;F as calculated is still too long.
Since the maximum speed Vmax of the system is
reached already, the last measure to obtain a satisfactory programming of the control
system is to increase the preliminary target torque level to MPT5. Now,
the resultant installed torque becomes MP5, and the angular position
of the down shift point between the first step and the second step becomes &PHgr;P5.
The distance between this point and the aimed final position &PHgr;f
is satisfactory short to result in an optimum cycle time without risking overshoot
in the applied pretension torque. This means that the obtained angular position
&PHgr;P5 differs from the aimed final position &PHgr;F by
less than 20 - 50%.
It is also possible to use the installed torque MP
as reference criteria when determining a satisfactory end status of the first tightening
step. Accordingly, the installed torque MP should amount to a 50 - 80
% fraction of the desired final torque MF.
When applying the self-programming method according to
the invention on a relatively stiff joint, as illustrated in Fig. 2, the strategy
chosen by the control system is somewhat different from the strategy used in the
above described embodiment.
After an initial very slow tightening process, starting
by a first tightening step at the speed v1 and a following acceleration
in a second step until the final level MF is reached, the torque rate
or stiffness of the screw joint is detected and calculated.. Since the joint, according
to the initial detecting and calculating process, has a steep torque angle characteristic,
the self-programming strategy will be to increase stepwise the rotation speed during
the first tightening step while aiming at the same preliminary target MPT1.
As illustrated in the diagram in Fig. 2, the initial speed v1 is stepwise
increased successively to v2, v3, v4, v5,
v6, and finally to vmax.
During this speed increase, the actually installed torque
in the joint is increased from MP1 to MP2 , MP3
, MP4 , MP5, MP6 and MP7. Since the
angular position of the joint corresponding to the installed torque MP7
is still not close enough to the desired final angular position &PHgr;P
and since the rotation speed is not possible to increase any further, the preliminary
torque target is increased one step to MPT2. This results in an increase
of the installed torque to MP8, and the obtained angular position of
the joint after the first tightening step is &PHgr;P8. The resultant
distance between this angular position &PHgr;P8 and the aimed final
position &PHgr;F is acceptably short, which means that the second tightening
step and, accordingly, the overall cycle time will be satisfactory short.
At this point, the programming is automatically locked,
which means that all subsequent tightening processes on the same type of screw joint
will be carried out in the same way, i.e. using the maximum system speed Vmax
up to the preliminary torque target MPT2, and completing the tightening
process at a low speed up to the desired final pretension level MF. This
means that the final angle of rotation to be performed during the second tightening
step, namely from the obtained position &PHgr;8 to the final aimed position
&PHgr;F, is short enough to provide a satisfactory short overall cycle
Above there have described self-programming processes of
a power nutrunner control system in two types of screw joints having different torque
rates or torque growth characteristics, and there have been described two different
strategies chosen by the system itself for obtaining a satisfactory programming.
This choice of strategy is made automatically by the system itself after having
detected and calculated during the initial low speed processes the torque growth
and friction characteristics of the actual type of screw joint. In case of a soft
joint, the programming process may be speeded up by stepping up at the same time
the rotation speed and the preliminary target torque level. In a case of a stiff
torque growth characteristic, the preliminary torque target level is kept constant,
at least to begin with, while stepping up the rotation speed to see what value is
obtained of the installed torque or the angular position. If the maximum speed of
the system does not suffice to reach an acceptable angular position, the preliminary
torque target has to be stepped up as well.
In the above examples, torque has been used as a measurement
for pretension level of the screw joint, and angular positions of the screw joint
have used to determine the down shift point during tightening. The invention, however,
is not limited to the use of these two parameters for governing the process. Instead
of torque, axial load in the screw joint could be used as an indication on the pretension
Neither is the invention limited to a tightening process
divided into two distinct steps where the rotation is completely stopped between
the two steps. The process could as well be performed in two different speed phases
where the rotation of the screw joint is not stopped at the down shift point, a
momentary speed reduction between the first tightening step and the second step
may be enough.