SYSTEM FOR DYNAMICALLY CONTROLLING THE TORQUE OUTPUT OF A PNEUMATIC TOOL

Background of the Invention

The present invention relates to the control of torque or power from pneumatic tightening tools, and more specifically, to high speed pneumatic tools, such as impact and impulse tools, for purposes of tightening desired fasteners.

Impact and impulse tools are currently used extensively to tighten non-critical bolts in automotive and other industrial applications. Such tools provide very high torque to weight ratios, are very fast and have very low reaction torque since they effectively hammer the bolt tight. Unfortunately, however, the impacting action of the tools makes it difficult to control the tightening process since it is not possible to make accurate torque measurements, as it is with continuously operating tools. Consequently, such tools are rarely used in critical applications where bolts are required to be tightened precisely to a specified load or torque.

Techniques have been developed for performing direct load measurements in fasteners utilizing ultrasonic transducers which are removably, or preferably permanently attached to the fasteners. Examples of such techniques can be found, for example, in U.S. Patent No. 6,990,866

(Kibblewhite) ; U.S. Patent No. 6,009,380 (Vecchio et al . ) ; U.S. Patent No. 5,220,839 (Kibblewhite); U.S. Patent No. 5,018,988 (Kibblewhite et al . ) ; U.S. Patent No. 4,899,591 (Kibblewhite); and U.S. Patent No. 4,846,001 (Kibblewhite), each of which is incorporated by reference as if fully set forth herein. It has been found that such techniques make it possible to directly control the installation load of various different types of fasteners using all types of assembly tools, including impact and impulse tools. Certain characteristics associated with impact

and impulse tools, however, make them less desirable for use in critical applications. Firstly, if the tools are sized to tighten bolts quickly, to minimize assembly time, the angle of rotation per impact, and consequently the load increase per impact, can be large at the time that the specified load or torque is reached. Since the tools cannot be stopped during an impact, this results in significant tool overrun (i.e., final loads which exceed the specified loads), even when high speed solenoid valves are used to stop the tool.

Secondly, the rundown speed of such tools is extremely high, typically above 6,000 rpm. When these tools are used with prevailing torque lock nuts, locking fasteners or thread forming fasteners, rundown at these speeds can cause excessive localized heating in the threads of the fastener, resulting in undesirable changes in friction conditions or the degradation of friction coatings. This has been found to be common with the use of prevailing torque lock nuts in the aerospace industry, for example.

Summary of the Invention

A primary objective of the present invention is to eliminate the above-mentioned undesirable characteristics of pneumatic tightening tools, allowing such tools to be used for high speed assembly of critical bolts to precise loads. In accordance with the present invention, this is accomplished by dynamically controlling the output power of a pneumatic tool during a tightening cycle using an electronically controlled air pressure regulator to reduce the tightening rate, or the load increase per impact in the case of an impact or impulse tool, to enable the tool to be stopped precisely at a specified stopping load or torque.

In a preferred mode for torque fasteners, the output power of a pneumatic tool is dynamically controlled during the tightening cycle using an electronically

controlled air pressure regulator to minimize the speed of rotation during rundown, to minimize heating effects with prevailing torque fasteners, and to then increase the power from the tool, as required, to provide the torque to reach a specified stopping load or torque.

In another preferred mode, the maximum air pressure supplied to a pneumatic tool is limited, using an electronically controlled air pressure regulator, depending on the expected torque required to tighten the fastener to a specified load or torque.

The foregoing improvements are further described with reference to the detailed description which is provided hereafter, in conjunction with the following drawing.

Brief Description of the Drawing

The single figure is a schematic representation of a pneumatic tool in combination with a system for dynamically controlling the output power of the pneumatic tool during a fastener tightening cycle.

Description of Preferred Embodiments

Referring to the single figure provided, a preferred embodiment of the present invention generally includes a fastener 1 which has been fitted with an ultrasonic transducer 2, a tool such as the illustrated impact wrench 3 which ha.s been modified to measure load in the fastener 1 during tightening using the ultrasonic transducer 2, an electronic control 4 for making load measurements in the fastener 1 and for making control decisions based on the load measurements which have been made, and an electronically controlled air pressure regulator 5 associated with the supply line 6 which delivers pressurized air to the impact wrench 3 to dynamically control the air pressure supplied to the

impact wrench 3 during tightening and to stop the impact wrench 3 by reducing the supplied air pressure to zero. The fastener 1 of the preferred embodiment of the present invention is preferably a load indicating fastener with a permanent ultrasonic transducer 2, such as is described, for example, in the above-referenced U.S. Patents No. 6,990,866; No. 5,220,839; No. 4,899,591; and No. 4,846,001. However, if desired, the fastener 1 can also be a convention fastener with a removable ultrasonic transducer suitably applied to the fastener. Although the fastener 1 selected for illustration in the drawing is a threaded bolt, it is to be understood that any of a variety of different types of fasteners can be used in accordance with the present. invention, other than the fastener 1 which has been shown for illustrative purposes.

The impact wrench 3 used to tighten the load indicating fastener 1 is preferably modified with a spring biased pin 7 to permit electrical contact with the ultrasonic transducer 2 for purposes of making load measurements in the fastener 1 during tightening. Such modified tools are described, for example, in the above-referenced U.S. Patents No. 5,018,988 and No. 4,899,591. While the impact wrench 3 has been selected for illustration in the drawing, it is to be understood that any of a variety of different types of tightening tools can be used in accordance with the present invention, other than the impact wrench 3 which has been shown for illustrative purposes.

The impact wrench 3 is electrically connected to an electronic control 4 which includes ultrasonic load measurement circuitry, as is described, for example, in the above-referenced U.S. Patent No. 6,009,380, for purposes of making precise high speed ultrasonic load measurements in the fastener 1 during tightening, for load control purposes, as is described, for example, in the above-referenced U.S. Patent No. 6,990,866.

The electronically controlled air pressure

regulator 5 is a high-speed regulator which can preferably change the air pressure delivered to the impact wrench 3 within the amount of time available between impacts. An example of an electronically controlled air pressure regulator which can provide such a function is the PAR-15 valve manufactured by Parker Pneumatic.

In a preferred mode of operation, the electronic .control 4 first establishes a maximum allowable air pressure setting for the fastener 1 being tightened based on the capacity of the tool (the impact wrench 3) and the expected maximum torque required to tighten the fastener 1. The electronic control 4 preferably continuously measures load from the load indicating fastener 1 during tightening. The electronic control 4 computes a tightening rate or an increase in load over a time interval such as, for example, an increase in load during the time for the impact tool to deliver two impacts. After each load measurement and load rate calculation, the electronic control 4 makes a decision whether to increase the air pressure, decrease the air pressure, or leave the air pressure at its current setting, based on the load measurement and load rate calculation.

If the tool is being used with prevailing torque fasteners, it can be desirable to perform the rundown of the fastener 1 at a reduced speed. In such cases, the electronic control 4 is preferably caused to operate by first adjusting the air pressure to a predetermined low pressure setting which is sufficient to rotate the fastener 1 until loading commences. As soon as loading commences, which is indicated when the measured load reaches a predetermined minimum rundown load setting, the electronic control 4 then increases the air pressure to a normal tightening pressure, such as the predetermined maximum allowable air pressure for the fastener 1.

As the tightening process continues, the electronic control 4 continuously makes load measurements and load rate calculations. Based on a comparison with an

optimized load rate verses load characteristic stored for the tool type utilized (the selected impact wrench 3) , the electronic control 4 increases, decreases or leaves unchanged the air pressure setting. As the tightening load approaches the stopping load, for example at 90% to 95% of the stopping load, the electronic control 4 reduces the air pressure so that the load increase per impact is minimal, for example, less that 2% of the stopping load per impact. As soon as the stopping load is reached, the air pressure is reduced to zero, stopping the tool before the next impact.

Consequently, tightening overrun is minimal, i.e., less than 2% in the above example.

When the tool is required to tighten as quickly as possible, as is usually the case on automotive assembly lines, for example, and assuming there is no requirement for reduced rundown speed, then the tool preferably starts at its maximum allowable air pressure setting and the control process thereafter proceeds as previously described.

As an example of the foregoing operations, the system illustrated in the single figure can be operated to tighten a fastener with a permanent ultrasonic transducer by making load measurements during tightening of the fastener with an impact wrench, and by dynamically determining the tightening load rate to be applied to the fastener by the impact wrench.

The tightening rate is measured in terms of the increase in load over a period corresponding to 2 impacts, divided by the target load for the tightened fastener, which is preferably implemented in terms of measurement updates. In the present example, the air pressure regulator can be set to one of 16 air pressure levels. A dynamic power control strategy will then be determined by one of a number of predefined power tables, which are used to determine whether to maintain, increment or decrement by 1 the air pressure setting based on load and load rate measurements.

The index into the table will preferably be the current load

(i.e., a 5% range), and the table will contain a minimum load rate and a maximum load rate for the load. If the load rate is less than the minimum, the air pressure setting will be incremented by 1 (up to ^" the maximum available tightening power) , and if greater, the air pressure setting will be decremented by 1. The following Table illustrates a typical predefined power table for performing the previously described dynamic power control strategy.

Table

Current Load Table Index Inc . if Rate < % Load Dec . if Rate > % Load ( % of tarσet) ( % load / 5) Increase / 2 Imøacts Increase / 2 Iπvoacts

0-5 0 10 255

5-10 1 10 255

10-15 2 10 255

15-20 3 10 255

20-25 4 10 255

25-30 5 10 255

30-35 6 10 255

35-40 7 10 255

40-45 8 10 255

45-50 9 10 255

50-55 10 7 20

55-60 11 7 20

60- 65 12 7 15

65-70 13 7 15

70-75 14 7 15

75-80 15 6 10

80-85 16 6 10

85- 90 17 6 10

90- 95 18 3 5

95 + 19 2 3

User settings for the foregoing system can include the selection of a power table (by number) , the time between

impacts delivered (for example, in 10ms increments), rundown load (% of target) , rundown power setting, and maximum usable torque from the tool. Note that a maximum tightening power setting will be calculated from- the maximum usable torque and the maximum torque specified for a particular application.

A fast tightening mode can be initiated at a maximum tightening power setting, with no incrementing above this level. At every measurement update (for example, 12ms) load rate is calculated and the power setting is maintained, decremented or incremented according to the table until the target load is reached.

A slow rundown mode, for prevailing torque fasteners, can be initiated with the rundown power setting, and can proceed until the appropriate rundown load (%) is reached. At this point, the power is increased to a maximum tightening power setting and is continued as defined in the selected power table, as for the fast tightening.

It will be appreciated by one skilled in the art that the above-described method of controlling tightening rate during tightening is applicable to types of pneumatic tools other than the illustrated impact wrench 3, such as impulse tools and continuous tightening pneumatic tools. It will be further appreciated that the above-described method can be used with convention fasteners and removable ultrasonic transducers, or conventional fasteners with tools and electronic controls for measuring torque and for determining torque rate, instead of load and load rate, in a similar manner to that previously described, to minimize heating with prevailing torque fasteners or to minimize torque overrun. Accordingly, it is to be understood that various changes in the details, materials and arrangement of parts which have been herein described and illustrated in order to explain the nature of this invention may be made by those skilled in the art within the principle and scope of the invention as expressed in the following claims.