Pneumatic tools are used in a wide range of industries. In the automotive industry pneumatic tools are used, inter alia, for driving bolts or nuts and the like during assembly of components. One application involves tightening a set of wheel nuts on an assembly line.

Conventional pneumatic tools include a pressure switch which changes state of an output signal when the tool reaches a preset torque. The output signal is interfaced to a controller which includes an error proofing counting function. The counting function is adapted to provide a warning to the operator if he misses a wheel nut. One problem with prior art controllers is that they are unable to determine if a nut was actually run down prior to the tool reaching the preset torque and shutting down. This is significant because an operator may unintentionally perform multiple tightening sequences on one nut thereby bypassing the error proofing counting function.

An object of the present invention is to provide a pneumatic tool controller which alleviates the disadvantages of the prior art.

The present invention is based on the recognition that pneumatic tools use compressed air at a known rate when they are operating. When the tool shuts down after it reaches the preset torque, air flowing through the tool is reduced significantly. Typical flow rates for air tools vary between 120-600 litres per minute with a reduction of about 50% at shut down. Air flow at the normal or substantially normal flow rate for a predetermined period of time can serve as an indication that the nut was run down prior to the tool shutting off. The predetermined period of time may be estimated by timing a nut tightening sequence. The controller may be adapted to interpret a lack of indication that the nut was run down as a false start caused by an operator accidentally performing a tightening sequence on a previously tightened nut. A false start may not increment the counting function. However, the controller may increment the counting function when an output signal from the pressure switch is preceded by a flow-rate that is above a set threshold and is of a predetermined time duration. The predetermined time duration may be set below the time actually required to perform a nut tightening sequence but

should not be set so low that it is not distinguishable from a false, start. The set threshold for the flow rate may also be set at or below the typical rate of air usage for the tool in question but should be set sufficiently above the shut down rate so that it is readily distinguishable from the latter.

The controller may be provided in any suitable manner and by any suitable means. Preferably the controller includes a programmable device such as a PLC. Air flow may be measured in any suitable manner and by any suitable means. An air flow sensor such as a mass air flow (MAF) sensor used for calculating air/fuel ratio in internal combustion engines has been found suitable for this purpose. The MAF sensor makes use of the hot-film anemometry principle. A controlled current flowing through a thin film element causes it to heat up until it gains equilibrium. When air flow occurs across the element convective heat loss causes the heating current to increase as the element attempts to regain equilibrium. The current increase is converted to an analogue output via a signal conditioning circuit. The air flow sensor may be connected to the controller via a suitable interface such as an analogue card.

The controller may also receive a signal from the pressure switch which changes state when the pneumatic tool reaches a preset torque.

According to one aspect of the present invention there is provided a controller for a pneumatic tool including: means for measuring rate of air flow to said tool ; means for comparing said air flow to a first threshold; means for timing said air flow ; means for comparing to a second threshold a duration of time that said airflow remains above said first threshold; means for determining that said tool has reached a preset torque; and means for producing a warning when said duration of time is below said second threshold and said determining means indicates that said tool has reached said preset torque.

According to a further aspect of the present invention there is provided a method of operating a pneumatic tool including the steps of: measuring rate of air flow to said tool ; comparing said air flow to a first threshold; timing said air flow ;

comparing to a second threshold a duration of time that said airflow remains above said first threshold; determining when said tool has reached a preset torque; and producing a warning when said duration of time is below said second threshold and said tool has reached said preset torque.

A preferred embodiment of the present invention will now be described with reference to the accompanying drawings wherein: Fig. 1 shows one embodiment of pneumatic tool controller according to the present invention; Fig. 2 shows details of terminal connections to the PLC; Fig. 3 shows the output of the MAF sensor and pressure switch relative to a threshold; Fig. 4 shows a close up of the sensing element of the MAF sensor; Fig. 5 shows a program flow chart associated with the controller ; and Fig. 6 shows a ladder diagram associated with the program.

Fig. 1 shows a pneumatic tool controller including PLC 10, air flow sensor 11 and pressure switch 12. A power supply comprising modules 13,14 provides DC power to the apparatus. Modules 13,14 provide 12V DC each and their outputs are connected in series to supply 24 volts DC to PLC 10. Module 14 also supplies 12 volt DC to air flow sensor 11. Air flow sensor 11 comprises a MAF sensor manufactured by Siemens Ltd. The MAF sensor is as used in association with fuel injection systems of Holden Vectra engines to measure intake air flow for calculating air/fuel ratio. The MAF sensor is fitted to an enclosure that is capable of handling the pressure of the main air line (7 bar) to the pneumatic tool. Testing of the MAF sensor on the bench showed a clear air flow signal (0 to 6 volts) with a reduction of flow at tool shut down (refer Fig. 3).

When the air flow signal is over a set threshold for a predetermined time PLC 10 looks at the output of pressure switch 12. The threshold is set at or below a typical rate of air usage for tool in question. As indicated above, the threshold should be set sufficiently above the shut down air flow rate to distinguish it from the latter. The predetermined time may be set at or below the period of time taken to run down a nut but should not be set so low that it is not distinguishable from a false start.

Referring to Fig. 3 the air flow signal shows two runs of the pneumatic tool marked A, B which are well above the flow threshold T1. Each run is for a time duration in excess of 500 milli seconds indicating that the nut was not previously run down. The third run of the tool marked C is of much shorter duration than runs A and B indicating a false start. Little air is used because the nut was previously run down. Each run of the tool is accompanied by an output pulse (shown in broken outline) from pressure switch 12 indicating that the tool has reached the preset torque. It is apparent from Figure 3 that the error proofing counting function would be misled in the case of run C if it relied only on the signal from the pressure switch. However, reference to the air flow signal differentiates run C as a false start over runs A and B which are true runs. The counting function is therefore only incremented when an output from pressure switch 12 is accompanied by a signal from air flow sensor 11 that is above the flow threshold T1 for a predetermined period of time T2 eg. 500 milli seconds.

PLC 10 may be a PICOTM controller manufactured by Allen Bradley. The PICO PLC is an electronic control relay with built in logic, timer, counter and real-time clock functions. The PICOT PLC is a combined control and input device that can be programmed using ladder diagrams (refer Fig. 6). PICOTM Model No. 1760-L12 BWB can also receive analog signals at two inputs and evaluate the signals with analog comparators. This avoids the need for a separate analogue interface. The PICOT"^ PLC can also be programmed from a PC using PICO soft programming software.

Referring to Figs. 1 and 5, PLC 10 is programmed to compare the analog output signal from air flow sensor 11 with a preset flow threshold (T1) representing a normal air usage rate for the tool in question (step 50).

Assuming that the output signal is greater than the flow threshold a timer is started (step 51). The timer output is compared to a time threshold (T2) representing a duration of time that the tool requires to perform a nut tightening sequence (step 52). Assuming that the timer output is greater than the time threshold (T2) and an output is received from pressure switch 12 (step 53), PLC 10 switches on lamp 15 indicating an"OK Cycle" (step 54). PLC 10 then checks to see that the analog output signal from air flow sensor 11 has fallen below the preset flow threshold (T1) (step 55) before resetting itself.

Assuming that the timer output is not greater than the time threshold (T2) and an output is received from pressure switch 12 (step 57), PLC 10 switches on an alarm including fail lamp 16 and buzzer 17 (step 58). This warns the operator that he has performed a multiple tightening sequence on a nut which is then corrected by locating the nut that was not run down. Buzzer 17 will continue to sound until an output from pressure switch 12 is no longer received at which time buzzer 17 is switched off (step 59) and the system returns to step 52. Buzzer 17 will sound each time that an incorrect nut is selected for running down until the operator locates the correct nut.

Finally, it is to be understood that various alterations, modifications and/or additions may be introduced into the constructions and arrangements of parts previously described without departing from the spirit or ambit of the invention.