GB1571994A - Electric nutrunner - Google Patents

Description

(54) ELECTRIC NUTRUNNER (71) We. ATLAS COPCO AKTIEBOLAG, of Nacka, Sweden, a Swedish Company, do hereby declare the invention for which we pray that a Patent may be granted to us and the apparatus by which it is to be performed to be particularly described in and by the following statement: This invention refers to an electric nutrunner for tightening bolt joints to a desired tightening torque.

In tightening a bolt joint two different phases may be observed. The first phase comprises the initial threading sequence, which can be said to be tightening without increasing the torque. In the second phase, the tightening sequence proper, the components of the joint are clamped together, and the torque will then have to increase continuously so as to make the tightening continue. These two phases are illustrated in Figure 1, wherein the torque M is shown as a function of the angle of rotation 0, with portion I referring to the intial threading sequence whereas portion II refers to the tightening sequence.

In mechanical tightening. i.e. tightening with a motor-powered nutrunner, the initial threading is carried out at a high angular velocity. During the tightening proper, the angular velocity of the driving shaft will decrease progressively down to zero. This is illustrated in Figure 2. wherein the angular velocity d0/dt is shown as a function of time t, with the portion I representing the initial threading sequence whereas the portion II represents the tightening sequence.

Thus, during the tightening, the rotating parts of the nutrunner and the joint are braked with the torque generated by the motor being supplemented by a deceleration torque d2Oidt2, the magnitude of which is dependent on the inertia factor J of the rotating components and the rate of the deceleration - d20/dt2. Therefore, the torque delivered to the joint can generally be described by the following expression: MF = MM - J d20/dt2 (1). wherein MF = the torque delivered to the joint, MM = the torque generated by the motor at the driving shaft, J = the total of the rotational moment of inertia of all of the rotating components reduced to the driving shaft of the machine, o = the angle of rotation of the driving shaft of the machine, and t = time.

In an ordinary pneumatic nutrunner the torque delivered by the motor is determined by the pressure of the supply air. In Figure 3 the torque M is shown as a function of time t for a hard joint, with the relationship being indicated by characteristic I, and a soft joint, with which the relationship is illustrated by characteristic II. Thus the torque MF = MM - J d20/dt2 delivered to the joint will be dependent on the hardness of the joint as this hardness influences the dvnamic additive torque - J d20/dt2, which is dependent on the rate of the braking. Said additive torque is substantial in hard joints but practically negligible in soft joints. The fact that the total torque MF is dependent on the hardness of the joint comprises a drawback whose elimination is being attempted in various ways.

One method of avoiding the above-mentioned drawback is to disconnect the motor drive at a certain level of the delivered torque of the motor and if possible to select this level such that it is adjusted to the specific joint hardness in each individual case. To accomplish this, a nutrunner is provided with some device for measuring the delivered torque directly or indirectly. and at a predetermined level Mn turning off or disengagement of the motor is initiated. The tightening sequence of a hard joint will then have the appearance of Figure 4, the upper portion of which illustrates the torque M as a function of time t, whereas its lower portion shows the angular velocity dO/dt as a function of time.If tn designates the point at which disengagement or stopping of the motor is initiated, a torque of Mn - J (d20/dt2)n will have been delivered to the joint at that point. However, the final torque M5 delivered to the joint will be greater. At the point tnn the motion of the motor will not have ceased, and the driving shaft will have an angular velocity of (d0/dt)n. Consequently, the rotating system will have a stored kinetic energy equal to Wn =1/2 J (dl3/dt)2n Depending on the design of the system, this energy will in its entirety or in part be supplied to the joint as an additive torque Mx.

Consequently, the resulting final torque M5 will be M5 = Mn - J (d20/dt2)n + Mx According to the invention there is provided an electric nutrunner for tightening bolt joints to a predetermined desired tightening torque, comprising a control device and a motor, the control device including a current growth sensing circuit arranged to provide compensation for a variation in the terminating torque as a result of varying joint hardness, so as to provide the predetermined desired torque, by controlling power supplied to the motor according to the sensed current through the motor, the control device being arranged to produce a pulse for electrically braking the motor when tightening is to be terminated.

The motor may be braked by means of a controllable torque provided by passing a reverse current through one or more motor windings when the pulse occurs.

Precise control of the tightening sequence may be achieved by means of equipment consisting of a nutrunner and an associated control system up to a final torque of M5 which is not affected by the hardness of the joint. This equipment may be such that - Mn can be measured with great exactness, - the term - J (d2(3/dt2)n is minimized not only by a low value of J but also by compensating for the varying joint hardness by the control equipment - Mx is minimized not only by the low value of J but also by a portion of the kinetic energy 1/2 J (dO/dt)2n being braked away.

The above-mentioned precise control may be achieved by utilizing an electric nutrunner having a low moment of inertia. a drive circuit for the motor of the nutrunner designed in such manner that the motor is braked by short-circuiting, wherein the tightening is interrupted, and a specific circuit in which the growth of the torque can be sensed in order that the additive torque - J (d20/dt2)n may be compensated. As an example of a particularly appropriate motor a so-called permanently magnetized direct-current motor may be mentioned (PMLs motor), which has the characteristic that its delivered torque is directly proportional to the drive current, i.e. Mmotor = k. i, wherein k is a motor constant and i is the motor current.Thus it is simple to measure Motor by measuring the current, which for example may be expressed as the voltage drop over a resistor in series with the motor. By sensing the growth of the current di/dt during the tightening, for instance by measuring the voltage drop over an inductor in series with the motor, it becomes possible to compensate for the term - J d-0/dt2. This will be described more specifically below.

As a result of new magnetic materials being rapidly developed, for example of the type of rare earth metal/cobalt, it has become possible to design motors having substantially smaller dimensions than with magnetic materials of the types presently in use.

By selecting a PMLs motor of the so-called moving coil type, i.e. a motor in which solely the winding and not the iron core rotates, a system having an extremely low moment of inertia is achieved, i.e. the term Mx becomes small.

The invention will be further described, by way of example, with reference to the accompanying drawings, in which: Figure 1 illustrates the torques of the two phases that may be observed in tightening a bolt joint: Figure 2 shows the angular velocity as a function of time for initial threading and tightening, respectively, by means of a motorpowered nutrunner; Figure 3 illustrates the torque M delivered to the joint as a function of time t for a hard and soft joint, respectively; Figure 4 illustrates the tightening sequence of a hard joint when the motor is shut off or disengaged; Figure 5a shows a perspective view of a motor of a type that may be utilized as a drive source for nutrunners in accordance with the invention, with the rotor being removed from the motor: Figure 5b shows a perspective view of the rotor for the motor illustrated in Figure Sa:: Figure 6 shows a basic circuit diagram of the motor and of the starting and control circuits connected thereto; Figure 7 shows a more detailed embodiment of the circuit of Figure 6: Figure 8 shows a modification of the circuit of Figure 7; Figure 9 shows a circuit diagram of a circuit that is utilized in tightening I nut and a screw.

respectively, to a predetermined torque in accordance with the above as well is a circuit intended for compensation of the joint hardness respectively; Figure IO shows a portion of the circuit of Figure 7 in more detail; Figtn' 11 shows an example of the potentiometer adjustment as a function of the torque; and Figure 12 schematically shows the nutrunner in its complete state.

The motor preferably utilized is a motor of so-called moving coil type. of which Figures 5a and Sf) show an example. Figure Sa the designation la refers to an external housing, 11) illustrates permanent magnets disposed in said housing. and Ic shows a stationary iron core in the housing. said core being provided with an opening ld through which the shaft of the rotor illustrated in Figure Sb subsequently is to be passed. with the rotor winding being introduced into the annular opening le of Figure Sa. A motor of the relevant type is characterised in that it does not have iron in the rotor.The iron core is stationary, and the only rotating parts of the moving coil motor are the rotor winding, the commutator, the shaft and the bearings. As a result of the described design, the motor has an extremely low moment of inertia, which is utilized in the tightening sequence, and a very low inductance, therehv attaining a small electric time constant. A permanently magnetized direct-current motor of the type indicated has a linear relationship between torque and current which provides very simple control possibilities.However, it is not necessary to have this linear relationship. hut is sufficient to have a well-defined relationship between torquc and current. which means that the present invention is not restricted to motors of the above-mentioned type.

Figure 6 shows an example of a basic circuit diagram of the motor and of the circuits influencing the latter. The motor 3 is connected to a constant. induction-free source of direct voltage 4 l ia an external resistor 5. The input voltage is to be chosen such that the motor has the required number of revolutions per minute in frec-mnning. The external resistor 5 and the rotor resistance result in the current being restricted during starting and in electrical braking.

The motor and thus the nutrunner are started by the actuation of a starting device 6, whereby current flows from the source 4 of direct voltage through fuse 7. starting device 6, resistor 5. motor 3. and a control device 8. When the control device X provides the information that the tightening sequence is to be terminated, a thyristor 9 (or a corresponding controlled switch) is fired, whereby on one hand the driving current is conducted past the motor 3 through the lead 10 and on the other hand the motor is short-circuited and consequently braked strongly. In consequence of the low moment of inertia of the motor, the amount of energy stored in the rotating portions of the nutrunner when the motor is shut off is small.As the result of the electric braking only a certain portion of this small stored energy goes out to the joint. whereas the remaining part is dissipated in the resistor 5 in the form of heat.

Figure 7 shows a more detailed circuit diagram corresponding to the diagram of Figure 6 for a particular case. The motor 3 is connected to the constant, induction-free direct-voltage source 4 lia external resistors 11 and 13, which for example may have resistances of 1.1 ohms. and a circuit 12 to be described hereinafter. As an example. it may be assumed that the input voltage of the selected motor is approximately 74 volts in order for the motor to have the required number of revolutions per minute in free-running. The resistances of the mentioned external resistors 11 and 13, the circuit 12, and the motor resistance, which in the relevant case may be for example 1.2 ohms. limit the current to 35-40 amperes during starting or electrical braking.

In starting, a thyristor 14 is triggered by means of a pushbutton 15'. whereby a current from the direct-voltage source 4 flows to fuse 15, motor 3, resistors 11 and 13, circuit 12, and thyristor 14. If the current through the motor increases to approximately 36 amperes the motor will stop as the result of the voltage drop across the resistor 13 becoming so great that a thyristor I 16 is triggered i/a four diodes 17 connected in series, whereby a capacitor 20 is discharged through the thyristor 14. which is thus turned off. During this discharge in the backward direction. thyristor 16 also is turned off.

When the tightening sequence is completed the motor is to stop. A thyristor 18 is activated as described hereinafter. whereby the motor and resistor 11 are short-circuited and the motor is braked strongly. Simultaneously. driving current by-passes the motor, through the stop thyristor 1X. and through circuit 12. The current through resistor 13 will then be approximately 36 amperes. thereby causing the protection circuit consisting of the main thyristor 14 and the thyristor 16 to be actuated.

The stopping thyristor 18 may be triggered in two different manners, namely (1) by the stopping button 19 being depressed or (2) as the result of a triggering pulse being provided from a control device 8.

Figure 8 shows a circuit diagram in which the motor is decelerated by controlling the braking current i.e. the current resulting from the motor back e.m.f; so that a controllable torque is provided when the control device 8 gives a braking command. When a pulse from control device 8 occurs. a trigger voltage is developed across a resistor 41 resulting in the potential on the base of a transistor 42 being increased. Current flows through the resistor 41 into transistor 42. A greater current will then flow into a transistor 44 through the amplifying circuit consisting of a resistor 40 and transistor 42. The motor 3 thus behaves as a generator and drives a current through transistor 44 and resistor 45. A zener diode 43 is connected to resistor 45 to the base of transistor 42.When the voltage drop across the resistor 45 is equal to the voltage drop across the zener diode 43 the maximum value of the braking current in the motor will be attained. The base voltage of transistors Q and 44 is relativelv small. so that the braking current is substantially equal to the zener voltage divided by the resistance of resistor 45. Thus, as long as the counter-emf of motor 3 exceeds the voltage drop of transistor 44 and resistor 45 this maximum current will brake the motor with a constant torque.

The electric braking may also be achieved by providing the motor with auxiliary windings which are particularly intended for the braking function and whose connection to a voltage source is controlled by control device 8.

Figure 9 shows the circuit diagram of the control device of Figure 7. As a well-defined relationship exists between torque and current in the motor, the current through the motor is sensed as the voltage across the shunt resistor 12 of Figure 7. This voltage is conveyed over terminals C and D to the comparator circuit illustrated in Figure 9. in which the voltage drop across circuit 12 is compared to a reference voltage of a potentiometer 21 in a comparator 22 comprising an operational amplifier of a type known pel se. said amplifier being provided with positive feedback z a resistor 29.When equivalence is reached. a capacitor 23 connected to the collector of a transistor 24 is discharged through lead B to the control grid of stopping thyristor IX. which causes the motor to be short-circuited. The resistors 3()-36 and the zener diodes 37 and 38 are components for providing the necessary voltage drops and voltage limitations. The setting S of the potentiometer 21 as a function of the delivered torque M of the motor is illustrated in Figure 11.

As was mentioned by way of introduction, a feature of the control device is the ability to compensate for variations in joint hardness. In the embodiments described, this is achieved by the circuit 12. which comprises an inductance-free resistor 12' having a resistance R5 of, for example. 1.1 ohms in series with an inductor Ls as illustrated in Figure 1(). Hereby a voltage drop across the resistor 12' and the inductor Ls is attained which is Rs . i + Ls di/dt, i.e. consideration is also taken of the time dependency of the current. and hence of the torque. For instance. let it be assumed that the reference voltage of the potentiometer 21 is preset to Up volts.This implies that for a joint with very low hardness, i.e. a soft joint, the current increases very slowly and the term di/dt is negligible. This, in turn, means that the motor is short circuited and comes to a stop when the voltage drop across the resistor 12' is equal to Rs.ip = Up for the current being equal to Up/Rs. Let us now assume that a hard joint is tightened with the reference voltage being the same. Up. In this case the term L di/dt is not negligible.The braking of the motor will now be initiated when the voltage across the resistor 12' and the inductor L, is equal to the same value Up. that is Up = Ras . i + Ls di/dt However Up will be reached for a current i having a lower value then iJr for the soft joint as the term LK di/dt contributes to the voltage drop. Thus, the harder the joint the lower the current for which the motor braking is initiated. However the same predetermined torque will have been reached for both the soft and the hard joints because for the hard joint the dynamic torque ( J d20/dt2) described above is added. This fact can also be described by the equations governing the tightening.

As previously shown, the total torque delivered to the joint at the point of disengagment or initiated stopping of the motor is M,, -J (d2 0/dot2 ),. From the two differential equations governing the tightening. namely: J .d2H/dt2 = k.i U = R.i + k. dH/dt where J = moment of inertia of the rotating parts, k = the torque/current constant of the motor (Nm/A), kh = joint stiffness (Nm/rad), U = supply voltage, R = total circuit resistance, and o = angle of rotation of the nut.

the total torque Mn - J.d20/dt2 can be rewritten as k.i. + R.J./k di/dt.

The voltage drop across the resistor 12' and the inductor L5 is R5. i + L5. di/dt There thus exists a correspondence between the total torque and the voltage drop so that, if the inductor is given a value L5= RsR.J./k2, the voltage drop across Rs and L5 is an analogue representation of the total torque delivered to the joint at the point of disengagement or initiated stopping of the motor regardless of the joint stiffness.

Figure 12, finally. shows the nutrunner schematically, wherein the designation 25 refers to a tachometer connected to the motor 3, the designation 26 refers to a gear, the designation 27 refers to the driving shaft, and 28 is a nut socket for tightening. Thus, in this case the nutrunner has been provided with a mechanical gear so that an appropriate torque is achieved.

The invention is not restricted to the embodiments described above and illustrated in the drawings, and these embodiments merely comprise examples of the invention and its application.

WHAT WE CLAIM IS: 1. An electric nutrunner for tightening bolt joints to a predetermined desired tightening torque. comprising a control device and a motor, the control device including a current growth sensing circuit arranged to provide compensation for a variation in the terminating torque as a result of varying joint hardness, so as to provide the predetermined desired tightening torque by controlling power supplied to the motor according to the sensed current through the motor, the control device being arranged to produce a pulse for electrically braking the motor when tightening is to be terminated.

2. An electric nutrunner as claimed in claim 1, in which the motor is braked by short-circuiting as the result of the actuation of a controlled switch when said pulse occurs.

3. An electric nutrunner as claimed in claim 1, in which the motor is braked by means of a controllable torque provided by passing a reverse current through one or more motor windings when the pulse occurs.

4. An electric nutrunner as claimed in any one of the preceding claims, in which said current growth sensing circuit comprises an inductance-free resistor connected in series with the motor and an inductor connected in series with the resistor and in which the voltage across the resistor and inductor is supplied to a comparator, wherein it is compared to a preset reference voltage. with an output signal being delivered from the comparator upon the occurrence of equivalence so as to interrupt the tightening sequence of the nutrunner by braking the motor 5. An electric nutrunner as claimed in any one of claims 1-4, in which the motor comprises a permanently magnetized direct-current motor having an iron-free rotor.

6. An electric nutrunner substantially as hereinbefore described with reference to and as shown in the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (6)

**WARNING** start of CLMS field may overlap end of DESC **. where J = moment of inertia of the rotating parts, k = the torque/current constant of the motor (Nm/A), kh = joint stiffness (Nm/rad), U = supply voltage, R = total circuit resistance, and o = angle of rotation of the nut. the total torque Mn - J.d20/dt2 can be rewritten as k.i. + R.J./k di/dt. The voltage drop across the resistor 12' and the inductor L5 is R5. i + L5. di/dt There thus exists a correspondence between the total torque and the voltage drop so that, if the inductor is given a value L5= RsR.J./k2, the voltage drop across Rs and L5 is an analogue representation of the total torque delivered to the joint at the point of disengagement or initiated stopping of the motor regardless of the joint stiffness. Figure 12, finally. shows the nutrunner schematically, wherein the designation 25 refers to a tachometer connected to the motor 3, the designation 26 refers to a gear, the designation 27 refers to the driving shaft, and 28 is a nut socket for tightening. Thus, in this case the nutrunner has been provided with a mechanical gear so that an appropriate torque is achieved. The invention is not restricted to the embodiments described above and illustrated in the drawings, and these embodiments merely comprise examples of the invention and its application. WHAT WE CLAIM IS:

1. An electric nutrunner for tightening bolt joints to a predetermined desired tightening torque. comprising a control device and a motor, the control device including a current growth sensing circuit arranged to provide compensation for a variation in the terminating torque as a result of varying joint hardness, so as to provide the predetermined desired tightening torque by controlling power supplied to the motor according to the sensed current through the motor, the control device being arranged to produce a pulse for electrically braking the motor when tightening is to be terminated.

2. An electric nutrunner as claimed in claim 1, in which the motor is braked by short-circuiting as the result of the actuation of a controlled switch when said pulse occurs.

3. An electric nutrunner as claimed in claim 1, in which the motor is braked by means of a controllable torque provided by passing a reverse current through one or more motor windings when the pulse occurs.

4. An electric nutrunner as claimed in any one of the preceding claims, in which said current growth sensing circuit comprises an inductance-free resistor connected in series with the motor and an inductor connected in series with the resistor and in which the voltage across the resistor and inductor is supplied to a comparator, wherein it is compared to a preset reference voltage. with an output signal being delivered from the comparator upon the occurrence of equivalence so as to interrupt the tightening sequence of the nutrunner by braking the motor

5. An electric nutrunner as claimed in any one of claims 1-4, in which the motor comprises a permanently magnetized direct-current motor having an iron-free rotor.

6. An electric nutrunner substantially as hereinbefore described with reference to and as shown in the accompanying drawings.