Automated screw driving device

A fully automated hand held screw driving device comprises an automatic feeding mechanism of screws to the screwdriver from an integral storage magazine, an automatic speed control mechanism for controlling the rotary speed of the screwdriver, an automatic force control mechanism for controlling the seating force of the screwdriver bit on the screw, an adjustable depth control mechanism for controlling the final screw depth in the work surface, and an adjustable seating torque control mechanism for controlling the final screw head seating torque. The screws are spirally wound on a replaceable bobbin removably mounted in the magazine. The device can accommodate a full range of practical screw sizes and can be fitted with exchangeable bits for use with screws having standard recessed star or square heads and various bolt heads. A central microprocessor is used to control all operating functions of the screw driving device.

RELATED APPLICATIONS 
This application claims priority on U.S. Provisional Application No. 
60/025,726 filed on Sep. 11, 1996 (now abandoned). 
BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to hand held electric screwdrivers and, more 
particularly, to a screw driving device for driving screws, of varying 
head configurations and sizes, into work surfaces in a fully automatic and 
controlled manner. 
2. Description of the Prior Art 
The process of fastening sheet construction materials is generally achieved 
using hand held electrical screw guns which are manually fed with screws, 
one by one. Current proposals to improve this procedure through partial 
automation generally utilize a manually operated ratchetting mechanism to 
feed screws, attached in series to a belt, into position in front of a 
screwdriver bit. The screwdriver is then actuated and manually pushed 
forward to engage the screw and drive it into the work surface. U.S. Pat. 
No. 5,109,738 issued on May 5, 1992 to Marian et al. and U.S. Pat. No. 
5,167,174 issued on Dec. 1, 1992 to Fujiyama et al. both describe such 
belt fed screw driving machines. U.S. Pat. No. 5,154,242 issued on Oct. 
13, 1992 to Soshin et al. describes a manually fed screwdriver with a 
multi-stage tightening torque control. This screwdriver allows for a high 
speed screw driving phase and a slow speed final tightening phase; it also 
controls torque by monitoring motor temperature to correct for variations 
in the magnetic characteristic of the motor due to temperature variations. 
All control functions in the machine are microprocessor based. 
SUMMARY OF THE INVENTION 
It is therefore an aim of the present invention to provide an improved 
screw driving device. 
It is also an aim of the present invention to provide a fully automated 
electric hand held screw driving device. 
It is a further aim of the present invention to provide a screw driving 
device comprising a mechanism for feeding a screw from a storage magazine 
to a location opposite the screwdriver bit, a mechanism which causes the 
screwdriver bit to engage the head of the screw which is then rotatably 
driven by a motor into the working surface. 
It is a still further aim of the present invention to provide a screw 
driving device further comprising an automatic speed control mechanism for 
controlling the rotary speed of the screwdriver, an automatic force 
control mechanism for controlling the seating force of the screwdriver bit 
on the screw, an adjustable depth control mechanism for controlling the 
final screw depth in the work surface, and an adjustable seating torque 
control mechanism for controlling the final screw head seating torque. 
It is a still further aim of the present invention to provide a screw 
driving device adapted to engage a full range of practical screw sizes and 
to be fitted with exchangeable bits for use with screws having standard 
recessed star or square heads and various bolt heads. 
It is a still further aim of the present invention to provide a screw 
driving device comprising a central microprocessor to control all 
operating functions of the screw driving device. 
Therefore, in accordance with the present invention, there is provided a 
screw driving device for driving screws into work pieces, comprising 
housing means, magazine means adapted to carry a plurality of screws, a 
screwdriver bit in said housing means, first motorized displacement means 
for positioning one of the screws opposite said screwdriver bit in an 
operational position of the screw, second motorized displacement means for 
rotatably driving said screwdriver bit, third motorized displacement means 
for translationally displacing said screwdriver bit between a screw 
driving position and at least one retracted position and coaxially to the 
screw in said operational position, drill switch means adapted when 
actuated to cause, in synchronization, said first displacement means to 
bring a screw to said operational position, said third displacement means 
to displace said screwdriver bit into engagement with the screw, and said 
second displacement means to rotate said screwdriver bit and thus the 
screw while said third displacement means progressively advances the 
rotating screw such that it engages a work piece. 
Also in accordance with the present invention, there is provided a method 
for driving screws into work pieces using a screw driving device having a 
housing containing a translationally and rotatably displaceable 
screwdriver bit and a plurality of screws, comprising the step of: 
(a) with said screwdriver bit being sufficiently retracted, feeding one of 
the screws to a location opposite said screwdriver bit such that it 
extends substantially coaxially therewith; 
(b) displacing translationally said screwdriver bit towards the screw and 
into engagement therewith; and 
(c) rotating said screwdriver bit and the screw while translationally 
advancing said screwdriver bit towards the work piece such that the screw 
engages the work piece; 
wherein above steps (a), (b) and (c) take place automatically and in a 
synchronized manner upon actuation of a switch means. 
Further in accordance with the present invention, there is provided a 
replaceable bobbin for use in a screw driving device, comprising a 
plurality of screws detachably mounted on carrier tape means and adapted 
to be driven by the screw driving device and to be detached thereby from 
said carrier tape means, said bobbin being removably installable in the 
screw driving device and including spindle means and at least one flange 
means, said screws extending substantially perpendicularly to said carrier 
tape means and in a substantially parallel and successive manner 
therealong with said carrier tape means being spirally wound around said 
spindle means. 
Still further in accordance with the present invention, there is provided a 
screw driving device for driving screws into work pieces, comprising 
housing means adapted to carry a plurality of screws detachably mounted on 
a screw carrier tape means, said screw carrier tape means defining index 
notch means, a screwdriver bit in said housing means which is rotatable 
for driving the screws one-by-one into work pieces, displacement means for 
positioning one of the screws opposite said screwdriver bit in an 
operational position of the screw such that said screwdriver bit can then 
be engaged to the screw, said displacement means comprising motorized 
rotatable roller means adapted to drive said screw carrier tape means and 
switch means for selectively operating or stopping said roller means, 
whereby said roller means displace said carrier tape means to bring the 
screw in said operational position and are stopped by signal means 
resulting from said switch means being actuated by said notch means.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In accordance with the present invention, FIG. 1 illustrates a hand held 
fully automatic screw driving device D which should be particularly useful 
in the construction industry where sheet material, such as plywood, 
plasterboard and sheet metal are routinely fastened to large surfaces. 
Obviously, the screw driving device D can be used in a number of other 
applications where a screwdriver is required. 
Before describing the present screw driving device D in details and with 
reference to the accompanying drawings, a general description of the 
present invention now follows. 
The present screw driving device D provides a definite and substantial 
improvement over the prior art which consists of manual and electric 
screwdrivers and, at the upper end of the scale, of semi-automatic screw 
feed and automatic multi-stage screw driving control. Indeed, the screw 
driving device D constitutes a fully automatic apparatus which 
incorporates an improved screw feed mechanism, an improved multi-stage 
screw driving control and an automated mechanism for the forward motion 
and for the retraction of the rotatable and translationally displaceable 
screwdriver unit. 
The automatic screw driving device operates in the following manner. A 
quantity of screws is attached to a specially designed plastic carrier 
tape which is spirally wound onto an expendable bobbin and housed in a 
hollow circular magazine integrally mounted at the front of the 
screwdriver. The screw carrier tape is clamped between a pair of tape 
drive rollers or rotating cylinders which are used to advance the tape and 
thus the screws to a position located opposite, i.e. in front, of the 
screwdriver unit of the screw driving device. Precise positioning of the 
screws in front of the screwdriver unit is achieved by a limit switch, 
mounted under the screw carrier tape, this limit switch sensing the index 
notches defined in the carrier tape to determine the position of the 
screws. The tape drive cylinders are operated by a reduction gear train, 
which is coupled to the main drive motor via an electric clutch. 
When a screw is in position in front of the screwdriver unit, the magnetic 
clutch decouples the tape drive gear train from the main drive motor. When 
the screw driving device is held firmly against a flat surface, a safety 
switch operator mounted at the front face of the machine is depressed 
thereby allowing the screw driving action to begin when a trigger switch, 
which is mounted in the hand grip of the device, is also depressed. The 
main motor starts to rotate the screwdriver unit including the bit 
provided at the front end thereof. Simultaneously, a secondary motor moves 
the screwdriver bit translationally forward. As the screwdriver bit moves 
forward, it forces the screw out of the carrier tape and into a screw 
guide tube, the screw guide tube serving to hold the carrier tape in place 
as the screw is forced out of the tape. The screw is held firmly on the 
screw bit by a magnetic sleeve mounted just behind the tip of the 
screwdriver bit. 
As the screw advances and engages the work piece, the screwdriver bit is 
held firmly against the screw at constant force by torque control of the 
secondary motor via a central controller unit. A control switch setting 
allows the choice of either depth of penetration or maximum seating torque 
control to terminate the screw driving sequence. To achieve this, a 
precision linear potentiometer is mounted on the screwdriver unit's shaft 
to provide a continuous indication of screw head location to the central 
processor. If depth of penetration control is selected, which is suitable 
for materials such as wood or plasterboard, then the screw is driven to 
the selected depth, provided the maximum safe torque limit is not 
exceeded. If torque control is selected, suitable where hexagonal bolt 
headed screws with or without washers are used in hard materials, then as 
the screw head approaches the work surface, as sensed by the linear 
potentiometer, the screwdriver rotation is slowed to a crawl. The main 
screwdriver motor is switched from rotary speed control to torque control 
until the screw head is engaged to the work surface at a preselected 
torque level. 
When either of the above operating modes is completed, the main motor stops 
its screw driving action and the secondary motor translationally withdraws 
the screwdriver bit to the start position. The screw carrier tape is then 
advanced until the next screw is in drive position. The screw driving 
device must then be removed from the work surface, or the hand trigger 
control switch must be released before the screw driving cycle can be 
repeated. 
Screwdriver forward force and seating torque are controlled indirectly by 
using the basic DC motor equation: 
EQU T=kI.sub..function. I.sub.a 
where: T represents torque; 
k is the motor proportionality coefficient; 
I.sub..function. is the motor field current; and 
I.sub.a is the motor armature current. 
The motor proportionality coefficients for both primary and secondary 
motors are recalculated at each operating cycle of the screw driving 
device so as to compensate for temperature effects on the magnetic 
circuits of the armature and field. A lookup data table is then utilized 
to determine the exact value of I.sub.71 I.sub.a required to achieve the 
selected torque accurately. All control functions in the system are 
implemented using feedback techniques. 
Referring now specifically to FIG. 1, the screw driving device D includes a 
casing or shell 1 comprising therein a rack 2 in meshed engagement with a 
pinion 3, first and second DC motors M1 and M2, respectively, a 
potentiometer 5 including a sliding actuator rod 4, a coupling chuck 6 and 
a screwdriver bit 7 detachably engaged therein. The first DC motor M1 is 
adapted to impart rotary motion to the screwdriver bit 7 via a reduction 
gear train 10 and a drive sleeve 11. The drive sleeve 11 is mounted in 
bearings 9. The front end of the screwdriver bit 7 is provided with an 
integral tip 13 and forward seating force for the screwdriver tip 13 into 
the head of a screw 15 is provided by the second DC motor M2 via a 
reduction gear train 24 which rotatably drives the pinion 3 which itself 
translationally displaces the rack 2. The rack 2 is mounted in bearings 
23. The rotary motion of the screwdriver bit 7 is decoupled from the 
forward seating force drive mechanism or rack 2 by a thrust bearing 22. 
The screwdriver bit 7 and the forward drive mechanism 2 are joined at the 
coupling chuck 6. Obviously, the screwdriver bit 7 is detachable from the 
chuck 6 such that it can be selectively replaced with any of a series of 
similar screwdriver bits which have different tips adapted for engagement 
with various configurations of screw heads, e.g. recessed star (i.e. 
"philips") or square (i.e. "robertson") heads or bolt heads, for instance 
of the hexagonal type. For instance, a removable door can be provided on 
the side wall of the casing 1 closest to the screwdriver bit 7 (see left 
casing wall on FIG. 2A or lower casing wall on FIG. 3) such as to allow 
access to the screwdriver bit 7, generally between the chuck 6 and the 
proximal end of the drive sleeve 11. With reference to FIG. 1, the bit 7 
can thus be grasped and moved to the right, thereby disengaging it from 
the chuck 6 such that it can be then slid through the drive sleeve 11 and 
the front end of the device D (with the magazine 26 being open and the 
screw 15 being displaced slightly to allow the bit 7 to be pulled out of 
the device D). An electric cord 31 provides power to the motors M1 and M2. 
The first DC motor M1 is also coupled to the tape drive mechanism 14 via an 
electromagnetic clutch 20 and a reduction gear train 21. The screws 15 are 
mounted into a plastic carrier tape 33 which is spirally wound on an 
expendable bobbin 48 (see FIGS. 2B and 2C) removably fitted into the 
storage magazine 26 and comprising a tubular spindle 51 and a circular 
flange 52 provided at one end of the spindle 51 and extending at right 
angles to a rotation axis thereof. (as best seen in FIGS. 1, 2B and 2C). 
The carrier tape 33 is wound spirally around the spindle 51 in such a way 
that the wound carrier tape 33 extends in a single plane which is 
perpendicular to the spindle 51 (see FIGS. 1, 2B and 2C); in fact, the 
carrier tape 33 and the heads of the screws 15 are located adjacent to or 
against the flange 52 (FIG. 2C) with the screws 15 extending substantially 
parallelly to the spindle 51. The spirally wound carrier tape 33 and its 
support bobbin 48 are mounted in the screw magazine 26 by means of a 
centering pin 25 engaged in spindle 51, the bobbin 48 being free to rotate 
around the pin 25. Access to the front part of the shell 1 of the screw 
driving device D is provided by a door 49 which opens outwardly by means 
of a hinge 32 thereby allowing for the insertion of the bobbin 48 and its 
screw spiral tape 33 into the screw magazine 26 (see FIG. 2C) and removal 
of the bobbin 48 for replacement thereof because it is empty or to change 
the screw type. 
The tape drive mechanism consists of a pair of cylinders 14 oppositely 
mounted on each side of the screw carrier tape 33 so as to hold the tape 
33 under pressure. The screw tape 33 is initially fed into the tape drive 
cylinders 14 by a leader tape 40 which is thinner than the screw tape 33; 
this allows the leader tape 40 to be inserted between the tape drive 
cylinders 14 and the screw tape 33 to be pulled between the cylinders 14. 
The screws 15 are brought into position in front of the screwdriver bit 7 
and its tip 13 by rotation of the tape drive cylinders 14 with the carrier 
tape 33 being supported upstream of the guide tube 18 by a guide wheel 36 
(FIG. 6) in order to ensure that the tape 33 is fed straight to the guide 
tube 18 and thus prevent it from jamming against the screw guide tube 18 
(see FIG. 2B). The position of the screw 15 is detected by a limit switch 
19 which senses the index notches 39 defined in the screw tape 33 (see 
FIG. 6). A screw guide tube 18 supported by a support 47 (see FIG. 2B) 
serves as a restraining mechanism for the screw tape 33 as the screw 15 is 
pushed out of the screw carrier tape 33 by the screwdriver bit 7. During 
the period when the screw 15 has been pushed out of the screw carrier tape 
33 and the screw tip has not yet engaged the work surface, the screw 15 is 
held onto the screwdriver bit 7 by a magnetic sleeve 12. The screw-less 
portion of the carrier tape 33, i.e. the tape portion extending downstream 
of the screwdriver bit 7 and then between the rollers 14 (see FIG. 6), is 
received in take-up tape holding chamber 47 (see FIG. 3). 
The gear trains 10, 21 and 24 are housed in hermetically sealed gearboxes 
(not shown) to protect their mechanisms from dirt and the like. 
As best seen in FIGS. 2A and 3, the screwdriver bit 7 and the screw 15 
aligned therewith are located in the upper right hand corner of the casing 
1, approximately 3/4" or less (i.e. basically as close as possible) away 
from the top and right walls thereof preferably with markings on these 
walls, to allow screws to be inserted close to corners and to facilitate 
the accurate positioning of the screws on the work piece. 
The operation of the screw driving device D is controlled by a central 
microprocessor 28 mounted in a hand grip 27. The architecture of the 
control system is shown in FIG. 7. The control system utilizes the 
following analog inputs. 
1) the maximum screw depth which is set by knob 41, a potentiometer setting 
which determines the depth to which the screw head is driven into the work 
surface; 
2) the screw rotary speed limit which is set by knob 42, a potentiometer 
setting which determines the maximum rotary speed of the screws 15 as they 
are driven into the work surface; 
3) the maximum screw torque which is set by knob 43, a potentiometer 
setting which determines the maximum torque to which the screws 15 are 
tightened when torque mode is selected; 
4) the position of the screwdriver bit 7, an input which is provided by the 
linear potentiometer 5 which continuously provides information regarding 
the position of the head of the screw 15 as it travels toward the work 
surface on the basis that the actuator rod 4 extends through the 
potentiometer 5 and is connected at its rear end to the rack 2 (see FIG. 
1) thereby continuously providing to the potentiometer 5 the relative 
axial position of the rack 2 and thus of the head of the screw 15; 
5) the control voltage 101 of the first motor M1 which provides a 
continuous reading of the voltage applied to the first motor M1; 
6) the field current 102 of the first motor M1 which provides a continuous 
reading of the first motor M1 field current; 
7) the armature current 103 of the first motor M1 which provides a 
continuous reading of the first motor M1 armature current; 
8) the control voltage 104 of the second motor M2 which provides a 
continuous reading of the voltage applied to the second motor M2; 
9) the field current 105 of the second motor M2 which provides a continuous 
reading of the second motor M2 field current; and 
10) the armature current 106 of the second motor M2 which provides a 
continuous reading of the second motor M2 armature current. 
The control system uses the following digital inputs. 
1) the rotary speed 100 of the first motor M1 which is a measurement of the 
first motor M1 rotary speed; 
2) hand trigger switch 29 provided on the handle 38 which indicates whether 
the hand trigger switch is in the "on" or "off" position; 
3) screw guide limit switch 16 actuated by switch actuator 17 which 
indicates whether or not the front of the screw driving device D is firmly 
pressed against the work surface on the basis that, by positioning the 
device D against the work piece, the actuator 17 is pushed into the screw 
guide tube 18 such as to be flush with the front wall of the casing 1 and 
actuate the limit switch 16; 
4) torque/depth switch 44 which selects whether the screw 15 will be driven 
into the work surface to a maximum selected torque or to a maximum 
selected depth; and 
5) screw position switch 19 which indicates whether or not the screw 15 is 
in the drive position. 
The control system uses the following analog outputs. 
1) first motor M1 control voltage 108 is a variable DC voltage used to 
control the first motor M1; and 
2) second motor M2 control voltage 109 is a variable DC voltage used to 
control the second motor M2. 
The control system uses the following digital outputs. 
1) indicator light 45 indicates that the screw magazine 26 is empty, or a 
fault has occurred in the screw tape transport mechanism; 
2) indicator light 46, i.e. screwdriver retraction fault, indicates that 
the screwdriver bit 7 is not properly retracted; and 
3) 50 is a control signal which acts to set or release the tape drive 
clutch mechanism 20. 
FIGS. 10A to 10H constitute a logic flow diagram which illustrates how the 
screw driving device D is controlled. A normal operating sequence of the 
device D would proceed as follows: when electrical power is supplied to 
the device D, the control initiates at 120 (FIG. 10A); if the screw 
detection switch 19 detects a screw 15 in the drive position, the logic 
moves on to 121 to ensure that the screwdriver bit 7 is fully retracted; 
if either condition 19 or 121 does not hold true, the logic moves to the 
screwdriver retraction and screw positioning mode which will be described 
subsequently. If a screw 15 is detected in the drive position and the 
screwdriver bit 7 is fully retracted, the logic requires that either the 
hand trigger switch 29 or the screw guide limit switch 16 be switched off 
and on in sequence (by removing the device D sufficiently from the work 
piece to allow switch actuator 17 to return, under spring bias, to its 
extended position shown in FIGS. 1, 3, 4, and 5) so that the screw driving 
cycle is interrupted and the screw driving device D is moved to a new 
position, this logic being represented by sequence 29, 16, 16, or 16, 29, 
29. If the above conditions are true, the first motor M1's starting 
sequence is initiated at 42 and the second motor M2's starting sequence is 
initiated at 43. The maximum rotary speed limit of the first motor M1 is 
read from the selector switch 42. At 123, the first motor M1 is started 
and ramped toward the maximum M1 rotary speed limit. At 43, the maximum 
torque limit of the second motor M2 is determined; at 124, the second 
motor M2 is started and the speed thereof is controlled, using feedback, 
with a voltage ramp so that the resulting forward motion of the 
screwdriver bit 7 is higher than the forward motion of a screw as 
determined by the current rotary speed of the first motor M1. At 125, the 
armature resistance of the second motor M2 is calculated from the relation 
: 
EQU R.sub.a2 =V.sub.a2 /I.sub.a2 
where: R.sub.a2 is the M2 armature resistance; 
V.sub.a2 is the M2 armature voltage; and 
I.sub.a2 is the M2 armature current, 
given that the rotary speed of the second motor M2 is very small. 
Reference should be made to FIG. 8 for a further illustration of the 
control sequence. At 0% screw head position, the screwdriver tip 13 
engages the head of the screw 15 and the screw 15 is forced out of the 
screw carrier tape 33. The screw tip now moves toward the work surface at 
a speed which is higher than the equivalent forward travel of the screw 15 
due to its rotary motion. When the screw tip engages the work surface, the 
screw forward rate of travel of necessity slows to the equivalent rate due 
to the rotary speed of the screwdriver bit 7. This reduction in forward 
speed of the second motor M2 is detected at 127 (FIG. 10B); at 200 (FIG. 
10D), the motor proportionality coefficient of the second motor M2 is 
recalculated from the equation: 
EQU k.sub.2 =(V.sub.a2 -I.sub.a2 R.sub.a2)/.omega..sub.2 I.sub..function.2 
where: k.sub.2 is the M2 proportionality coefficient; 
V.sub.a2 is the M2 armature voltage; 
I.sub.a2 is the M2 armature current; 
R.sub.a2 is the M2 armature resistance; 
.omega..sub.2 is the M2 rotary speed; and 
I.sub..function.2 is the M2 field current, 
given that the motor control ramp rate is sufficiently small to make 
inductive and inertial effects minimal. 
At 201, the calculated value k.sub.2 is used to determine, from a look up 
data table stored in read only memory, the required armature-field current 
product for control of the torque of the second motor M2 to a maximum 
value and thereby the seating force of the screwdriver bit 7 onto the 
screw 15 to a maximum value. I.sub.a2 and I.sub..function.2 are measured 
and used in a feedback control of motor torque based on the DC motor 
equation: 
EQU T.sub.2 =k.sub.2 I.sub.a2 I.sub..function.2 
where:T.sub.2 is the M2 motor torque. 
The recalculation of k.sub.2 for every operating cycle of the screw driving 
device D allows for the dynamic compensation of the effect of temperature 
variations on the magnetic characteristic of the motor armature and field. 
This compensation procedure provides for stable and accurate control of 
the screwdriver bit seating force. 
FIG. 9 shows the forward drive motor M2 torque versus % screw head position 
curve. 
At 203, the M1 speed ramp is continued and at 204/205 the system pauses 
until the maximum M1 speed is reached. At 44, the system branches to 
either the position mode or the torque mode. Assuming the position mode is 
selected, the following sequence occurs. At 206, the screw position is 
monitored, when the screw position reaches 85%, M1, rotary speed is ramped 
to 20% of maximum at 207, this being to slow the rotation of the screw 15 
for the approach to final seated position. At 208, the screw position is 
monitored for 100% seated position, and when the 100% position is reached 
a stop sequence is initiated at 258. Time delay 209 and control sequence 
210/214 serve to stop the machine if the full seated position cannot be 
reached. 
If at 44 the torque mode is selected, the following sequence occurs. At 
250, the screw position is monitored, and when the screw position reaches 
80%, intermediate coefficients C.sub.1 & C.sub.2 are calculated for the 
purpose of calculating K.sub.1, the M1 proportionality coefficient, later 
in the cycle, when R.sub.a1 the motor armature resistance becomes 
available. 
EQU C.sub.1 =V.sub.1 /.omega..sub.1 I.sub..function.1 & C.sub.2 =I.sub.a1 
/.omega..sub.1 I.sub.71 1 
where: V.sub.1 is the M1 control voltage; 
.omega..sub.1 is the M1 rotary speed; 
I.sub..function.1 is the M1 field current; and 
I.sub.a1 is the M1 armature current. 
At 252, the screw position is monitored until it reaches 85%; at 253, 
V.sub.1 is ramped down to a level where M1 stalls. At 254, R.sub.a1 is 
calculated from the relation: 
EQU R.sub.a1 =V.sub.1 /I.sub.a1 
where: R.sub.a1 is the M1 armature resistance. 
At 255, k.sub.1 is calculated from the equation: 
EQU k.sub.1 =C.sub.1- C.sub.2 R.sub.a1 
where: k.sub.1 is the M1 proportionality coefficient. 
M1 torque is given by the DC motor equation: 
EQU T.sub.1 =k.sub.1 I.sub.a1 I.sub..function.1 
where: T.sub.1 is the M1 motor torque. 
The maximum required tightening torque is read at 43 and with k.sub.1 
available the required I.sub.a1 I.sub..function.1 product is determined 
from a data table stored in read only memory. At 257, V.sub.1 is ramped to 
produce the required I.sub.a1 I.sub..function.1 product, under feedback 
control, to accurately apply the maximum required tightening torque to the 
screw 15. 
The recalculation of k.sub.1 for every operating cycle of the machine 
allows for the dynamic compensation of the effect of temperature 
variations on the magnetic characteristic of the motor armature and field. 
This compensation procedure provides for stable and accurate control of 
the screwdriver seating torque. 
Motors M1 and M2 are then stopped in the sequence 258/261. The cycle then 
goes to entry point 2 (FIG. 10C). 
At 129, the position of the screwdriver bit 7 is determined; if the 
screwdriver bit 7 is not retracted, the M2 retraction mode is initiated at 
130; if the retraction mode is not completed within a time limit, M2 is 
stopped at 132 and a fault indication appears at 46. If the retraction 
mode is successful, then the retraction mode is stopped at 133. At 134, 
the magnetic clutch 20 engages the tape drive gears 21 to the first motor 
M1. At 134, the screw tape drive mode is initiated, and the screw position 
switch 20 determines that the carrier tape 33 is moving and that another 
screw 15 is loaded into position within a time limit set by time delay 
136. If time delay 136 times out, the M1 tape drive mode is stopped at 137 
and an empty indication appears at 45. If the screw positioning operation 
has been successful, the tape drive mode is stopped at 138 and the 
magnetic clutch 20 is released at 139. The system is now ready for another 
cycle, which can be initiated by either releasing the hand trigger switch 
29 and sliding the screw driving device D to another location without 
releasing the actuator 17 and thus the front safety switch 16, or by 
lifting the device D away from the work surface and placing it at another 
location without releasing the hand trigger switch 29. 
It is readily understood from the foregoing that the screw driving device D 
of the present invention provides a fully automated electric screwdriver 
which, for instance, eliminates the need for any manual translational 
displacement of the screwdriver bit until it engages the screw.