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Two-hand control unit
In the sequences described in previous paragraphs, the safety elements that require use of both hands in order to
initiate the start cycle command, have not been incorporated.
The general rules delineate the safety requirements in the workplace, both within the operating range of the
equipment and beyond. The equipmentmanufacturerwill determine themost suitable conditions for EM operation,
suchas the interruption of electrical andpneumatic power, positioning ofmoving components. Regarding the cycle
start command, the manufacturer of the systemmust decide whether to implement a traditional start system or
Two-Hand
device, based on the position of the operator and themachine or itsmechanical components.
The Two-Hand control unit permits an output signal only in the presence of two simultaneous inputs (given by the
simultaneous pressingof bothbuttons by theoperator). Thepneumatic circuit responsible, generally represented in
the boxwith a dashed green line, is integrated into a single blockwhich is connected to the outputs of the buttons.
in this casemanual valves
P1
and
P2
.
A certain distance is required between the two valves so as to prevent them from being operated with a single
hand.When they are activated the output signal reaches both the OR function as well as the AND function.
The output signal of theOR function, via a unidirectional flow regulator with a fixed calibration, is used to regulate
the filling time of the capacity connected in series, so as to delay the operation of the bistable 3/2-way valve.
This delay, even if small, permits the signal of the AND function to reach the 3/2-way valve faster, thereby opening
it at the same time as the buttons are activated. If one of the two buttonswere to be activatedwith a slight delay,
the output signal of the ORwould reach the 3/2-way valvemaintaining it in the closed position.
At the release of one of two buttons the output of the AND function is interrupted and the 3/2-way valve closes
when reached by the signal from the OR.
Consider themovement of two cylinders with the sequence:
A+ / B+ / A – / B –
Figure 104
Required conditions for the
I.C.
command:
• simultaneous signals from the two buttons
P1
and
P2
.
• If one of the two buttons is always active (blocked by external means) the start of the cycle is not possible.
• If one of the two buttons is released during stroke
A +
, the cylinder must reverse its stroke returning to its
negative end position.
• The operator can only release the buttons once limit switch
a1
is reached.
• The buttons can only be operatedwhen cylinder
B
is in negative end position.
Choice of control valves:
• prerequisite for
Phase 1
; cylinder
A
must be connected to a 5/2-waymonostable valve.
•For cylinder
B
, it is preferable touseabistable valvedue to thepresenceof intermediatephases betweenpositive
and negative strokes.
The limit switch
b0
feeds the two buttons
P1
and
P2
, their outlets are connected to the two inlets of the Two-
hand control unit, whose output is connected to
I.C.
in the flow diagram. The
I.C.
signal, in addition to operating
the pilot port of themonostable 5/2-way valve that enables themovement
A+
, also feeds limit switch
a1
, which
is not activated at present.
When the piston rod/piston of cylinder
A
reaches limit switch
a1
, there is an output signal which operates: the
main valve of cylinder
B
formovement
B+
and the opening of the3/2-waymemory valve,whose outlet substitutes
that of the two-hand unit. The buttons cannow be released. The piston rod/piston of cylinder
B
, when terminating
the positive stroke, activates limit switch
b1
whose output repositions the 3/2-way valve. This valve, by closing,
removes the pilot signal of the valve of cylinder
A
and the power supply of limit switch
a1
. In the absence of this
signal, the monostable function of the valve initiates the return stroke
A –
and the subsequent activation of
a0
.
The output signal from limit switch
a0
operates the valve of cylinder
B
which allows the stroke
B –
, the cycle
terminates.
This illustrated exercise is purelydemonstrativeanddoes not adhere toany current regulations regarding safety etc.
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