The basics of automationUnclassified


1- Introduction

The GEMMA (Guide for the Study of Marching and Stopping Modes) is a graphic guide developed by the Agency for the Development of Applied Productics (ADEPA) in April 1981.It constitutes a method of approaching Marching Modes and System Stops, it is a complementary aid tool to GRAFCET which enables the needs in terms of running and stopping of an automated system to be clearly and fully expressed.

The GEMMA is a structured document (tool-method) ready to be completed by its user in order to follow a guided and systematic approach


2- Basic concepts of GEMMA

The GEMMA defines the state in which the control part of the automated system is located. First, we can say that GEMMA is divided into two areas:

  • Control part without energy
  • Control part under energy
Basic concepts of GEMMA

In the non-energy PC zone, the control part is not supplied with energy. In this part there are no modes treated by the command part. Only actions called reflex or external actions can be carried out.

For the control part area under energy and according to the needs of the automated system to be studied, we choose to use certain operating and stopping modes. Indeed, the running and stopping modes can only be perceived and processed in the presence of a control part in working order. To each of these modes corresponds a “state rectangle” arranged on the graphic according to a precise structure.

3- Structuring of Gemma

3.1- Production – non-production

An additional distinction is made in the PC under energy zone. The production zone and the non-production zone are distinguished by a double dotted line of the production zone. The Start and Stop Modes inside the “Production” dotted lines correspond to states for which the machine is producing.


The system is in “production” if the added value for which the system was designed is obtained. the system is “out of production” otherwise

3.2- The families of the states of running and stopping modes (The procedures)

Each family of procedures corresponds to a zone of Gemma. There are three families:

  • The operating procedures grouping together the F states. We do not necessarily produce in all the F states. Preparatory modes for production, adjustments or tests, can be part of this family.
  • The shutdown procedures grouping together the A states, all the modes leading to or translating to a shutdown state, normal shutdowns and restarting procedures.
  • Failure procedures, grouping together the D states taken in the event of failure of the operating part.
The procedures

The GEMMA graphic guide shows the rectangles-states in which the different states of Start and Stop (M / A) taken by the machine will be expressed.

3.2.1 : F states


F1<normal production>In this state the machine produces normally: this is the state for which it was designed.
We can often make a grafcet which we call “base grafcet” correspond to this state.
Note: This state does not necessarily correspond to automatic operation.
F2<preparation walk>This state is used for machines requiring preparation prior to normal production:
Preheating of the tools, filling, various start-ups …
F3<closing march>This is the condition required for certain machines which must be emptied, cleaned … at the end of the day or at the end of the series.
F4<verification walk in the
This state makes it possible to check certain functions or certain movements on the machine without respecting the order of
cycle progress.
F5<verification walk in order>In this state, the production cycle can be explored at the production rate desired by the person carrying out
the cheking process
F6<test walk>Control, sorting, measuring machines, etc. include sensors that must be adjusted or calibrated: this
state allows the different operations.

3.2.2 – States A


A1<Stop in initial state>This is the idle state of the machine. It generally corresponds to the initial situation of Grafcet.
A2<Stop requested at end of cycle>When a stop is requested, the machine continues to produce until the end of the cycle; state A2 is therefore a transient state
to state A1
A3<Stop requested in
determined state>
The machine continues to produce until a stop at a position other than the end of the cycle; it is a transient state towards A4
A4<Stop Obtained>The machine is then stopped in a state other than the end of cycle.
A5<Preparation for recovery
road after failure>
It is in this state that we carry out all the operations (disengaging, cleaning …) necessary for a restoration.
road after failure
A6<Put PO in initial state>With the machine in A6, the operative part is manually or automatically returned to the initial position for a
restart in the initial state.
A7<Put PO in determined state>With the machine at A7, the operative part is returned to position for restarting in a position other than the initial state.

3.2.3 – D states


D1<Emergency stop>This is the state taken during an emergency stop: not only stops are provided for, but also release cycles,
the procedures and precautions necessary to avoid or limit the consequences of the failure.
D2<Diagnosis and / or treatment
It is in this state that the machine can be examined after failure and that a treatment can be given allowing
D3<Production all the same>It is sometimes necessary to continue production even after a machine failure: this will result in degraded production, forced or assisted by operators not provided for in normal production.

4- Method of using GEMMA

The study of the walking and stopping modes is planned from the design of the machine and integrated into its realization. After the establishment of the normal production GRAFCET (GPN), the GEMMA graphic guide is implemented for the selection of running and stopping modes.

The process has two phases:

  • The inventory of the different modes envisaged for the system and the highlighting of the sequences which connect them.
  • Determination of the conditions for switching from one mode to another.

4.1- Selection of Walk and Stop Modes :

The machine PO being defined, as well as the Grafcet of the normal production cycle (GPN), it is necessary to select and specify the necessary M / A modes.

Walk and Stop Modes

For a given machine, it is important to examine the case of each state rectangle:

  • If the proposed mode is retained, it will be specified in the “rectangle – state”.
  • A “rectangle-state” retained is completed in a handwritten manner: by specifying the operation performed specific to the machine studied; by highlighting the oriented link (s); by indicating the condition (s) of evolution
  • If the proposed mode is not necessary for the machine, a cross will be entered in the “rectangle – state” to signify that it is not retained.
rectangle-state-not retained

4.2- Look for changes from one state to another.

As the on and off modes have been selected and explained, the transition from one state to another should be specified.
The development of these passage conditions makes the design of the control panel possible and possibly leads to the addition of additional sensors.
The transition from one state to another is carried out in 2 ways:

  • Or with an evolution condition.
  • Either without condition of evolution
changes in a state

There are two states found on all machines:

  • state A1 (initial state) or (idle state) of the machine.
  • state F1, mode of (normal production) for which the machine was designed.

Starting from each of the two states, A1 and F1, it is interesting to find the evolutions towards other states:

  • We can start by starting the machine, that is to say going from A1 to F1, asking ourselves the question: is a preparation run (F2) necessary?
  • You can stop the machine as desired:
    – at the end of the cycle at circuit F1 – A2 – A1
    – in another position at circuit F1- A3 – A4

We will examine the cases of failure:

  • with (emergency stop) D1;
  • with (production all the same) D3;
  • etc.

4.3- GEMMA operational loops

On the GEMMA one characterizes several “loops”. A loop is a succession of states characterizing the functioning of the system. Indeed, it is only possible to pass from one state to another if the evolution conditions are respected, but it is sometimes impossible to pass from one state to another without using an intermediate state. This intermediate state will make it possible to reach the final state without risk for the system.

Normal walking loop:

The A1 >> F1 >> A2 >> A1 loop is the normal walking loop. This loop describes the normal operation of the system, then at the end of the cycle when the production cycle is stopped, the system returns to its initial position and will therefore be ready for a next cycle or series of cycles.

Normal walking loop

Adjustment step loop:

The A1 >> F4 >> A6 >> A1 loop is the adjustment run loop. The system leaves state A1 (stop under initial conditions) and goes to F4 (Checking out of order) which allows the operator to be able to test the actuators, pre-actuators, sensors, etc., of the system, in a mess. Once the checks have been carried out, the system goes from state F4 to state A6 (Putting the P.O. in the initial state). In this state the system will reach the initial conditions. When the conditions are met, the system will go from state A6 to state A1.

Adjustment step loop

The F1> D1> A5> A6> A1> F1 loop is the safety stop loop. This loop makes it possible to manage all the successive states of an automated system from an emergency stop during normal production to the resumption of normal production.
One particularity of box D1 is interesting. An arrow end is grafted onto the link arrow between state F1 and state D1. This arrow associated with its comment which means that this box is accessible from all states of Gemma. In other words, whatever the state in which the Gemma is located, if the conditions necessary to go into state D1 are met then the system will go into D1

Safety stop loop

5 – Design of a sequential automation:

For a given process, we must take into account:

  • Industrial constraints (profitability, safety),
  • The nature and types of sensors and therefore the signals delivered by the sensors,
  • The nature of the actuators and therefore the control signals accepted by the actuators.

The establishment of the specifications is the most important part especially for the arrangement of the sensors. It also determines:

  • The choice of I / O interfaces,
  • The number and type of I / O,
  • The usefulness or not of a field bus.

The establishment of the automation sequences is what is called the normal functioning GRAFCET or also level 1 GRAFCET. It is the diagram of the actions to be carried out according to the information received.
The operational and technological analysis takes into account the constraints imposed by the sensors and the actuators, as well as the operating modes of the automation (on, off, manual, automatic, safety). This is called the control GRAFCET or even level 2 GRAFCET which represents the complete automation to be produced. At this stage, depending on the problems encountered, it may be necessary to return to the specifications and / or to the GRAFCET level 1.

sequential automation

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