Menu 40 − AMC Homing System

Mode: RFC‑A

Homing system

Homing is a method used to detect a particular position, usually with some type of mechanical switch (a "home switch"). The position feedback (the slave position in the AMC) is then modified so that it has a known position at the point where the "home switch" position was detected. In the example below the scaling has been set up so that the AMC user units are millimetres.

The total movement is from 0mm to 1000mm moving from left to right. Initially the system starts at a position of 100mm from the left, but the AMC is operating in incremental mode with a position feedback device that does not give absolute position, and so the slave position (AMC Slave Position (33.004)) is zero. Homing is started when AMC Reference Select (34.007) is set to 5 (Home) and in this case the system will move to the right by 300mm until the "home switch" is detected. At this point the mechanical position will be 400mm, but the slave position will be 300mm because it has increased from its initial value of 0mm by 300mm. In this example the slave position should be 0mm at the left-hand end and 1000mm at the right-hand end, and so the slave position should be 400mm at the "home switch" transition. To achieve this an offset is applied to the slave position. The user knows that the "home switch" transition is at 400mm, and must indicate this by setting AMC Home Position (40.004) = 400. The drive will then perform the following calculation to modify the slave offset:

AMC Slave Offset (33.003) = AMC Home Position (40.004) - AMC Slave Position (33.004) = 400mm - 300mm = 100mm

Because the system has moved by 300mm the slave position will be 300mm just before the "home switch" transition, and so after the transition:

AMC Slave Position (33.004) = 300mm + (400mm - 300mm) = 400mm

So now the slave position is 400mm at the transition, the absolute position is known and the position changes from 0mm to 1000mm over the range of travel. Because the slave position is always the sum of the slave position feedback and its offset, the above will always work even if the initial value of AMC Slave Position (33.004) is non-zero. Normally this step change of the slave position would cause unwanted movement because this change would be seen as a position error by the position controller. To avoid this effect both the input and output of the profile generator are also adjusted by the same change applied to the slave offset.

The homing sequence can include up to three phases:

Phase 1: Home to switch
The system operates in speed control (with maximum speed in either direction of AMC Home Maximum Speed (40.003)) until it detects a valid "home switch" transition. Phase 1 does not have to be present in the homing sequence (i.e. AMC Home Mode (40.002) = 1). In this case the sequence starts in Phase 2. If Phase 1 is not present then Phase 2 must be present to detect the "home switch" transition.

Phase 2: Home to freeze
The system operates in speed control (with maximum speed in either direction of AMC Home Maximum Speed (40.003)) until it detects a freeze event. Phase 2 is optional and can follow on from Phase 1 to give a more accurate home position than is possible from the initial "home switch" transition. Alternatively it can be used alone to detect the home position. 

Phase 3: Home offset
The system operates in position control (with a maximum speed in either direction of AMC Home Offset Maximum Speed (40.006)) until the system moves to a position that is a distance from the home position defined by AMC Home Offset Position (40.007).

Each of these phases is described in more detail below. It is possible to see the progress through these phases by monitoring AMC Homing Sequence State (40.024).

Phase 1: Home to switch
There are a number of possible homing modes (see AMC Home Mode (40.002)), but homing Mode 5 is used as an example to describe this phase. AMC Home Switch (40.012), AMC Home Positive Limit (40.013) and AMC Home Negative Limit (40.014), which can be controlled for example by digital inputs, are used to define the movement of the system in an attempt to produce the required "home switch" transition and to set the home position. The left-hand end of the movement is the negative end and the right-hand end is the positive end. In Mode 5 AMC Home Negative Limit (40.014) is not active in controlling the movement. If this switch becomes active an error has occurred, and so AMC Reference Select (34.007) is automatically set to 0 (Stop) and the home sequence is aborted. The direction of movement depends on the state of the "home switch". Three example starting points are shown on the diagram below. When the switch is low (Starting Points 1 and 3) the movement is forwards and when the switch is high (Starting Point 2) the movement is backwards. The home position is detected when a transition occurs on the negative edge (the one closest to the negative end) of AMC Home Switch (40.012). This can occur with the system moving either forwards (negative end to positive end) or backwards (positive end to negative end), and so it corresponds to a rising transition in the forwards direction or a falling transition in the reverse direction as shown.

The diagram below shows the movement from each starting point until the "home switch" transition is detected at the end of Phase1.

If the initial position is on the negative side of the required negative edge transition (Starting Point 1) then the movement will be forwards toward the required negative edge transition and the system will complete Phase 1 successfully.

If the initial position is where the "home switch" is high (Starting Point 2) again the movement will be backwards toward the required negative edge transition and the system will complete Phase 1 successfully.

However, if the initial position is beyond the positive edge of the region where the "home switch" is high (Starting Point 3) the resulting forward movement will be away from the required negative edge transition and unless further action is taken Phase 1 will fail. To prevent this, if AMC Home Positive Limit (40.013) becomes active it reverses the direction of motion. This state is latched and overrides the state of  AMC Home Switch (40.012) so that the backwards movement continues until the negative edge of the home switch is reached to complete Phase 1. 

If Phase 2 is not required (AMC Home On Freeze (40.011) = 0) the home position has now been found and the slave position offset must be modified to make the physical position where the transition occurred equal to the required home position as described previously. The slave offset is modified so that

AMC Slave Offset (33.003) = AMC Home Position (40.004) - AMC Slave Position (33.004) 

The system then proceeds to Phase 3.

Phase 2: Home to freeze
Phase 2 is either used alone or follows on from Phase 1 to give a more accurate home position than can be achieved in Phase 1. The home system runs with a sampling time of 4ms, and in Phase 1 the "home switch" transition is detected in the sample after it occurs. This means there will be an error in the home position that is dependent on the speed when the "home switch" is detected. 

The freeze system captures the position of the transition independently of the system samples, and so the result is more accurate and does not include a significant speed dependent error. The freeze signal can be derived from a marker pulse, which will occur once per revolution in a rotary system as in the example shown below, or from a sensor connected via a digital input to the drive, or from an option module freeze system.

At the start of Phase 2 the freeze flag of the slave position freeze source is cleared. For example, if the drive F1 freeze is being used as the source then F1 Freeze Flag (03.104) is cleared. This has the effect of also clearing AMC Slave Freeze Flag (33.010) so that a new freeze position can be detected. 

The direction used to approach the freeze transition is defined by AMC Home Direction (40.001), where 0 corresponds to the forward direction and 1 to the backwards direction. In the example shown below the initial position is where the "home switch" is active, and so the initial direction is backwards. At the end of Phase 1 the slave offset is not modified, but the direction of movement changes to forwards because AMC Home Direction (40.001) = 0 and the system continues in the forward direction until the freeze is detected. The slave offset is then modified so that

AMC Slave Offset (33.003) = AMC Home Position (40.004) - AMC Slave Freeze Position (33.006)

The system then proceeds to Phase 3.

Phase 3: Home offset
So far during the homing sequence the system has been operating in speed control with the maximum speed in either direction defined by AMC Home Maximum Speed (40.003). When the required transition is detected at the end of the previous phase (i.e. Phase 1 or Phase 2) the system will be moving at a speed up to this maximum speed. If the reference selector (AMC Reference Select (34.007)) is simply set to 0 (Stop) the system would slow to a standstill under control of the profile, and so its final position would be away from the home position. 

The position required at the end of the homing sequence is defined as the distance from the home position by AMC Home Offset Position (40.007). If this is set to zero then the required position at the end of the sequence is the home position itself. During Phase 3 the system operates in position control, with a maximum speed of AMC Home Offset Maximum Speed (40.006) to move to a distance of AMC Home Offset Position (40.007) from the home position. 

In the example given below AMC Home Offset Position (40.007) is a positive non-zero value longer than the distance required to stop at the speed at the end of the previous phase. Therefore the system continues to move in the forwards direction until it stops at the required position. The user can define how close the slave position must be to the required position to terminate Phase 3 with AMC Home Complete Window (40.010).

The example given is for AMC Home Mode (40.002) = 5. This demonstrates the principles used for all the other homing modes.

AMC Home Direction (40.001) defines the direction of movement during Phase 2 (home to freeze) when the system operates under speed control to detect the freeze input transition, where 0 is forwards and 1 is backwards.

AMC Home Mode (40.002) is used to select the required homing mode. Each of the modes is described below. It should be noted that only Phase 1 is described for Modes 2 to 8. In these modes Phase 3 always occurs and Phase 2 occurs if AMC Home On Freeze (40.011) = 1.  

Mode (-1): Home direct
AMC Slave Offset (33.003) is simply updated to make present position become the AMC Home Position (40.004) without any movement. The AMC Reference Select (34.007) is changed immediately back to 0 (Stop).

Mode 0: Manual
Only Phase 1 is allowed in this mode, and so AMC Home On Freeze (40.011) has no effect. In the general description at the start of this menu it was stated that if either AMC Home Positive Limit (40.013) or AMC Home Negative Limit (40.014) become active then this is latched and the defined movement overrides the movement defined by AMC Home Switch (40.012). Manual mode operates differently to the other modes in that the activation of the limits is not latched and the direction is defined by the state of these switches as given in the table below.

It is assumed that the initial position is where the home switch is not active so that the home position can be detected on the change from not active to active at the appropriate edge of AMC Home Switch (40.012). AMC Home Positive Limit (40.013) and AMC Home Negative Limit (40.014) can be used as jog controls, for example, to move the system until the home position is detected.

Mode 1: Home on freeze flag
Phase 1 is not used in this mode, but the home position is always detected in Phase 2 when the AMC Slave Freeze Flag (33.010) becomes active. (Unlike all the following modes it is not necessary to set AMC Home On Freeze (40.011) to 1.) Once the home position has been detected the system moves on to Phase 3.

Mode 2: Home on positive limit
During Phase 1 AMC Home Positive Limit (40.013) defines the direction of movement so that the system moves towards the point where it changes state.  

Mode 3: Home on negative limit
During Phase 1 AMC Home Negative Limit (40.014) defines the direction of movement so that the system moves towards the point where it changes state.

Mode 4: Home on home switch
During Phase 1 AMC Home Switch (40.012) defines the direction of movement so that the system moves towards the point where it changes state.

Mode 5: Home on negative edge of home switch with positive limit
AMC Home Switch (40.012) is connected to a "home switch" that can be approached from either direction. The negative edge of the "home switch" is used to define the home position. From Starting Points 1 and 2 the defined movement is towards the home position. However, the movement from Starting Point 3 is away from the home position, and so AMC Home Positive Limit (40.013) must be connected to some form of limit switch so that when the positive end of the system is reached the direction is reversed.

Mode 6: Home on positive edge of home switch with positive limit
This operates in the same way as Mode 5 except that the home position is at the positive edge of the "home switch". To achieve this the direction off a high "home switch" is changed from backwards to forwards.
 

Mode 7: Home on negative edge of home switch with negative limit
This operates in the same way as Mode 5 except that a negative limit is used to reverse the direction when the movement is away from the required home switch transition. To achieve this the direction off a low "home switch" is changed from forwards to backwards.



Mode 8: Home on positive edge of home switch with negative limit
This operates in the same way as Mode 5 except that the home position is at the positive edge of the "home switch" and a negative limit is used to reverse the direction when the movement is away from the required home switch transition. To achieve this the direction of movement off a low or high "home switch" are both reversed.




Mode 9: User defined mode
This is a flexible mode that allows different aspects of the homing sequence to be defined by the bits in AMC Home Mode Setup (40.022).  

During Phases 1 and 2 the homing system operates in speed control and AMC Home Maximum Speed (40.003) is the maximum speed used during these phases. The homing system will set the speed reference at the input to the profile generator to ±AMC Home Maximum Speed (40.003) to try and reach the home switch. Depending on the acceleration rate and the distance and time required to reach the "home switch" it may or may not be possible to reach the defined maximum speed.

AMC Home Position (40.004) defines the slave position after Phase 1 (or Phase 2 if being used) has been completed. The slave position offset is modifed so that the slave position is equal to AMC Home Position (40.004) at the end of Phase 1 (or Phase 2 if being used). See the introduction for this menu for more details.

AMC Home Complete (40.005) is set to zero when a homing sequence is started. It is set to one again at the end of Phase 1 if Phase 2 is not being used, or otherwise at the end of Phase 2.

During Phase 3 the homing system operates in position control. AMC Home Offset Maximum Speed (40.006) defines the maximum speed that can be used in either direction during this phase.

The offset from the home position during Phase 3 of the homing sequence, i.e the distance of the final target position from the home position is defined by AMC Home Offset Position (40.007).

AMC Home Offset Complete (40.008) is set to zero when a homing sequence is started. It is set to one again when Phase 3 is completed and the system has reached its target position.

AMC Home Maximum Allowed Move (40.009) defines the maximum distance the slave is allowed to move to complete the home sequence. The slave position is stored at the start of the homing sequence. If the distance moved from this initial position up to the point where the home position has been detected (at the end of Phase 1 if Phase 2 is not being used, or at the end of Phase 2) becomes greater than AMC Home Maximum Allowed Move (40.009) then AMC Reference Select (34.007) is set to 0 to abort the homing sequence. If AMC Home Maximum Allowed Move (40.009) is set to zero then this feature is disabled and there is no limit on the possible movement.

AMC Home Complete Window (40.010) is used to define the window allowed in detecting that the AMC Home Offset Position (40.007) has been reached at the end of Phase 3 of the homing sequence. Phase 3 is complete when |AMC Profile Input Position (38.006) - AMC Slave Position (33.004)| ≤ AMC Home Complete Window (40.010)

In Home Modes 2 to 8, if AMC Home On Freeze (40.011) = 1 then Phase 2 is used to detect the home position using the selected slave position freeze source. Otherwise the home position detected from the "home switch" in Phase 1 and the homing sequence moves directly from Phase 1 to Phase 3. In Home Modes 0, 1 and 9 AMC Home On Freeze (40.011) has no effect. 

AMC Home Switch (40.012) can be used as a home switch input. AMC Home Mode (40.002) describes how this is used in different homing modes. 

AMC Home Positive Limit (40.013) can be used as a home switch or a limit switch. AMC Home Mode (40.002) describes how this is used in different homing modes. 

AMC Home Negative Limit (40.014) can be used as a home switch or a limit switch. AMC Home Mode (40.002) describes how this is used in different homing modes.

AMC Home Positive Limit Selector (40.015) selects the source that controls AMC Home Positive Limit (40.013) as follows:

AMC Home Negative Limit Selector (40.016) selects the source that controls AMC Home Negative Limit (40.014) as follows:

This detector is intended to indicate when the system has reached a mechanical end stop and is trying to move against it, producing more than a specific level of torque in rotary system (or force in a linear system). When Final Torque Reference (04.003) is greater or equal to AMC Home Hard Stop Torque Threshold (40.018) it is assumed that the drive is operating in the forwards direction and has hit a physical limit. AMC Home Hard Stop Torque Threshold (40.018) should be set to a level higher than the torque seen in normal operation so that the detector only becomes acitve when the system is at the mechanical stop. A time delay is provided that is reset to zero each time the torque reference is below the threshold and counts up when it is above the threshold, so that the limit condition must exist for the delay time before the limit becomes active. If AMC Output Invert (39.013) = 0 then positive torque will produce forward motion and will cause the system to hit the positive limit. In this case the positive and negative outputs of the hard stop detector are connected to AMC Home Positive Limit (40.013) and AMC Home Negative Limit (40.014) respectively. If AMC Output Invert (39.013) = 1 then positive torque will produce backwards motion and the connections are reversed.

See AMC Home Torque Limit Active (40.017).

See AMC Home Torque Limit Active (40.017).

AMC Home Required (40.020) can be used to enable the "home before motion" function which ensures that the system is homed before any motion other than the homing sequence is allowed. If AMC Home Required (40.020) = 0 then the "home before motion" function is disabled, and AMC Home Required Flag (41.017) and bit 14 of AMC Status (41.002) are held at zero.

If AMC Home Required (40.020) = 1 then the "home before motion" function is enabled. AMC Home Slave Source (40.021) indicates the source of the slave position that was used when the last homing sequence was completed successfully. If this is not the same as the currently selected slave source (i.e. AMC Home Slave Source (40.021) is not equal to AMC Slave Source Select (33.001)) then AMC Home Required Flag (41.017) and bit 14 of AMC Status (41.002) are set to one, and it is not possible to produce any movement unless AMC Reference Select (34.007) is used to select a homing sequence. At the end of a successful homing sequence the value of AMC Home Slave Source (40.021) is automatically made equal to AMC Slave Source Select (33.001), which sets AMC Home Required Flag (41.017) and bit 14 of AMC Status (41.002) to zero and enables all the motion references to be used. If an absolute position feedback device is providing the slave position and therefore absolute mode is selected (AMC Absolute Mode Enable (31.002) = 1) then it is not necessary to repeat the homing sequence each time the drive is power cycled. In addition to AMC Home Slave Source (40.021), indicating the slave source used during homing, AMC Slave Offset (33.003) is updated to make the slave position correspond to AMC Home Position (40.004) at the physical point where the home switch transition was detected. To make the results of the home sequence valid after subsequent power cycling AMC Home Slave Source (40.021) and AMC Slave Offset (33.003) must be saved. To achieve this AMC Home Slave Source (40.021) is a power-down save parameter, and AMC Slave Offset (33.003) is automatically saved at the end of a successful homing sequence if absolute mode is selected (AMC Absolute Mode Enable (31.002) = 1). The drive must also ensure that when "home before motion" is enabled that a homing sequence is carried out when necessary. To achieve this AMC Home Slave Source (40.021) is reset to -1 (None), which does not correspond with any valid slave source, under various conditions. The table below gives all the conditions and actions used to make the "home before motion" system operate correctly.

*The "AMC Disabled" condition is true when AMC Select (31.001) = 1 AND ( AMC Enable (41.001) = 0 OR Reference On (01.011) = 0 OR AMC Position Feedback Initialised Flag (41.018) = 0).

Note that power-down save parameters are saved when the 24V control supply is removed from Digitax HD products, and so AMC Home Slave Source (40.021) is always saved when the power is removed. With all other products power-down save parameters are only saved when the power is removed if User Supply Select (06.072) = 0 AND Low Under Voltage Threshold Select (06.067) = 0 AND Backup Supply Mode Enable (06.068) = 0. If these conditions are not met then the user should save parameters at the appropriate time to store AMC Home Slave Source (40.021).

Note that setting AMC Home Required (40.020) = 0 does not reset AMC Home Slave Source (40.021) = -1 (None), and so it is possible to temporarily disable the "home before motion" function to move the system and then enable it again without forcing the system to be homed again. 

See AMC Home Required (40.020).

When AMC Home Mode (40.002) is set to mode 9 the bits in AMC Home Mode Setup (40.022) are used to set up different parts of the homing sequence. The table below shows the purpose of each bit and then a more detailed description is given below the table.

b0: Home direct
If this bit is one it overrides all other bits except for bit 1 and causes the homing sequence to terminate immediately after it is started with no movement and sets AMC Slave Offset (33.003) = AMC Home Position (40.004) - AMC Slave Position (33.004). This has the effect of setting the home position at the present physical position of the system and leaving the system at the home position without any movement. If this bit is zero it has no effect.    

b1: Do not update the home position
If this bit is one then the required homing sequence is completed as normal except that AMC Slave Offset (33.003) and the input/output of the profile are not modified when the home position is detected. This has the effect of causing all the movement of the homing sequence in Phase 1 and/or Phase 2 as applicable, and then during Phase 3 the AMC Profile Input Position (38.006) is left at the value it had when the previous phase was completed. Therefore during Phase 3 the system simply moves to this position. 

b2: Direction off high home switch
b3: Direction off low home switch
These bits define the required direction of movement off the home switch during Phase 1. "b2 Direction off high home switch" defines the direction when the selected "home switch" is active where 0 is forwards and 1 is backwards, and "b3 Direction off low home switch" defines the direction when the selected "home switch" is inactive. It should be noted that if either the positive limit or negative limit is enabled, and becomes active and latched then the direction defined by the latch limit will override the direction defined by the home switch. This is shown in the example below.

The table below shows how the positive and negative limits are latched.

b4-5: Home switch selector
The table below shows the "home switch" selected by these bits. If AMC Slave Freeze Flag (33.010) is selected then the homing sequence omits Phase 1 and starts in Phase 2.

b6: Positive limit enabled
If this bit is zero the positive limit is disabled, and if this limit becomes active AMC Reference Select (34.007) is set to zero to terminate the homing sequence. If this bit is one the positive limit is enabled and will cause the required movement to be backwards away from this limit as shown in the example for b2 and b3 above. 

b7: Negative limit enabled
If this bit is zero the negative limit is disabled, and if this limit becomes active AMC Reference Select (34.007) is set to zero to terminate the homing sequence. If this bit is one the negative limit is enabled and will cause the required movement to be forwards away from this limit. 

b8-9: Home transition
b10: Home switch side
b11: Home on edge in either direction
b8-9 define the required home transition as follows:

b10 defines the home switch side as 0 = positive side and 1 = negative side. The positive side is the edge nearest to the positive end of the system.

Bits b8 to b11 combine to define the "home detection direction" and the transitions required to detect the home switch to end Phase 1 of the homing sequence. First the "home detection direction" is defined as follows:

1. If "b11 Home on edge in either direction" is one then the "home detection direction" is the same as "b3 Direction off low home switch".
2. If the "home switch transition" is either high state or low state then the "home detection direction" is the same as "b3 Direction off low home switch".
3. Otherwise the "home detection direction" is given in the table below. The "home detection direction" and the home transition will then automatically select the required side of the home switch.

Next the transitions that can be used to detect the required home switch edge has been passed or the required static state has been detected are defined.

If "b11 Home on edge in either direction" = 1 or the "home switch transition" is either high state or low state then if either the state/transition for the forward or backwards direction are detected in the appropriate direction Phase 1 is complete and the "home switch" has been located. This means that the required state has occurred, or if a transition is required the correct edge has been passed in either direction.

Otherwise if "b11 Home on edge in either direction" = 0 and a transition is required it is necessary to pass the home switch in the correct direction. This means that the home switch will always be approached in the same direction when Phase 1 is completed which can give more consistent results. The transition is detected in either direction as the required transition for both forwards and backwards is specified, but if the direction is opposite to the "home detection direction" the required movement is reversed so that the home switch edge will be passed again in the opposite direction to complete Phase 1.

b12: Home on freeze
AMC Home On Freeze (40.011) has no effect when Home Mode 9 is selected, but is replaced by this bit. If this bit is one Phase 2 is included, otherwise the homing sequence changes directly to Phase 3 at the end of Phase 1. 

b13-14: Direction in Phase 2
The direction of movement during Phase 2 is given in the table below. 

A homing sequence can be aborted and restarted by changing AMC Reference Select (34.007) from 5 (Homing) to another value and back again to 5. This can also be achieved by setting AMC Home Restart (40.023) to one. This parameter is automatically reset to zero and the homing sequence restarted from the beginning of the sequence. Any information obtained during the previous home sequence will be ignored.   

AMC Homing Sequence State (40.024) gives the state of the homing sequence.