Store Multiturn

Overview

• 
  
• If the AKD2G drive is replaced, stored multiturn data will be lost.

The Store Multiturn saves multiturn counts from a feedback to memory and restores it on power cycle. Multiturn counts are the number of revolutions a feedback device has traveled in one direction. A feedback can have multiturn counts ranging from 0 (none) to 2³² bits of multiturn count. A typical multiturn absolute feedback has 2¹² multiturn counts.

FB#.STOREMULTITURN.BITS specifies the number of single turn-bits to use to determine if the motor has moved. Kollmorgen recommends using as many bits as possible to make the machine safe.

There are several limitations due to the drive saving multiturn counts.

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• • Linear encoders are not supported.
  • For use with HIPERFACE DSL encoders, see FB#.SETMULTITURNS.

Normal Multiturn Rollover Behavior

With a normal multiturn encoder, there are 12 bits of revs between rollovers, meaning rev -2048 to rev +2048 (or 0 to +4096 if unsigned). Moving past the rollover (rev +2048) and rebooting the drive will cause the drive to change the position to a negative value (rev -2048 plus 1 count).

Moving only up to rev +2048 minus 1 count will remain on the original side of the rollover (it will not roll over). Moving 1 more count (to rev +2048) and rebooting will cause the position to roll over to rev -2048 plus 1 count.

Track Rollovers

To prevent rollovers, set FB#.STOREMULTITURN.ENABLE = 1. Whenever the drive is power cycled after passing multiturn rollovers, the drive will keep track of the rollovers and retain the correct position.

Movement Detection

This feature also enables a movement detection check. If the motor moved more than a specified amount while the drive was powered off, it will throw a fault (F4x28 – Failed to restore multiturn bits).

Clearing the fault

Fault F4x28 can only be cleared by rebooting the drive again and making sure the motor does not move more than the specified amount while the drive is powered off. When the drive boots up, the fault will be cleared.

Movement Threshold

The movement threshold is set by FB#.STOREMULTITURN.BITS, with values ranging from 2 to 8. It specifies the number of most significant bits of the single turn feedback position. The maximum allowable movement is defined by the binary value of the most significant “FB#.STOREMULTITURN.BITS + 1” bits must not change by more than a value of 1.

Based on this definition, the maximum allowable movement is not constant. It will vary with the initial position at power down. Hence, there is a range of possible maximum allowable movement distances depending on the actual position at power down. The high and low ends of the range, however, are constant, and can be used to estimate a useable setting for the application.

• High end of range: Exceeding this movement will cause a fault, regardless of initial position.

• Low end of range: NOT exceeding this movement will NOT cause a fault, regardless of initial position.

A movement within this distance range may or may not cause a fault, depending on the actual position at power down.

The following formulas can be used to calculate the high and low ends of the ranges in 32bit counts.

High end of range:

Low end of range:

The following tables show the high and low ends of the range of possible maximum allowable movement distances per setting of FB#.STOREMULTITURN.BITS.

Allowable Movement

FB#. STOREMULTITURN .BITS	32 bit counts		Revs		Degrees
	High †	Low ‡	High †	Low ‡	High †	Low ‡
2	1073741823	536870912	0.2499999998	0.1250000000	89.99999992	45
3	536870911	268435456	0.1249999998	0.0625000000	44.99999992	22.5
4	268435455	134217728	0.0624999998	0.0312500000	22.49999992	11.25
5	134217727	67108864	0.0312499998	0.0156250000	11.24999992	5.625
6	67108863	33554432	0.0156249998	0.0078125000	5.62499992	2.8125
7	33554431	16777216	0.0078124998	0.0039062500	2.81249992	1.40625
8	16777215	8388608	0.0039062498	0.0019531250	1.40624992	0.703125

† The high end of the range will fault.
‡ The low end of the range will not fault.

Limitations

When using a multiturn absolute feedback device, Store Multiturn allows the drive to detect and fault if the motor moves while the power is off.

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• If the motor moves while the machine is powered off, then the drive could have an incorrect position value on restarting causing unexpected movement when the machine restarts.

When changing feedbacks, turn the multiturn feature off and then back on to reset the stored position. It is possible to install a new feedback within the single-turn range as the previous feedback and the drive does not detect a change.

If the motor moves while the drive is off, the drive will fault on power up and multiturn counts are reset. For example, if a running machine stops due to a power outage then the drive faults on power up and the stored multiturn counts reset.

Keyword	Description
FB#.STOREMULTITURN.ENABLE	Enables or disables the store multiturn feature.
FB#.STOREMULTITURN.BITS	Sets the number of single-turn bits used to determine if the position stored is valid after power cycle.

AKD1G Keyword Equivalents

The two capture units are now called CAP1 and CAP2 instead of CAP0 and CAP1.

AKD1G Keyword	AKD2G Equivalent
FB1.EXTENDEDMULTITURN	FB#.STOREMULTITURN.ENABLE

Examples

Typical Uses of Store Multiturn

Store Multiturn can be used if the existing absolute feedback counts do not meet the travel required in one direction.

Example Configuring FB#.STORE.MULTITURN.BITS

The drive uses 2⁶⁴ counts for position. One bit, the sign bit, determines direction, 31 bits determine the number of revolutions (multiturn bits), and 32 bits are used to determine where you are in a revolution (single turn bits). If configured to use 2 bits, of the most significant single-turn counts, to determine if the motor has moved, then the motor is allowed move up to while the drive is powered down without generating a fault. Keep in mind that the allowable movement is based on the position value at power down. The motor is allowed to move up to 45 degrees without generating a fault, and between 45 degrees and 90 degrees depending on the initial position value. If it moves more than 90 degrees, it will definitely generate a fault. For some machines this may be minimal movement and for others this may be unsafe; therefore, it is configurable.

Example

FB#.STOREMULTITURN.BITS = 3, means single turn bits 28, 29, 30 and 31 must not change more than a value of 1. While the drive is powered off, the motor is only allowed to move a distance of somewhere in the range of 22.5 degrees to just shy of 45 degrees. The actual threshold will depend on the starting position.

If the single turn position value at power down is 656,175,559 counts (32 bit counts/rev).

656,175,559 = 0010 0111 0001 1100 0111 0001 1100 0111

The 4 most significant bits = 0010 = 2 dec. The value of these bits is only allowed to change by 1, meaning one of the two following conditions:

• Increase to 3, with all lower bits = 1

• Decrease to 1, with all lower bits = zero

Increasing:

Max allowable position:	0011 1111 1111 1111 1111 1111 1111 1111	= 1,073,741,823
Power down position:	0010 0111 0001 1100 0111 0001 1100 0111	= 656,175,559
Difference =	0001 1000 1110 0011 1000 1110 0011 1000	= 417,566,264 counts
	= 0.0972222220 revs
	= 34.9999999254 deg

Decreasing:

Power down position:	0010 0111 0001 1100 0111 0001 1100 0111	= 656,175,559
Min allowable position:	0001 0000 0000 0000 0000 0000 0000 0000	= 268,435,456
Difference =	0001 0111 0001 1100 0111 0001 1100 0111	= 387,740,103 counts
	= 0.0902777777 revs
	= 32.4999999906 deg

See also

FB#.SETMULTITURNS