Motion control

[Vanisp]

Dear Mr.Jochen,

This is a new post regarding Contour moves.

As the velocity of a contoured move is determined by the distance between the position points in the contour array. Request you to kindly help us out in sorting out few problems.

Our application is Dual axes linear motion of 1 meter length. Each axis consists of  an AC Drive, Linear motor and linear incremental encoder of resolution 25 microns (1 mm = 25 counts). The maximum speed of the system is 2 m/s.

We could achieve the maximum speed in Absolute position mode. And could successfully make a contour move of different patterns upto 0.5 m/s in Absolute contouring. But unable to achieve beyond 0.5 m/s.

1. Kindly give us sample points in counts where we can achieve 2 m/s. 

2. The contour move always starts from zero, Can we make contour moves which start from a Non-zero point. Also the contour move should be continuous.

Thanking you,

Waiting for your reply.

Regards.

[Jochen]

Hello Vanisp,

it's very kind from you, that you address me directly, but as this is a public forum there is a whole community monitoring it.

Anyway. Could you please  explain a bit more in detail what happens if you exceed the velocity limit of 0.5 m/s? Do you get an error? Could you please post a log file that contains your contouring data?

Contouring operations are always relative to the start position. So e. g. if you move your axis to position 10000 and start a contouring operation, that starts with "0, 1, 4, 8,..." in the position buffer, this doesn't mean, that the axis jumps back to these absolute positions. Instead the absolute positions of this move start with "10000,10001, 10004, 10008,..." (this example is valid for Absolute Contouring Mode. Please refer to the documentation of "Set Operation Mode" for more details).

For continuous contouring operations please refer to the example "Buffered Contouring(Rose).vi". In fact the maximum number of contouring points for a single contouring operation is 2^32 values. With a update period of 20 ms this equals to 497 days of continuous operation, which should be enough for the vast majority of applications.

Kind regards,

Jochen Klier

National Instruments

Message Edited by Jochen on 01-27-2010 01:39 PM

[Vanisp]

Dear Sir,

We get an error message stating "overcurrent" from the drive when I increase the velocity (higher than 0.7 m/s). Kindly have a look at the contour array. The contour array generates figure 8 pattern. Please let me know if any further details are required.

29.40000	0.660
1041.900	91.140
1854.090	293.670
2630.220	607.230
3355.170	1025.760
4014.780	1541.130
4596.270	2143.260
5088.300	2820.480
5481.270	3559.590
5767.560	4346.190
5941.620	5164.950
6000.000	6000.000
5985.390	6418.530
5868.900	7247.460
5638.170	8052.120
5297.700	8816.820
4854.090	9526.710
4316.040	10167.960
3693.960	10728.060
3000.000	11196.150
2247.630	11563.110
1451.520	11821.770
627.1800	11967.120
-209.400	11996.340
-1041.900 11908.860
-1854.090	 11706.330
-2630.220	 11392.770
-3355.170	 10974.240
-4014.780	 10458.870
-4596.270	 9856.740
-5088.300	 9179.520
-5481.270	 8440.410
-5767.560	 7653.810
-5941.620	 6835.050
-6000.000 6000.000
-5941.620 5164.950
-5767.560	 4346.190
-5481.270	 3559.590
-5088.300	 2820.480
-4596.270	 2143.260
-4014.780 1541.130
-3355.170	 1025.760
-2630.220	 607.230
-1854.090	 293.670
-1041.900	 91.140
209.4000	-3.660
1041.900	-91.140
1854.090	-293.670
2630.220	-607.230
3355.170	-1025.760
4014.780	-1541.130
4596.270	-2143.260
5088.300	-2820.480
5481.270	-3559.590
5767.560	-4346.190
5941.620	-5164.950
6000.000	-6000.000
5941.620	-6835.050
5767.560	-7653.810
5481.270	-8440.410
5088.300	-9179.520
4596.270	-9856.740
4014.780	-10458.870
3355.170	-10974.240
2630.220	-11392.770
1854.090	-11706.330
1041.900	-11908.860
209.4000	-11996.340
-627.180	-11967.120
-1451.520	 -11821.770
-2247.630	 -11563.110
-3000.000	 -11196.150
-3693.960	 -10728.060
-4316.040	 -10167.960
-4854.090	 -9526.710
-5297.700 -8816.820
-5638.170	 -8052.120
-5868.900	 -7247.460
-5985.390	 -6418.530
-5985.390	 -5581.470
-5868.900	 -4752.540
-5638.170 -3947.880
-5297.700	 -3183.180
-4854.090	 -2473.290
-4316.040	 -1832.040
-3693.960	 -1271.940
-3000.000	 -803.850
-2247.630	 -436.890
-1451.520	 -178.230
-627.180	-32.880
0.000000	0.000
0.000000	0.000

 Kindly help us out.

Regards,

Vani

[Jochen]

Vani,

if your drive runs into an overcurrent situation, this typically means, that the acceleration values are too high (current is proportional to torque and torque is proportional to acceleration).

Looking at your  contouring data, this might be the case (e. g. there is a jump from 29.4 to 1041.9 in the beginning). I don't know, what this jump means in terms of physical acceleration, but you should be able to calculate that.

Please double check the accelerations generated by your contouring profile and adjust them accordingly.

Kind regards,

Jochen

[Vanisp]

Dear Sir,

Please have a look at this contour array given in the attachment. This array was generated from MOTION ASSISTANT for the move constraints given below.

Velocity = 0.75 m/s (which is 30000 counts/s)

Acceleration = 3.75 m/s2 (150000 counts/s2)

Deceleration = 3.75 m/s2 (150000 counts/s2)

Here too we get the same error message. 

x-axis : 29, 39, 69, 119, 188 ,................

y-axis : 0, 1, 4, 8, 13, 19, .................

Waiting for your reply. 

Thanking you,

With regards,

Vani

[Jochen]

Vani,

overcurrent means too much torque. It's that simple. If you don't run into this problem with an Absolute Move, there might be differences between this operation mode and the contouring mode in terms of forces that occur in your mechanical system.

Maybe you need to optimize the tuning of your PID parameters. I could imagine, that the contouring profile introduces oscillations to your axes and you need to tune the control loops for better damping.

If tuning is the potential source of this issue, please refer to this document.

Kind regards,

Jochen

[Vanisp]

Dear Sir,

Please find an attachment of the step response of the system and kindly suggest.

Thanking you,

Regards,

S.P.Vani 

[Jochen]

The step response doesn't look too bad, but you could try to get a critically damped system (without overshot) by increasing Kd. If the system becomes a bit nervous, you could increase Td to 3 - 5.

As your system doesn't settle to the exact target position, I guess your Ki value is zero. Setting Ki to a low value (e. g. 1 or 2) and decreasing the integration limit to a quite low value (e. g. 100 to 200) could result in better static accuracy, but it also tends to add an overshot, so you should be careful with that.

[Vanisp]

Dear Sir,

As the mechanical system is not available with us, we are unable to fine-tune and check the step response. 

Can you please advice us what could be our next step which can be done. 

Thanking you,

Regards,

Vani 

[Vanisp]

Dear Sir,

I would also like to mention few details regarding the mechanical system. The total force required to run the system is 90 N (F=ma = 18 Kg * 5m/s2). 5m/s2 corresponds to 2,00,000 counts/s2 in MAX. This excludes external frictional forces and static friction. 

Motor Specification: -

Motor peak force = 213N

Max. force at speed 2 m/s = 134N (63% of 213). 

Here Duty cycle = 100% 

Max. force at 100% duty cycle = 40% of total available force (40% of 134) = 54N.

The max. available force = 54N and the required force = 90N. We are able to achieve upto 1m/s speed for this load. Is the acceleration too high for this application? Or do I have to procure a motor that has got a capacity to produce more force? 

Also with NO LOAD condition and acceleration 8 m/s2, we were able to achieve 1.7 m/s speed in absolute contouring mode. 

We will surely try to get a critically damped system after we get the mechanical system. Kindly advice what could be possibly done. 

Thanking you,

Regards,

Vani