Re: Linearity of DC motors is becoming a cause of concern.

Student Max wrote:

Linerarity of perm magnet motors is a fact, torque is directly related to
current. Back EMF is directly related to rotational speed. Bank on it.

Your mistake is to assume that "rotational speed" can be controlled by
voltage, and it can not.

Torque is controlled with current. Current is a factor of the effective
differential voltage between driver and back EMF and the motors internal
resistance.

To control motor speed, you need a closed loop system.


Respected Gentlemen,
We are building a robot for IEEE contest and are running across an
issue that I hope this community can help me us with.
We decided to use (2) 12V DC motor for our drive train.
The following is the spec of the motors:
12V Permanent Magnet
· All metal gears
· Weight: 7.26 oz (.454 lbs)
· 200 RPM
· 30:1 gear reduction
· Stall torque 194 oz-in
· Rated torque at max efficiency. 50 oz-in
· Speed under rated load 161 RPM
· Stall Current 3.3 Amps
· 78% efficiency
· Current at rated load 730 mA
· No Load current 115 mA

When connecting it with the OOBOT40-3 we should theoretically have 127
variable speeds. We assumed that it's not going to be that way but at
least expected a decent variability.

After doing an experiment - connecting the OOPIC to the motors and
getting the speeds by using a tachometer, here are the results.
OOPIC Speed vs forward motor speed.
60 - 83
65 - 93
70 - 102
75 - 110
80 - 117
85 - 123
90 - 129
100 - 137
110 - 147
120 - 153
130 - 161
140 - 162
150 - 174
155 - 187
160 - 188 (max)

OOPIC Speed vs reverse motor speed.
60 - 84
65 - 94
70 - 103
75 - 110
80 - 118
85 - 125
90 - 131
100 - 140
110 - 148
120 - 153
130 - 161
140 - 162
150 - 175
155 - 187
160 - 188

We are controlling the motors by using an H-bridge. The PWM frequency
is 2 KHz.

Is there a possibility to gain some more variability without adding
major components to the design since we are only 30 days away from the
competition and the Profs are going to chew me out if I propose some
gadget to be added.

Any help, comment, suggestion, will be really appreciated

Best Regards
Sahim.

.

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