ENERGY:
Batteries supply the power and they're always rated in Ampere Hours. An ampere hour is the
current you can draw out of a battery for an hour before it goes dead. Other
ratings include the automobile world's "cold cranking amps". For
a given chemistry these bear a relationship to the basic ampere hours. It is
important thing to know is how much peak current can you draw out of the battery for a
very short time and still get a reasonable voltage out of it.... this is like "cold
cranking amps". One example of gel cell chemistry was a 7 Ampere Hour (AH) 12V
unit that delivered about 38 peak Amperes @ 11 volts for 5 seconds. A 7 Ampere Hour Nicad
chemisty battery would have significantly higher peak amps.
MULTIPLE MOTORS:
Most robots use a pair of DC PM motors; a right side motor and a left side motor. Sometimes
vehicles use two right motors and two left motors. If there is more than one motor for a
side consider how to wire the motors. Motors that are hard coupled via gears, sprockets
or shafts must be wired in series to force them to share the torque load. That
means the battery voltage required is motor voltage times number of motors series'ed. The
current load is that of a single motor. For hard coupled designs consider that using
one proper sized motor is usually more efficient than coupling two smaller motors.
On the other hand if there are two right side motors each independently coupled to it's
own wheel, the motors should be wired in parallel. Parallel wired motors will be powered
by a battery voltage equal to the motor rated voltage but the current draw will increase
by the number of motors on a side.
MOTOR CONTROL: Virtually all purchases for winning competitive "tank style"
steered 'bots use our RDFR series which is a dual controller for vehicles
with a right side motor and a left side motor. We have provided an
easy Selection Guide to help you chose the right RDFR controller model.
Detailed specifications about the RDFR series is at
RDFR speed controls with
popular models for particular weight classes noted. Pricing can be found on our
Price List. Although we can only
offer limited individual advice and engineering services for robot design
for competitive robot events this page is a brief discussion of some basic
design considerations about batteries, speed controls, motors and gearing.
RDFR ratings: The operating voltage range is specified in real volts and cannot be
exceeded even momentarily without damage. Batteries come off the charger at
higher than normal voltage and this must be considered. For example the common 1.2 volt
Nicad might give 1.5 V when it comes off the charger. Therefore, a RDFR23 rated at 30
volts can't use more than 20 cells, 19 Nicads would be a wiser choice. The current rating
for each half of an RDFR, like the half that controls the right side of the vehicle, is
the spec sheet current rating. There is a continuous current rating and a higher 5 second
starting current rating based upon perfect heat sinking, 100% PWM duty cycle (read fully
on), and infrequent starting surges. These ratings are lowered when both halves are
operated, when duty cycles like 95% are commanded, when the competitive driving style is
constant direction reversing, or when the heat sinking is less than ideal. Because of the
number of variables Vantec cannot assist in precisely determining this de-rating but for
short runs like a BattleBot event it is not significant if
the unit is properly
mounted.
>When the motors are coasting, particularly during electro-dynamic braking, they act as
generators. The power in the inertia of a 200 lb robot going 20 mph must be absorbed
by something. It is absorbed by the chemistry of your rechargeable battery. In
this situation the batteries act as a voltage clamp protecting the controller. The
battery chemistry must be healthy for this to occur. Don't add a series diode in line in
the battery line because it will block the clamping action and the controller will fail.
Some users place a large 100,000 ufd "computer" electolytic capacitor
across the power going into the controller, a good idea, to further smooth the input
voltage and clamping action.
MOTOR RATINGS:
Motors are rated many ways but the voltage and operating current are the most important
for choosing the battery and speed controller. A current rating may be given in motor
literature or on a label. What voltage and mechanical load conditions were applied for the
given ampere current rating? A "No Load" current rating is not useful for sizing
speed controllers or batteries. The normal continuous current rating while
under realistic mechnical load is the
absolute minimum continuous Vantec current spec required. When you know the true effective continuous current for all
the motors and the needed running time then the AmpHour battery size required can be
calculated. But the starting current of all of the motors must also be considered in
determining the battery AH size; remembering battery peak ampere limits. A larger than
calculated AH battery simply extends the running time with a weight penalty. In a properly
designed system it is the motor load that determines the current draw, not the battery AH
rating.
GEARING:
If the mechanical load on the motor does NOT exceed the load the motor manufacturer used
to determine continuous amperes then the system should work. The load is controlled
by wheel or track size, and gearing. Gearing is hard to physically do and hard to
determine the correct ratio. Frequently, a guess is made, the gears or sprockets purchased
and installed, and then the unlucky builder discovers the motor never revs up but instead
is lugged down drawing excessive current. Since gearing is hard to change but wiring is
easy the unlucky builder doubles the voltage to speed up the vehicle.
But now the lugged
down motors draw twice the current (Twice NOT half!). Just after the robot finally goes
fast for a minute the controller and motor are destroyed.
A better way is to design a gearing mechanism that can be easily changed. One idea is to
use a gearhead motor with a NO LOAD output RPM 4X your wheel target speed. When you load
the motor down to an efficient RPM of about 60% of that No Load speed you'll need 2.4:1
sprockets to complete the gearing. Pick sets of gears or sprockets that can be
interchanged to modify that ratio plus and minus over a wide range. Finally, verify proper
gearing by measuring the motor currents with a cheap automotive ampere meter. Fuses
can be used as inexpensive recording amp meters remembering they blow at currents over
their ratings. Different wheels, rubber compounds, driving surface and competitions may
call for different ratios, just like auto racing.
BIGGER:
If you are building a heavywight or super-heavyweight you may determine that there is no
Vantec RDFR controller big enough. These Dual controllers can be FACTORY supplied as
Singles with roughly 1.7X the list RDFR current rating. The price is the same as the
RDFR model, the model number is usually incremented by 1 to denote the higher current
capability. For example: A RDFR38E is 32 volts, 80 amps but when converted to a
single it is a RSFR39E, 32 volts, 130 amps. The singles may be operated
totally independently OR utilize the mixing function when ordered
as MIX1 and MIX2 pairs.
REALITY: These
professional competitions are like auto racing; a lot of money is invested, risked and
lost so that somebody, and we hope YOU, wins. |
