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BATTLEBOT Design Considerations

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.

Note: Our products are not safety devices nor for use in life-critical or life support systems. Specifications and price subject to change without notice. Patented. Some trade names, trademarks & content owned by others.

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