Radio Control Dual Forward & Reversing Speed Controls (RDFR series)

* MOBILE ROBOTS, TWIN SCREW BOATS
* TANK TYPE STEERING WITH RIGHT & LEFT MOTORS, CONTROLS 2 MOTORS
* SIMPLE SINGLE JOY STICK OPERATION
* SELECT BEST MIXING, GAIN, BRAKE & ACCELERATION CURVES
* SELECT NO MIXING FOR DUAL INDEPENDENT CONTROLLERS
* 4.5-140 VDC, 220 AMPS, 1.5 HP PER MOTOR/SIDE
* OPTO-ISOLATED R/C INPUTS PLUG IN LIKE A SERVO
* BRAKE / CLUTCH OUTPUTS

The RDFR DIRECTOR family of speed controls perform speed, direction and steering functions for vehicles powered by two independent electric motors employed as a right drive and a left drive; all from only two Radio/Control channels. Ideal for robots with tank tread drives or separate drive wheels. When used with a single spring centered joy stick: hands off is stop, up stick gets straight ahead, and down yields backwards. Pure right or left twirls the vehicle as the motors turn opposite directions. In between stick positions are completely proportional, including reverse. RDFRs eliminate heavy duty steering servos yet the steering signal is available. In twin-screw boats or subs maneuverability is enhanced by differential props combined with rudder steering. RDFRs have also been used to command proportional hydraulic valves to control hydraulic motors.

One R/C channel commands speed/direction and the other steering.

Five steering algorithms are jumper selectable: LINEAR, mild & moderate EXPOnential1 & 2, SKIP and CUSTOM. The SKIP algorithm mixes steering into the speed commands only near the extremes of rudder steering. In boats with rudders this enables maximum speed and stable torque roll forces while still offering maneuverability; especially useful for subs.

There is also a selection of gains. The factory CUSTOM option allows you to optimize these functions for your vehicle.

They may also be configured without mixing to be two independent speed controls. This lets you control your vehicle with two joysticks, one for each motor, and perform the turning algorithm with your thumbs. The RDFR is the only controller that gives you your choice of steering methods. Yet another configuration provides a dual output controller with a single switchable command.

RDFR DIRECTORs are compatible with most model R/C systems including PCM or FM and plug into your receiver like a servo. They're available with Futaba J or G, Airtronics, Deans, or JR connectors; please specify when ordering. The RDFR family replaces our older DFRM type.

Inside an RDFR are two rugged forward/reverse speed controls coupled together through special logic that generates the differential right and left motor rotation needed to guide the vehicle. The logic also validates the R/C signals. If they are out of tolerance the last known good command is used. However, a majority of faulty signals safely shuts the motors off. The RDFR DIRECTOR is designed without relays for professional applications requiring repetitive forward to reverse commands.
The optically isolated outputs are Pulse Width Modulated full H-bridge circuits originally designed for Permanent Magnet DC wheel chair motors. N-channel power MOSFETs are paralleled in each H-bridge leg to achieve the specified amperage. For example: the RDFR38 contains 48 70 Amp MOSFETs. In contrast to hobby controllers VANTEC's MOSFETs are all properly thermally mounted flat to the metal heat dissapating case with special insulating washers.
The driver PWModulates the bottom half of the bridge while holding the diagonal upper bridge leg on in order to effect efficient re-circulating currents in the motor.

Sequenced electro-dynamic braking shunts the motor by modulating both top legs of the bridge. With a command to "stop" the brake is gently ramped from 0 to 100% duty cycle. When an R/C command that changes direction is received the brake is quickly sequenced to first bring the motor to a halt, then the reversing PWM power is ramped up to the commanded speed. This forced sequencing minimizes motor "plugging" and stress on mechanical components. The implementation and timing of these functions is user selectable.

Output current through the MOSFETransistors may compression limited above a threshhold by PWM duty cycle limiting in RDFR21-23 models. There's a threshhold adjustment trimpot for each output. Some short circuit protection is provided.
OPTIONAL BRAKE RELEASE or CLUTCH ENGAGEMENT: A 2 Amp output current sink turns on whenever there's any R/C "motion" command. With a "stop" R/C command it goes off after a short delay. This option is available independently for each motor or as a single output logically OR'd.
Simultaneous operation of both halves at max ratings may require additional heat sinking; usually the metal frame of your vehicle is sufficient. No special heatsinks are required. The RDFR comes a detailed instruction manual.

Warranty Information:
One year warranty for repair on a fixed price basis according to the repair deposit fee listed on the Vantec site's price list at the time of submission to Vantec for repair, limited to units which are not abused and one such repair per unit. All repairs must include proper credit card information and have prior approval as evidenced by a Return Material Authorization number or they will be returned to the sender.

Quantity Disounts available:
If you plan to purchase 4 or more of the same RDFR speed controllers at once, you may qualify for a special discount.


More information specific for Combat Robotics use

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.   Knowledge is winning power.

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. Sometimes they also have other ratings. In the automobile world there are "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 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.  Check the regulations of your event for outlawed battery chemistries.

Most robots use a pair of 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 and 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.

RDFR ratings.  The operating voltage range is specified in real volts and cannot be exceeded even momentarily without damage to the unit. 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 the 3 minutes of a BattleBot event it is not significant if the unit is properly mounted.

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 mechnical load current rating is the absolute minimum continuous Vantec current spec required; remember to de-rate the controller for two motors and the true effective continuous current under competitive driving conditions, etc. When you know  the true effective continuous current for all the motors and the needed running time then the AH 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.

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 people 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.

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.

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. If you are interested in configuring your Vantec ESC this way, please .

While VANTEC does offer a limited warranty but as a practical matter that warranty is seldom applicable to competitive and particularly destructive robot contests.   These professional competitions are like auto racing; a lot of money is invested, risked and lost so that somebody, and we hope that is YOU,  wins.